Lint round 5+6: staticcheck and errcheck to zero (#2130)

* refine the plan

* Fix all staticcheck lint findings (uncapped)

golangci-lint's default max-same-issues=3 was hiding most of the backlog:
the true pre-fix count was 69 staticcheck findings, not 34. This fixes all
of them, driving staticcheck to zero:

- ST1023/QF1011 (37): omit explicit types inferred from the RHS
- S1009/S1031 (15): drop redundant nil checks before len()/range
- SA9003 (9): remove comment-only empty branches, keeping the comments
- QF1007 (3): merge conditional assignment into declaration
- QF1006 (3): lift break conditions into loop conditions
- QF1001 (3): apply De Morgan's law / name the negated predicate

Also updates plans/lintfixes.md with the cap discovery and the corrected
errcheck picture (1202 uncapped, ~949 of them fmt.Fprint*).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Drive errcheck to zero: config for bulk categories, propagate real errors

Adds .golangci.yml with errcheck exclude-functions for fmt.Fprint* (usage
printers), (*bufio.Writer).Write/WriteString (sticky errors, surfaced at the
now-checked final Flush), and (*strings.Builder).WriteString; pins
max-issues-per-linter/max-same-issues to 0 so CI reports true counts.

Real error paths now propagate instead of being dropped:
- Finalize{Reader,Writer}Options in join/put/filter/split/tee and the
  repl/script entry points: 'mlr join -i badformat' now errors instead of
  silently using wrong separators
- final output-stream Flush in pkg/stream: write failure no longer exits 0
- DSL emit/print/dump redirect writes, matching their sibling branches
- CSV writer WriteCSVRecordMaybeColorized, close-time Flush in file output
  handlers, ENV[...] Setenv, REPL record-write and redirect-close errors
- termcvt write-side Close before rename (had "TODO: check return status")

The rest are deliberate ignores, marked with _ = and a comment where the
reason isn't obvious: unset-of-missing-path no-ops, read-side closes,
mid-stream FlushOnEveryRecord, init-time strftime registrations, in-memory
usage-capture pipes, and regtest-harness env/temp-file teardown.

golangci-lint now reports 0 issues on ./cmd/mlr ./pkg/... with all caps off.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.golangci.yml

@@ -0,0 +1,28 @@
+# Configuration for golangci-lint, used by .github/workflows/golangci-lint.yml
+# and available locally via `golangci-lint run ./cmd/mlr ./pkg/...`.
+# Tracking issue for lint cleanup: https://github.com/johnkerl/miller/issues/2109
+
+version: "2"
+
+linters:
+  settings:
+    errcheck:
+      exclude-functions:
+        # Writes to stdout/stderr for usage and error messages; there is
+        # nothing useful to do if these fail.
+        - fmt.Fprint
+        - fmt.Fprintf
+        - fmt.Fprintln
+        # bufio.Writer errors are sticky: once a write fails, all subsequent
+        # writes and the next Flush report the same error. Record-writer
+        # output goes through a final checked Flush in pkg/stream.
+        - (*bufio.Writer).Write
+        - (*bufio.Writer).WriteString
+        # Documented to always return a nil error.
+        - (*strings.Builder).WriteString
+
+issues:
+  # Report all findings: the defaults (50/3) hide large categories and make
+  # fix progress impossible to track. See plans/lintfixes.md.
+  max-issues-per-linter: 0
+  max-same-issues: 0

cmd/mlr/main.go

@@ -55,7 +55,7 @@ func main() {
 			fmt.Fprintln(os.Stderr, os.Args[0], ": ", "Could not start CPU profile: ", err)
 			return
 		}
-		defer handle.Close()
+		defer func() { _ = handle.Close() }()
 
 		if err := pprof.StartCPUProfile(handle); err != nil {
 			fmt.Fprintln(os.Stderr, os.Args[0], ": ", "Could not start CPU profile: ", err)

pkg/auxents/hex.go

@@ -68,7 +68,7 @@ func hexMain(args []string) int {
 
 			hexDumpFile(istream, doRaw)
 
-			istream.Close()
+			_ = istream.Close()
 			if !doRaw && len(args) > 1 {
 				fmt.Println()
 			}

pkg/auxents/lecat.go

@@ -54,7 +54,7 @@ func lecatMain(args []string) int {
 
 			lecatFile(istream, doColor)
 
-			istream.Close()
+			_ = istream.Close()
 		}
 	}
 	return 0

pkg/auxents/termcvt.go

@@ -96,9 +96,11 @@ func termcvtMain(args []string) int {
 
 			termcvtFile(istream, ostream, inputTerminator, outputTerminator)
 
-			istream.Close()
+			_ = istream.Close()
-			// TODO: check return status
+			if err := ostream.Close(); err != nil {
-			ostream.Close()
+				fmt.Fprintf(os.Stderr, "mlr termcvt: %v\n", err)
+				os.Exit(1)
+			}
 
 			err = os.Rename(tempname, filename)
 			if err != nil {
@@ -120,7 +122,7 @@ func termcvtMain(args []string) int {
 
 			termcvtFile(istream, os.Stdout, inputTerminator, outputTerminator)
 
-			istream.Close()
+			_ = istream.Close()
 		}
 	}
 	return 0
@@ -144,6 +146,9 @@ func termcvtFile(istream *os.File, ostream *os.File, inputTerminator string, out
 
 		// This is how to do a chomp:
 		line = strings.TrimRight(line, inputTerminator)
-		ostream.Write([]byte(line + outputTerminator))
+		if _, err := ostream.Write([]byte(line + outputTerminator)); err != nil {
+			fmt.Fprintf(os.Stderr, "mlr termcvt: %v\n", err)
+			os.Exit(1)
+		}
 	}
 }

pkg/auxents/unhex.go

@@ -39,7 +39,7 @@ func unhexMain(args []string) int {
 				os.Exit(1)
 			}
 			unhexFile(istream, os.Stdout)
-			istream.Close()
+			_ = istream.Close()
 		}
 	}
 
@@ -83,7 +83,10 @@ func unhexFile(istream *os.File, ostream *os.File) {
 					os.Exit(1)
 				}
 				byteArray[0] = byte(scanValue)
-				ostream.Write(byteArray)
+				if _, err := ostream.Write(byteArray); err != nil {
+					fmt.Fprintln(os.Stderr, "mlr unhex:", err)
+					os.Exit(1)
+				}
 			}
 		}
 	}

pkg/bifs/arithmetic.go

@@ -718,26 +718,26 @@ func BIF_mod_exp(input1, input2, input3 *mlrval.Mlrval) *mlrval.Mlrval {
 // * empty-null always loses against numbers
 
 func min_f_ff(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = input1.AcquireFloatValue()
+	a := input1.AcquireFloatValue()
-	var b float64 = input2.AcquireFloatValue()
+	b := input2.AcquireFloatValue()
 	return mlrval.FromFloat(math.Min(a, b))
 }
 
 func min_f_fi(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = input1.AcquireFloatValue()
+	a := input1.AcquireFloatValue()
-	var b float64 = float64(input2.AcquireIntValue())
+	b := float64(input2.AcquireIntValue())
 	return mlrval.FromFloat(math.Min(a, b))
 }
 
 func min_f_if(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = float64(input1.AcquireIntValue())
+	a := float64(input1.AcquireIntValue())
-	var b float64 = input2.AcquireFloatValue()
+	b := input2.AcquireFloatValue()
 	return mlrval.FromFloat(math.Min(a, b))
 }
 
 func min_i_ii(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a int64 = input1.AcquireIntValue()
+	a := input1.AcquireIntValue()
-	var b int64 = input2.AcquireIntValue()
+	b := input2.AcquireIntValue()
 	if a < b {
 		return input1
 	}
@@ -756,8 +756,8 @@ func min_b_bb(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
 }
 
 func min_s_ss(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a string = input1.AcquireStringValue()
+	a := input1.AcquireStringValue()
-	var b string = input2.AcquireStringValue()
+	b := input2.AcquireStringValue()
 	if a < b {
 		return input1
 	}
@@ -883,26 +883,26 @@ func BIF_minlen_within_map_values(m *mlrval.Mlrmap) *mlrval.Mlrval {
 }
 
 func max_f_ff(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = input1.AcquireFloatValue()
+	a := input1.AcquireFloatValue()
-	var b float64 = input2.AcquireFloatValue()
+	b := input2.AcquireFloatValue()
 	return mlrval.FromFloat(math.Max(a, b))
 }
 
 func max_f_fi(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = input1.AcquireFloatValue()
+	a := input1.AcquireFloatValue()
-	var b float64 = float64(input2.AcquireIntValue())
+	b := float64(input2.AcquireIntValue())
 	return mlrval.FromFloat(math.Max(a, b))
 }
 
 func max_f_if(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a float64 = float64(input1.AcquireIntValue())
+	a := float64(input1.AcquireIntValue())
-	var b float64 = input2.AcquireFloatValue()
+	b := input2.AcquireFloatValue()
 	return mlrval.FromFloat(math.Max(a, b))
 }
 
 func max_i_ii(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a int64 = input1.AcquireIntValue()
+	a := input1.AcquireIntValue()
-	var b int64 = input2.AcquireIntValue()
+	b := input2.AcquireIntValue()
 	if a > b {
 		return input1
 	}
@@ -921,8 +921,8 @@ func max_b_bb(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
 }
 
 func max_s_ss(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var a string = input1.AcquireStringValue()
+	a := input1.AcquireStringValue()
-	var b string = input2.AcquireStringValue()
+	b := input2.AcquireStringValue()
 	if a > b {
 		return input1
 	}

pkg/bifs/bits.go

@@ -223,8 +223,8 @@ func BIF_signed_right_shift(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
 // Unsigned right shift
 
 func ursh_i_ii(input1, input2 *mlrval.Mlrval) *mlrval.Mlrval {
-	var ua uint64 = uint64(input1.AcquireIntValue())
+	ua := uint64(input1.AcquireIntValue())
-	var ub uint64 = uint64(input2.AcquireIntValue())
+	ub := uint64(input2.AcquireIntValue())
 	var uc = ua >> ub
 	return mlrval.FromInt(int64(uc))
 }

pkg/bifs/datetime.go

@@ -448,18 +448,18 @@ func init() {
 	})
 
 	ss := strftime.NewSpecificationSet()
-	ss.Set('1', appender1)
+	_ = ss.Set('1', appender1)
-	ss.Set('2', appender2)
+	_ = ss.Set('2', appender2)
-	ss.Set('3', appender3)
+	_ = ss.Set('3', appender3)
-	ss.Set('4', appender4)
+	_ = ss.Set('4', appender4)
-	ss.Set('5', appender5)
+	_ = ss.Set('5', appender5)
-	ss.Set('6', appender6)
+	_ = ss.Set('6', appender6)
-	ss.Set('7', appender7)
+	_ = ss.Set('7', appender7)
-	ss.Set('8', appender8)
+	_ = ss.Set('8', appender8)
-	ss.Set('9', appender9)
+	_ = ss.Set('9', appender9)
-	ss.Set('N', appenderN)
+	_ = ss.Set('N', appenderN)
-	ss.Set('O', appenderO)
+	_ = ss.Set('O', appenderO)
-	ss.Set('s', appenderS)
+	_ = ss.Set('s', appenderS)
 
 	strftimeExtensions = strftime.WithSpecificationSet(ss)
 }

pkg/bifs/relative_time.go

@@ -27,10 +27,7 @@ func BIF_dhms2sec(input1 *mlrval.Mlrval) *mlrval.Mlrval {
 	remainingInput := input
 	var seconds int64
 
-	for {
+	for remainingInput != "" {
-		if remainingInput == "" {
-			break
-		}
 		var n int64
 		var rest string
 
@@ -89,10 +86,7 @@ func BIF_dhms2fsec(input1 *mlrval.Mlrval) *mlrval.Mlrval {
 	remainingInput := input
 	var seconds float64
 
-	for {
+	for remainingInput != "" {
-		if remainingInput == "" {
-			break
-		}
 		var f float64
 		var rest string
 

pkg/cli/option_parse.go

@@ -3458,7 +3458,7 @@ var MiscFlagSection = FlagSection{
 					fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
 					os.Exit(1)
 				}
-				defer handle.Close()
+				defer func() { _ = handle.Close() }()
 
 				lineReader := bufio.NewReader(handle)
 
@@ -3565,7 +3565,7 @@ var MiscFlagSection = FlagSection{
 			help: "Specify timezone, overriding `$TZ` environment variable (if any).",
 			parser: func(args []string, argc int, pargi *int, options *TOptions) {
 				CheckArgCount(args, *pargi, argc, 2)
-				os.Setenv("TZ", args[*pargi+1])
+				_ = os.Setenv("TZ", args[*pargi+1])
 				*pargi += 2
 			},
 		},

pkg/climain/mlrcli_mlrrc.go

@@ -47,7 +47,7 @@ func tryLoadMlrrc(
 	if err != nil {
 		return false
 	}
-	defer handle.Close()
+	defer func() { _ = handle.Close() }()
 
 	lineReader := bufio.NewReader(handle)
 

pkg/dsl/cst/blocks.go

@@ -63,7 +63,7 @@ func (root *RootNode) BuildStatementBlockNodeFromBeginOrEnd(
 	// With "parent":1,"children":[1], StatementBlockInBraces.Children[0] is the StatementBlock.
 	// Unwrap so we pass StatementBlock to BuildStatementBlockNode.
 	if astStatementBlockNode.Type == asts.NodeType(NodeTypeStatementBlockInBraces) {
-		lib.InternalCodingErrorIf(astStatementBlockNode.Children == nil || len(astStatementBlockNode.Children) < 1)
+		lib.InternalCodingErrorIf(len(astStatementBlockNode.Children) < 1)
 		astStatementBlockNode = astStatementBlockNode.Children[0]
 	}
 	statementBlockNode, err := root.BuildStatementBlockNode(astStatementBlockNode)

pkg/dsl/cst/builtin_functions.go

@@ -875,7 +875,7 @@ func (node *LogicalANDOperatorNode) Evaluate(
 
 	bout := node.b.Evaluate(state)
 	btype := bout.Type()
-	if !(btype == mlrval.MT_ABSENT || btype == mlrval.MT_BOOL) {
+	if btype != mlrval.MT_ABSENT && btype != mlrval.MT_BOOL {
 		return mlrval.FromNotNamedTypeError("&&", bout, "absent or boolean")
 	}
 	if btype == mlrval.MT_ABSENT {
@@ -957,7 +957,7 @@ func (node *LogicalOROperatorNode) Evaluate(
 
 	bout := node.b.Evaluate(state)
 	btype := bout.Type()
-	if !(btype == mlrval.MT_ABSENT || btype == mlrval.MT_BOOL) {
+	if btype != mlrval.MT_ABSENT && btype != mlrval.MT_BOOL {
 		return mlrval.FromNotNamedTypeError("||", bout, "absent or boolean")
 	}
 	if btype == mlrval.MT_ABSENT {

pkg/dsl/cst/dump.go

@@ -202,7 +202,9 @@ func (node *DumpStatementNode) Execute(state *runtime.State) (*BlockExitPayload,
 		}
 	}
 	outputString := buffer.String()
-	node.dumpToRedirectFunc(outputString, state)
+	if err := node.dumpToRedirectFunc(outputString, state); err != nil {
+		return nil, err
+	}
 	return nil, nil
 }
 
@@ -244,6 +246,5 @@ func (node *DumpStatementNode) dumpToFileOrPipe(
 	}
 	outputFileName := redirectorTarget.String()
 
-	node.outputHandlerManager.WriteString(outputString, outputFileName)
+	return node.outputHandlerManager.WriteString(outputString, outputFileName)
-	return nil
 }

pkg/dsl/cst/emit_emitp.go

@@ -537,7 +537,10 @@ func (node *EmitXStatementNode) executeNonIndexedNonLashedEmit(
 					nextLevelNames = append(nextLevelNames, pe.Key)
 					nextLevelValues = append(nextLevelValues, pe.Value.Copy())
 				}
-				node.executeNonIndexedNonLashedEmit(nextLevelNames, nextLevelValues, state)
+				err := node.executeNonIndexedNonLashedEmit(nextLevelNames, nextLevelValues, state)
+				if err != nil {
+					return err
+				}
 			}
 		}
 	}
@@ -625,7 +628,10 @@ func (node *EmitXStatementNode) executeNonIndexedLashedEmit(
 					nextLevelNames = append(nextLevelNames, pe.Key)
 					nextLevelValues = append(nextLevelValues, pe.Value.Copy())
 				}
-				node.executeNonIndexedNonLashedEmit(nextLevelNames, nextLevelValues, state)
+				err := node.executeNonIndexedNonLashedEmit(nextLevelNames, nextLevelValues, state)
+				if err != nil {
+					return err
+				}
 			}
 		}
 	}
@@ -821,13 +827,16 @@ func (node *EmitXStatementNode) executeIndexedNonLashedEmitAux(
 						return err
 					}
 				} else {
-					node.executeIndexedNonLashedEmitPAux(
+					err := node.executeIndexedNonLashedEmitPAux(
 						newrec,
 						[]string{names[i]},
 						[]*mlrval.Mlrmap{valueAsMap},
 						indices[1:],
 						state,
 					)
+					if err != nil {
+						return err
+					}
 				}
 			}
 		}
@@ -966,13 +975,16 @@ func (node *EmitXStatementNode) executeIndexedLashedEmitAux(
 		if len(indices) > 1 && nextLevelMaps[0] != nil {
 			// Recurse.  The leading map drives the iteration; we don't
 			// continue even if other maps aren't empty
-			node.executeIndexedLashedEmitAux(
+			err := node.executeIndexedLashedEmitAux(
 				newrec,
 				names,
 				nextLevelMaps,
 				indices[1:],
 				state,
 			)
+			if err != nil {
+				return err
+			}
 		} else { // end of recursion
 			for i, nextLevelValue := range nextLevelValues {
 				if nextLevelValue != nil {
@@ -1027,13 +1039,16 @@ func (node *EmitXStatementNode) executeIndexedNonLashedEmitPAux(
 						return err
 					}
 				} else {
-					node.executeIndexedNonLashedEmitPAux(
+					err := node.executeIndexedNonLashedEmitPAux(
 						newrec,
 						[]string{names[i]},
 						[]*mlrval.Mlrmap{valueAsMap},
 						indices[1:],
 						state,
 					)
+					if err != nil {
+						return err
+					}
 				}
 			}
 		}
@@ -1083,13 +1098,16 @@ func (node *EmitXStatementNode) executeIndexedLashedEmitPAux(
 
 		if nextLevelMaps[0] != nil && len(indices) >= 2 {
 			// recurse
-			node.executeIndexedLashedEmitPAux(
+			err := node.executeIndexedLashedEmitPAux(
 				newrec,
 				names,
 				nextLevelMaps,
 				indices[1:],
 				state,
 			)
+			if err != nil {
+				return err
+			}
 		} else {
 			// end of recursion
 			for i, nextLevel := range nextLevels {

pkg/dsl/cst/evaluable.go

@@ -17,7 +17,7 @@ import (
 
 func (root *RootNode) BuildEvaluableNode(astNode *asts.ASTNode) (IEvaluable, error) {
 	// Try BuildLeafNode first for terminals
-	if astNode.Children == nil || len(astNode.Children) == 0 {
+	if len(astNode.Children) == 0 {
 		if leaf, err := root.BuildLeafNode(astNode); err == nil {
 			return leaf, nil
 		}
@@ -97,7 +97,7 @@ func (root *RootNode) BuildEvaluableNode(astNode *asts.ASTNode) (IEvaluable, err
 
 	// Fallback: try BuildLeafNode for unhandled types (e.g. DirectFieldValue, IntLiteral).
 	// Only for leaf-like nodes (0 or 1 child); nodes with 2+ children are not leaves.
-	if astNode.Children == nil || len(astNode.Children) <= 1 {
+	if len(astNode.Children) <= 1 {
 		if leaf, err := root.BuildLeafNode(astNode); err == nil {
 			return leaf, nil
 		}

pkg/dsl/cst/for.go

@@ -182,9 +182,8 @@ func (node *ForLoopOneVariableNode) Execute(state *runtime.State) (*BlockExitPay
 			}
 		}
 
-	} else if indexMlrval.IsAbsent() {
-		// Data-heterogeneity no-op
 	}
+	// else if indexMlrval.IsAbsent(): data-heterogeneity no-op
 
 	// TODO: backwards compatibility with the C port means we treat this as
 	// silent zero-pass. But maybe we should surface it as an error. Maybe
@@ -372,9 +371,8 @@ func (node *ForLoopTwoVariableNode) Execute(state *runtime.State) (*BlockExitPay
 			}
 		}
 
-	} else if indexMlrval.IsAbsent() {
-		// Data-heterogeneity no-op
 	}
+	// else if indexMlrval.IsAbsent(): data-heterogeneity no-op
 
 	// TODO: backwards compatibility with the C port means we treat this as
 	// silent zero-pass. But maybe we should surface it as an error. Maybe
@@ -581,9 +579,8 @@ func (node *ForLoopMultivariableNode) executeOuter(
 			}
 		}
 
-	} else if mv.IsAbsent() {
-		// Data-heterogeneity no-op
 	}
+	// else if mv.IsAbsent(): data-heterogeneity no-op
 
 	// TODO: backwards compatibility with the C port means we treat this as
 	// silent zero-pass. But maybe we should surface it as an error. Maybe
@@ -675,9 +672,8 @@ func (node *ForLoopMultivariableNode) executeInner(
 			}
 		}
 
-	} else if mv.IsAbsent() {
-		// Data-heterogeneity no-op
 	}
+	// else if mv.IsAbsent(): data-heterogeneity no-op
 
 	// TODO: backwards compatibility with the C port means we treat this as
 	// silent zero-pass. But maybe we should surface it as an error. Maybe

pkg/dsl/cst/keyword_usage_json.go

@@ -33,16 +33,16 @@ func captureStdout(f func()) string {
 	done := make(chan string)
 	go func() {
 		var buf bytes.Buffer
-		io.Copy(&buf, r)
+		_, _ = io.Copy(&buf, r)
 		done <- buf.String()
 	}()
 
 	f()
 
-	w.Close()
+	_ = w.Close()
 	os.Stdout = old
 	s := <-done
-	r.Close()
+	_ = r.Close()
 	return s
 }
 

pkg/dsl/cst/leaves.go

@@ -18,9 +18,9 @@ func (root *RootNode) BuildLeafNode(
 ) (IEvaluable, error) {
 	// The BNF uses empty slice for terminals. It may also produce reduced literal nodes
 	// (float_literal, int_literal) with one child (the terminal).
-	lib.InternalCodingErrorIf(astNode.Children != nil && len(astNode.Children) > 1)
+	lib.InternalCodingErrorIf(len(astNode.Children) > 1)
 	sval := tokenLit(astNode)
-	if sval == "" && astNode.Children != nil && len(astNode.Children) == 1 {
+	if sval == "" && len(astNode.Children) == 1 {
 		sval = tokenLit(astNode.Children[0])
 	}
 

pkg/dsl/cst/lvalues.go

@@ -143,7 +143,8 @@ func (node *DirectFieldValueLvalueNode) UnassignIndexed(
 		name := node.lhsFieldName.String()
 		state.Inrec.Remove(name)
 	} else {
-		state.Inrec.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Inrec.RemoveIndexed(
 			append([]*mlrval.Mlrval{node.lhsFieldName}, indices...),
 		)
 	}
@@ -254,7 +255,8 @@ func (node *IndirectFieldValueLvalueNode) UnassignIndexed(
 		name := lhsFieldName.String()
 		state.Inrec.Remove(name)
 	} else {
-		state.Inrec.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Inrec.RemoveIndexed(
 			append([]*mlrval.Mlrval{lhsFieldName.Copy()}, indices...),
 		)
 	}
@@ -357,12 +359,12 @@ func (node *PositionalFieldNameLvalueNode) UnassignIndexed(
 		index, ok := lhsFieldIndex.GetIntValue()
 		if ok {
 			state.Inrec.RemoveWithPositionalIndex(index)
-		} else {
-			// TODO: incorporate error-return into this API
 		}
+		// TODO: incorporate error-return into this API for the non-int case
 	} else {
 		// xxx positional
-		state.Inrec.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Inrec.RemoveIndexed(
 			append([]*mlrval.Mlrval{lhsFieldIndex}, indices...),
 		)
 	}
@@ -475,12 +477,12 @@ func (node *PositionalFieldValueLvalueNode) UnassignIndexed(
 		index, ok := lhsFieldIndex.GetIntValue()
 		if ok {
 			state.Inrec.RemoveWithPositionalIndex(index)
-		} else {
-			// TODO: incorporate error-return into this API
 		}
+		// TODO: incorporate error-return into this API for the non-int case
 	} else {
 		// xxx positional
-		state.Inrec.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Inrec.RemoveIndexed(
 			append([]*mlrval.Mlrval{lhsFieldIndex}, indices...),
 		)
 	}
@@ -493,7 +495,7 @@ func (root *RootNode) BuildFullSrecLvalueNode(astNode *asts.ASTNode) (IAssignabl
 	lib.InternalCodingErrorIf(astNode.Type != asts.NodeType(NodeTypeFullSrec))
 	lib.InternalCodingErrorIf(astNode == nil)
 	// PGPG FullSrec has empty list as children
-	lib.InternalCodingErrorIf(astNode.Children != nil && len(astNode.Children) > 0)
+	lib.InternalCodingErrorIf(len(astNode.Children) > 0)
 	return NewFullSrecLvalueNode(), nil
 }
 
@@ -556,7 +558,8 @@ func (node *FullSrecLvalueNode) UnassignIndexed(
 	if indices == nil {
 		state.Inrec.Clear()
 	} else {
-		state.Inrec.RemoveIndexed(indices)
+		// unset of a non-existent path is a no-op
+		_ = state.Inrec.RemoveIndexed(indices)
 	}
 }
 
@@ -625,7 +628,8 @@ func (node *DirectOosvarValueLvalueNode) UnassignIndexed(
 		name := node.lhsOosvarName.String()
 		state.Oosvars.Remove(name)
 	} else {
-		state.Oosvars.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Oosvars.RemoveIndexed(
 			append([]*mlrval.Mlrval{node.lhsOosvarName}, indices...),
 		)
 	}
@@ -705,7 +709,8 @@ func (node *IndirectOosvarValueLvalueNode) UnassignIndexed(
 		sname := lhsOosvarName.String()
 		state.Oosvars.Remove(sname)
 	} else {
-		state.Oosvars.RemoveIndexed(
+		// unset of a non-existent path is a no-op
+		_ = state.Oosvars.RemoveIndexed(
 			append([]*mlrval.Mlrval{lhsOosvarName}, indices...),
 		)
 	}
@@ -717,7 +722,7 @@ type FullOosvarLvalueNode struct {
 func (root *RootNode) BuildFullOosvarLvalueNode(astNode *asts.ASTNode) (IAssignable, error) {
 	lib.InternalCodingErrorIf(astNode.Type != asts.NodeType(NodeTypeFullOosvar))
 	lib.InternalCodingErrorIf(astNode == nil)
-	lib.InternalCodingErrorIf(astNode.Children != nil && len(astNode.Children) > 0)
+	lib.InternalCodingErrorIf(len(astNode.Children) > 0)
 	return NewFullOosvarLvalueNode(), nil
 }
 
@@ -762,7 +767,8 @@ func (node *FullOosvarLvalueNode) UnassignIndexed(
 	if indices == nil {
 		state.Oosvars.Clear()
 	} else {
-		state.Oosvars.RemoveIndexed(indices)
+		// unset of a non-existent path is a no-op
+		_ = state.Oosvars.RemoveIndexed(indices)
 	}
 }
 
@@ -780,7 +786,7 @@ type LocalVariableLvalueNode struct {
 }
 
 func (root *RootNode) BuildTypedeclLocalVariableLvalueNode(astNode *asts.ASTNode) (IAssignable, error) {
-	lib.InternalCodingErrorIf(astNode.Children == nil || len(astNode.Children) < 2)
+	lib.InternalCodingErrorIf(len(astNode.Children) < 2)
 	typeNode := astNode.Children[0]
 	varNode := astNode.Children[1]
 	// PGPG Typedecl produces kw_int, kw_bool, etc. (no Typedecl wrapper node)
@@ -803,7 +809,7 @@ func (root *RootNode) BuildLocalVariableLvalueNode(astNode *asts.ASTNode) (IAssi
 	typeName := "any"
 	defineTypedAtScope := false
 	// PGPG: LocalVariable is terminal (children nil or empty). Miller had typed params with Children[0]=Typedecl.
-	if astNode.Children != nil && len(astNode.Children) > 0 { // typed, like 'num x = 3'
+	if len(astNode.Children) > 0 { // typed, like 'num x = 3'
 		typeNode := astNode.Children[0]
 		lib.InternalCodingErrorIf(typeNode.Type != asts.NodeType(NodeTypeTypedecl))
 		typeName = tokenLit(typeNode)
@@ -1062,7 +1068,9 @@ func (node *EnvironmentVariableLvalueNode) Assign(
 
 	sname := name.String()
 	svalue := rvalue.String()
-	os.Setenv(sname, svalue)
+	if err := os.Setenv(sname, svalue); err != nil {
+		return err
+	}
 	if sname == "TZ" {
 		err := lib.SetTZFromEnv() // affects the time library; notify it
 		if err != nil {
@@ -1093,7 +1101,7 @@ func (node *EnvironmentVariableLvalueNode) Unassign(
 		return
 	}
 
-	os.Unsetenv(name.String())
+	_ = os.Unsetenv(name.String())
 }
 
 func (node *EnvironmentVariableLvalueNode) UnassignIndexed(

pkg/dsl/cst/print.go

@@ -327,7 +327,9 @@ func (root *RootNode) buildPrintxStatementNode(
 
 func (node *PrintStatementNode) Execute(state *runtime.State) (*BlockExitPayload, error) {
 	if len(node.expressionEvaluables) == 0 {
-		node.printToRedirectFunc(node.terminator, state)
+		if err := node.printToRedirectFunc(node.terminator, state); err != nil {
+			return nil, err
+		}
 	} else {
 		// 5x faster than fmt.Print() separately: note that os.Stdout is
 		// non-buffered in Go whereas stdout is buffered in C.
@@ -347,7 +349,9 @@ func (node *PrintStatementNode) Execute(state *runtime.State) (*BlockExitPayload
 			}
 		}
 		buffer.WriteString(node.terminator)
-		node.printToRedirectFunc(buffer.String(), state)
+		if err := node.printToRedirectFunc(buffer.String(), state); err != nil {
+			return nil, err
+		}
 	}
 	return nil, nil
 }
@@ -387,6 +391,5 @@ func (node *PrintStatementNode) printToFileOrPipe(
 	}
 	outputFileName := redirectorTarget.String()
 
-	node.outputHandlerManager.WriteString(outputString, outputFileName)
+	return node.outputHandlerManager.WriteString(outputString, outputFileName)
-	return nil
 }

pkg/dsl/cst/root.go

@@ -368,12 +368,9 @@ func (root *RootNode) resolveFunctionCallsites() error {
 		if err != nil {
 			return err
 		}
-		if udf == nil {
+		// A nil udf here is unresolvable at CST-build time but perhaps a local
-			// Unresolvable at CST-build time but perhaps a local variable. For example,
+		// variable. For example, the UDF callsite '$z = f($x, $y)', and supposing
-			// the UDF callsite '$z = f($x, $y)', and supposing
+		// there will be 'f = func(a, b) { return a*b }' in scope at runtime.
-			// there will be 'f = func(a, b) { return a*b }' in scope at runtime.
-		}
-
 		unresolvedFunctionCallsite.udf = udf
 	}
 	return nil

pkg/dsl/cst/udf.go

@@ -563,13 +563,13 @@ func (root *RootNode) BuildUDF(
 	for i, parameterASTNode := range flatParams {
 		// PGPG: Parameter has one child (LocalVariable), or two (Typedecl, LocalVariable) for typed params.
 		var variableName string
-		var typeName string = "any"
+		typeName := "any"
-		if parameterASTNode.Children != nil && len(parameterASTNode.Children) == 2 {
+		if len(parameterASTNode.Children) == 2 {
 			// Typedecl LocalVariable -> [Typedecl, LocalVariable]
 			typeNode := parameterASTNode.Children[0]
 			nameNode := parameterASTNode.Children[1]
 			typeName = tokenLit(typeNode)
-			if typeName == "" && typeNode.Children != nil && len(typeNode.Children) > 0 {
+			if typeName == "" && len(typeNode.Children) > 0 {
 				typeName = tokenLit(typeNode.Children[0])
 			}
 			if typeName == "" {
@@ -579,14 +579,14 @@ func (root *RootNode) BuildUDF(
 			if string(nameNode.Type) == NodeTypeLocalVariable || nameNode.Type == asts.NodeType(NodeTypeLocalVariable) {
 				variableName = tokenLit(nameNode)
 			}
-		} else if parameterASTNode.Children != nil && len(parameterASTNode.Children) == 1 {
+		} else if len(parameterASTNode.Children) == 1 {
 			typeGatedParameterNameASTNode := parameterASTNode.Children[0]
 			if string(typeGatedParameterNameASTNode.Type) != NodeTypeLocalVariable &&
 				typeGatedParameterNameASTNode.Type != asts.NodeType(NodeTypeLocalVariable) {
 				lib.InternalCodingErrorWithMessageIf(true, "expected LocalVariable as parameter name")
 			}
 			variableName = tokenLit(typeGatedParameterNameASTNode)
-		} else if parameterASTNode.Children == nil || len(parameterASTNode.Children) == 0 {
+		} else if len(parameterASTNode.Children) == 0 {
 			// Direct LocalVariable (e.g. from with_prepended_children flattening)
 			if string(parameterASTNode.Type) != NodeTypeLocalVariable &&
 				parameterASTNode.Type != asts.NodeType(NodeTypeLocalVariable) {

pkg/dsl/cst/uds.go

@@ -254,13 +254,13 @@ func (root *RootNode) BuildAndInstallUDS(astNode *asts.ASTNode) error {
 	typeGatedParameterNames := make([]*types.TypeGatedMlrvalName, arity)
 	for i, parameterASTNode := range flatParams {
 		var variableName string
-		var typeName string = "any"
+		typeName := "any"
-		if parameterASTNode.Children != nil && len(parameterASTNode.Children) == 2 {
+		if len(parameterASTNode.Children) == 2 {
 			// Typedecl LocalVariable -> [Typedecl, LocalVariable]
 			typeNode := parameterASTNode.Children[0]
 			nameNode := parameterASTNode.Children[1]
 			typeName = tokenLit(typeNode)
-			if typeName == "" && typeNode.Children != nil && len(typeNode.Children) > 0 {
+			if typeName == "" && len(typeNode.Children) > 0 {
 				typeName = tokenLit(typeNode.Children[0])
 			}
 			if typeName == "" {
@@ -269,7 +269,7 @@ func (root *RootNode) BuildAndInstallUDS(astNode *asts.ASTNode) error {
 			if string(nameNode.Type) == NodeTypeLocalVariable || nameNode.Type == asts.NodeType(NodeTypeLocalVariable) {
 				variableName = tokenLit(nameNode)
 			}
-		} else if parameterASTNode.Children != nil && len(parameterASTNode.Children) == 1 {
+		} else if len(parameterASTNode.Children) == 1 {
 			typeGatedParameterNameASTNode := parameterASTNode.Children[0]
 			lib.InternalCodingErrorIf(typeGatedParameterNameASTNode.Type != asts.NodeType(NodeTypeLocalVariable))
 			variableName = tokenLit(typeGatedParameterNameASTNode)

pkg/entrypoint/entrypoint.go

@@ -163,14 +163,14 @@ func processFileInPlace(
 	// Get a handle with, perhaps, a recompression wrapper around it.
 	wrappedHandle, isNew, err := lib.WrapOutputHandle(handle, inputFileEncoding)
 	if err != nil {
-		os.Remove(tempFileName)
+		_ = os.Remove(tempFileName)
 		return err
 	}
 
 	// Run the Miller processing stream from the input file to the temp-output file.
 	err = stream.Stream([]string{fileName}, options, recordTransformers, wrappedHandle, false)
 	if err != nil {
-		os.Remove(tempFileName)
+		_ = os.Remove(tempFileName)
 		return err
 	}
 
@@ -178,7 +178,7 @@ func processFileInPlace(
 	if isNew {
 		err = wrappedHandle.Close()
 		if err != nil {
-			os.Remove(tempFileName)
+			_ = os.Remove(tempFileName)
 			return err
 		}
 	}
@@ -187,14 +187,14 @@ func processFileInPlace(
 	// it must be error-checked.
 	err = handle.Close()
 	if err != nil {
-		os.Remove(tempFileName)
+		_ = os.Remove(tempFileName)
 		return err
 	}
 
 	// Rename the temp-output file on top of the input file.
 	err = os.Rename(tempFileName, fileName)
 	if err != nil {
-		os.Remove(tempFileName)
+		_ = os.Remove(tempFileName)
 		return err
 	}
 

pkg/input/record_reader_csv.go

@@ -82,7 +82,7 @@ func (reader *RecordReaderCSV) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}
@@ -335,9 +335,8 @@ func (reader *RecordReaderCSV) maybeConsumeComment(
 		lib.InternalCodingErrorIf(len(csvRecord) != 1)
 		*recordsAndContexts = append(*recordsAndContexts, types.NewOutputString(csvRecord[0], context))
 
-	} else /* reader.readerOptions.CommentHandling == cli.SkipComments */ {
-		// discard entirely
 	}
+	// else, reader.readerOptions.CommentHandling == cli.SkipComments: discard entirely
 	return false
 }
 

pkg/input/record_reader_csvlite.go

@@ -122,7 +122,7 @@ func (reader *RecordReaderCSVLite) Read(
 						errorChannel,
 						downstreamDoneChannel,
 					)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_dcf.go

@@ -76,7 +76,7 @@ func (reader *RecordReaderDCF) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_dkvp_nidx.go

@@ -86,7 +86,7 @@ func (reader *RecordReaderDKVPNIDX) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}
@@ -216,7 +216,7 @@ func recordFromNIDXLine(reader *RecordReaderDKVPNIDX, line string) (*mlrval.Mlrm
 
 	values := reader.fieldSplitter.Split(line)
 
-	var i int = 0
+	i := 0
 	for _, value := range values {
 		i++
 		str_key := strconv.Itoa(i)

pkg/input/record_reader_dkvp_test.go

@@ -10,7 +10,8 @@ import (
 
 func TestRecordFromDKVPLine(t *testing.T) {
 	readerOptions := cli.DefaultReaderOptions()
-	cli.FinalizeReaderOptions(&readerOptions) // compute IPS, IFS -> IPSRegex, IFSRegex
+	err := cli.FinalizeReaderOptions(&readerOptions) // compute IPS, IFS -> IPSRegex, IFSRegex
+	assert.Nil(t, err)
 	reader, err := NewRecordReaderDKVP(&readerOptions, 1)
 	assert.NotNil(t, reader)
 	assert.Nil(t, err)

pkg/input/record_reader_dkvpx.go

@@ -62,7 +62,7 @@ func (reader *RecordReaderDKVPX) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_json.go

@@ -60,7 +60,7 @@ func (reader *RecordReaderJSON) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_pprint.go

@@ -122,7 +122,7 @@ func (reader *RecordReaderPprintFixedSplit) Read(
 						errorChannel,
 						downstreamDoneChannel,
 					)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}
@@ -351,7 +351,7 @@ func (reader *RecordReaderPprintBarredOrMarkdown) Read(
 						errorChannel,
 						downstreamDoneChannel,
 					)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_tsv.go

@@ -104,7 +104,7 @@ func (reader *RecordReaderTSV) Read(
 						errorChannel,
 						downstreamDoneChannel,
 					)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_xtab.go

@@ -88,7 +88,7 @@ func (reader *RecordReaderXTAB) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/input/record_reader_yaml.go

@@ -58,7 +58,7 @@ func (reader *RecordReaderYAML) Read(
 					errorChannel <- err
 				} else {
 					reader.processHandle(handle, filename, &context, readerChannel, errorChannel, downstreamDoneChannel)
-					handle.Close()
+					_ = handle.Close()
 				}
 			}
 		}

pkg/lib/halfpipe.go

@@ -40,7 +40,7 @@ func OpenOutboundHalfPipe(commandString string) (*os.File, error) {
 		return nil, err
 	}
 
-	go process.Wait()
+	go func() { _, _ = process.Wait() }()
 
 	return writePipe, nil
 }
@@ -87,7 +87,7 @@ func OpenInboundHalfPipe(commandString string) (*os.File, error) {
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
 		}
-		readPipe.Close()
+		_ = readPipe.Close()
 	}(process, readPipe)
 
 	return readPipe, nil

pkg/lib/readfiles.go

@@ -51,7 +51,7 @@ func LoadStringsFromDir(dirname string, extension string) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	names, err := f.Readdirnames(-1)
 	if err != nil {
@@ -80,7 +80,7 @@ func ReadCSVHeader(filename string) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer handle.Close()
+	defer func() { _ = handle.Close() }()
 	csvReader := csv.NewReader(handle)
 	header, err := csvReader.Read()
 	if err != nil {

pkg/mlrval/mlrmap_accessors.go

@@ -755,10 +755,7 @@ func (mlrmap *Mlrmap) Label(newNames []string) {
 
 	i := 0
 	numNewNames := len(newNames)
-	for {
+	for i < numNewNames {
-		if i >= numNewNames {
-			break
-		}
 		pe := mlrmap.pop()
 		if pe == nil {
 			break

pkg/mlrval/mlrmap_flatten_unflatten.go

@@ -179,7 +179,8 @@ func (mlrmap *Mlrmap) CopyUnflattened(
 		baseIndex := arrayval[0].String()
 		affectedBaseIndices[baseIndex] = true
 		// Use PutIndexed to assign $x["a"] = 7, or $x["b"] = 8, etc.
-		other.PutIndexed(
+		// Unflattening is best-effort: this API has no error return.
+		_ = other.PutIndexed(
 			CopyMlrvalArray(arrayval),
 			unflattenTerminal(pe.Value).Copy(),
 		)
@@ -226,7 +227,8 @@ func (mlrmap *Mlrmap) CopyUnflattenFields(
 			baseIndex := arrayval[0].String()
 			if fieldNameSet[baseIndex] {
 				// Use PutIndexed to assign $x["a"] = 7, or $x["b"] = 8, etc.
-				other.PutIndexed(
+				// Unflattening is best-effort: this API has no error return.
+				_ = other.PutIndexed(
 					CopyMlrvalArray(arrayval),
 					unflattenTerminal(pe.Value).Copy(),
 				)

pkg/mlrval/mlrval_yaml.go

@@ -197,14 +197,12 @@ func mlrvalToYAMLNode(mv *Mlrval) (*yaml.Node, error) {
 	case MT_ARRAY:
 		arr := mv.GetArray()
 		seqNode := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}
-		if arr != nil {
+		for _, elem := range arr {
-			for _, elem := range arr {
+			v, err := mlrvalToYAMLNode(elem)
-				v, err := mlrvalToYAMLNode(elem)
+			if err != nil {
-				if err != nil {
+				return nil, err
-					return nil, err
-				}
-				seqNode.Content = append(seqNode.Content, v)
 			}
+			seqNode.Content = append(seqNode.Content, v)
 		}
 		return seqNode, nil
 	case MT_MAP:

pkg/output/channel_writer.go

@@ -105,7 +105,8 @@ func channelWriterHandleBatch(
 			}
 
 			if writerOptions.FlushOnEveryRecord {
-				bufferedOutputStream.Flush()
+				// bufio.Writer errors are sticky; the final Flush in pkg/stream is checked
+				_ = bufferedOutputStream.Flush()
 			}
 
 		} else {

pkg/output/file_output_handlers.go

@@ -500,7 +500,9 @@ func (handler *FileOutputHandler) Close() (retval error) {
 		return retval
 	}
 
-	handler.bufferedOutputStream.Flush()
+	if err := handler.bufferedOutputStream.Flush(); err != nil {
+		return err
+	}
 	if handler.closeable {
 		return handler.handle.Close()
 	} // e.g. stdout

pkg/output/record_writer_csv.go

@@ -73,11 +73,14 @@ func (writer *RecordWriterCSV) Write(
 			fields[i] = pe.Key
 			i++
 		}
-		writer.WriteCSVRecordMaybeColorized(fields, bufferedOutputStream, outputIsStdout, true, writer.quoteAll)
+		err := writer.WriteCSVRecordMaybeColorized(fields, bufferedOutputStream, outputIsStdout, true, writer.quoteAll)
+		if err != nil {
+			return err
+		}
 		writer.needToPrintHeader = false
 	}
 
-	var outputNF int64 = outrec.FieldCount
+	outputNF := outrec.FieldCount
 	if outputNF < writer.firstRecordNF {
 		outputNF = writer.firstRecordNF
 	}
@@ -103,7 +106,5 @@ func (writer *RecordWriterCSV) Write(
 		fields[i] = ""
 	}
 
-	writer.WriteCSVRecordMaybeColorized(fields, bufferedOutputStream, outputIsStdout, false, writer.quoteAll)
+	return writer.WriteCSVRecordMaybeColorized(fields, bufferedOutputStream, outputIsStdout, false, writer.quoteAll)
-
-	return nil
 }

pkg/output/record_writer_pprint.go

@@ -2,7 +2,6 @@ package output
 
 import (
 	"bufio"
-	"fmt"
 	"strings"
 
 	"github.com/johnkerl/miller/v6/pkg/cli"
@@ -211,7 +210,8 @@ func (writer *RecordWriterPPRINT) writeHeterogenousListNonBarred(
 		}
 
 		if writer.writerOptions.FlushOnEveryRecord {
-			bufferedOutputStream.Flush()
+			// bufio.Writer errors are sticky; the final Flush in pkg/stream is checked
+			_ = bufferedOutputStream.Flush()
 		}
 	}
 }
@@ -351,7 +351,7 @@ func (writer *RecordWriterPPRINT) writeHeterogenousListBarred(
 				bufferedOutputStream.WriteString(colorizer.MaybeColorizeValue(s, outputIsStdout))
 			}
 			if pe.Next != nil {
-				bufferedOutputStream.WriteString(fmt.Sprint(bc.verticalMiddle))
+				bufferedOutputStream.WriteString(bc.verticalMiddle)
 			} else {
 				bufferedOutputStream.WriteString(bc.verticalEnd)
 				bufferedOutputStream.WriteString(writer.writerOptions.ORS)
@@ -373,7 +373,8 @@ func (writer *RecordWriterPPRINT) writeHeterogenousListBarred(
 		}
 
 		if writer.writerOptions.FlushOnEveryRecord {
-			bufferedOutputStream.Flush()
+			// bufio.Writer errors are sticky; the final Flush in pkg/stream is checked
+			_ = bufferedOutputStream.Flush()
 		}
 	}
 }

pkg/output/record_writer_tsv.go

@@ -75,7 +75,7 @@ func (writer *RecordWriterTSV) Write(
 		writer.needToPrintHeader = false
 	}
 
-	var outputNF int64 = outrec.FieldCount
+	outputNF := outrec.FieldCount
 	if outputNF < writer.firstRecordNF {
 		outputNF = writer.firstRecordNF
 	}

pkg/runtime/stack.go

@@ -454,7 +454,9 @@ func (frame *StackFrame) setIndexed(
 		leadingIndex := indices[0]
 		if leadingIndex.IsString() || leadingIndex.IsInt() {
 			newval := mlrval.FromMap(mlrval.NewMlrmap())
-			newval.PutIndexed(indices, mv)
+			if err := newval.PutIndexed(indices, mv); err != nil {
+				return err
+			}
 			return frame.set(stackVariable, newval)
 		}
 		return fmt.Errorf(
@@ -485,5 +487,6 @@ func (frame *StackFrame) unsetIndexed(
 	if value == nil {
 		return
 	}
-	value.RemoveIndexed(indices)
+	// unset of a non-existent path is a no-op
+	_ = value.RemoveIndexed(indices)
 }

pkg/stream/stream.go

@@ -101,7 +101,9 @@ func Stream(
 		}
 	}
 
-	bufferedOutputStream.Flush()
+	if err := bufferedOutputStream.Flush(); err != nil && retval == nil {
+		retval = err
+	}
 
 	return retval
 }

pkg/terminals/help/entry.go

@@ -821,12 +821,8 @@ func helpByExactSearch(things []string) bool {
 // We need to look various places, e.g. "sec2gmt" is the name of a verb as well
 // as a DSL function.
 func helpByExactSearchOne(thing string) bool {
-	found := false
-
 	// flag
-	if cli.FLAG_TABLE.ShowHelpForFlagWithName(thing) {
+	found := cli.FLAG_TABLE.ShowHelpForFlagWithName(thing)
-		found = true
-	}
 
 	// verb
 	if transformers.ShowHelpForTransformer(thing) {
@@ -857,12 +853,8 @@ func helpByApproximateSearch(things []string) bool {
 }
 
 func helpByApproximateSearchOne(thing string) bool {
-	found := false
-
 	// flag
-	if cli.FLAG_TABLE.ShowHelpForFlagApproximateWithName(thing) {
+	found := cli.FLAG_TABLE.ShowHelpForFlagApproximateWithName(thing)
-		found = true
-	}
 
 	// verb
 	if transformers.ShowHelpForTransformerApproximate(thing) {

pkg/terminals/help/entry_which.go

@@ -164,7 +164,8 @@ var whichStopwords = map[string]bool{
 // single-character tokens and stopwords.
 func whichTokenize(query string) []string {
 	words := strings.FieldsFunc(strings.ToLower(query), func(r rune) bool {
-		return !('a' <= r && r <= 'z') && !('0' <= r && r <= '9') && r != '-' && r != '_'
+		isWordRune := ('a' <= r && r <= 'z') || ('0' <= r && r <= '9') || r == '-' || r == '_'
+		return !isWordRune
 	})
 	var tokens []string
 	seen := map[string]bool{}

pkg/terminals/regtest/invoker.go

@@ -72,8 +72,8 @@ func RunDiffCommandOnStrings(
 ) {
 	actualOutputFileName := lib.WriteTempFileOrDie(actualOutput)
 	expectedOutputFileName := lib.WriteTempFileOrDie(expectedOutput)
-	defer os.Remove(actualOutputFileName)
+	defer func() { _ = os.Remove(actualOutputFileName) }()
-	defer os.Remove(expectedOutputFileName)
+	defer func() { _ = os.Remove(expectedOutputFileName) }()
 
 	// This is diff or fc
 	diffRunArray := platform.GetDiffRunArray(actualOutputFileName, expectedOutputFileName)

pkg/terminals/regtest/regtester.go

@@ -149,16 +149,16 @@ func (rt *RegTester) Execute(
 ) bool {
 	// Don't let the current user's settings affect expected results
 	for _, name := range envVarsToUnset {
-		os.Unsetenv(name)
+		_ = os.Unsetenv(name)
 	}
 	// If there is an accessible .mlrrc file, we don't want it to be read for the regression test.
-	os.Setenv("MLRRC", "__none__")
+	_ = os.Setenv("MLRRC", "__none__")
 
 	// This is important for multi-platform regression testing, wherein default floating-point
 	// output format has varying numbers of decimal places between the platform where
 	// the expected results were generated, and the platform where the actual values are being
 	// computed. For regression-test we OFMT from an environment variable.
-	os.Setenv("MLR_OFMT", "%.8f")
+	_ = os.Setenv("MLR_OFMT", "%.8f")
 
 	rt.resetCounts()
 
@@ -328,7 +328,7 @@ func (rt *RegTester) hasCaseSubdirectories(
 		fmt.Printf("%s: %v\n", dirName, err)
 		os.Exit(1)
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	names, err := f.Readdirnames(-1)
 	if err != nil {
@@ -461,12 +461,12 @@ func (rt *RegTester) executeSingleCmdFile(
 		if verbosityLevel >= 3 {
 			fmt.Printf("SETENV %s=%s\n", key, value)
 		}
-		os.Setenv(key, value)
+		_ = os.Setenv(key, value)
 	}
 	// This is so 'mlr' files can find the case-directory if they need it --
 	// typically, for redirected emit/dump/etc statements where we want
 	// the redirected-to files to be written into the case-directory.
-	os.Setenv("CASEDIR", caseDir)
+	_ = os.Setenv("CASEDIR", caseDir)
 
 	// Copy any files requested by the test. (Most don't; some do, e.g. those
 	// which test the write-in-place logic of mlr -I.)
@@ -496,9 +496,9 @@ func (rt *RegTester) executeSingleCmdFile(
 		if verbosityLevel >= 3 {
 			fmt.Printf("UNSETENV %s\n", key)
 		}
-		os.Setenv(key, "")
+		_ = os.Setenv(key, "")
 	}
-	os.Setenv("CASEDIR", "")
+	_ = os.Setenv("CASEDIR", "")
 
 	// The .postcmp needn't exist (most test cases don't have one) in which case
 	// the returned map will be empty.
@@ -542,7 +542,7 @@ func (rt *RegTester) executeSingleCmdFile(
 		// Write the .should-fail file
 		if actualExitCode == 0 {
 			// Remove it, if it exists.
-			os.Remove(expectFailFileName)
+			_ = os.Remove(expectFailFileName)
 		} else {
 			err = rt.storeFile(expectFailFileName, "")
 			if err != nil {
@@ -716,7 +716,7 @@ func (rt *RegTester) executeSingleCmdFile(
 		// Clean up any requested file-copies so that we're git-clean after the regression-test run.
 		for _, pair := range preCopySrcDestPairs {
 			dst := pair.second
-			os.Remove(dst)
+			_ = os.Remove(dst)
 			if verbosityLevel >= 3 {
 				fmt.Printf("%s: clean up %s\n", cmdFilePath, dst)
 			}
@@ -725,7 +725,7 @@ func (rt *RegTester) executeSingleCmdFile(
 		// Clean up any extra output files so that we're git-clean after the regression-test run.
 		for _, pair := range postCompareExpectedActualPairs {
 			actualFileName := pair.second
-			os.Remove(actualFileName)
+			_ = os.Remove(actualFileName)
 			if verbosityLevel >= 3 {
 				fmt.Printf("%s: clean up %s\n", cmdFilePath, actualFileName)
 			}

pkg/terminals/repl/entry.go

@@ -147,8 +147,14 @@ func ReplMain(args []string) int {
 		}
 	}
 
-	cli.FinalizeReaderOptions(&options.ReaderOptions)
+	if err := cli.FinalizeReaderOptions(&options.ReaderOptions); err != nil {
-	cli.FinalizeWriterOptions(&options.WriterOptions)
+		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", replName, err)
+		return 1
+	}
+	if err := cli.FinalizeWriterOptions(&options.WriterOptions); err != nil {
+		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", replName, err)
+		return 1
+	}
 
 	// --auto-flatten is on by default. But if input and output formats are both JSON,
 	// then we don't need to actually do anything. See also mlrcli_parse.go.
@@ -186,7 +192,11 @@ func ReplMain(args []string) int {
 		os.Exit(1)
 	}
 
-	repl.bufferedRecordOutputStream.Flush()
+	err = repl.bufferedRecordOutputStream.Flush()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", repl.replName, err)
+		os.Exit(1)
+	}
 	err = repl.closeBufferedOutputStream()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", repl.replName, err)

pkg/terminals/repl/verbs.go

@@ -200,9 +200,7 @@ func handleOpen(repl *Repl, args []string) bool {
 // something instead of waiting to show them an error only when they type
 // ':read'.
 func openFilesPreCheck(repl *Repl, args []string) bool {
-	if len(args) == 0 {
+	// Zero file names is stdin, which is readable.
-		// Zero file names is stdin, which is readable
-	}
 	for _, arg := range args {
 		fileInfo, err := os.Stat(arg)
 		if err != nil {
@@ -554,8 +552,9 @@ func skipOrProcessRecord(
 	// Strings to be printed from put/filter DSL print/dump/etc statements.
 	if recordAndContext.Record == nil {
 		if processingNotSkipping {
-			repl.bufferedRecordOutputStream.WriteString(recordAndContext.OutputString)
+			// Interactive terminal output: nothing useful to do on write failure
-			repl.bufferedRecordOutputStream.Flush()
+			_, _ = repl.bufferedRecordOutputStream.WriteString(recordAndContext.OutputString)
+			_ = repl.bufferedRecordOutputStream.Flush()
 		}
 		return false
 	}
@@ -617,8 +616,12 @@ func writeRecord(repl *Repl, outrec *mlrval.Mlrmap) {
 		}
 	}
 	// Write and flush immediately for REPL output.
-	repl.recordWriter.Write(outrec, nil, repl.bufferedRecordOutputStream, true /*outputIsStdout*/)
+	err := repl.recordWriter.Write(outrec, nil, repl.bufferedRecordOutputStream, true /*outputIsStdout*/)
-	repl.bufferedRecordOutputStream.Flush()
+	if err != nil {
+		fmt.Printf("mlr %s: %v\n", repl.replName, err)
+		return
+	}
+	_ = repl.bufferedRecordOutputStream.Flush()
 }
 
 func usageReadWrite(repl *Repl) {
@@ -642,7 +645,9 @@ func usageRedirectWrite(repl *Repl) {
 func handleRedirectWrite(repl *Repl, args []string) bool {
 	args = args[1:] // strip off verb
 	if len(args) == 0 {
-		repl.closeBufferedOutputStream()
+		if err := repl.closeBufferedOutputStream(); err != nil {
+			fmt.Printf("mlr %s: %v\n", repl.replName, err)
+		}
 		repl.setBufferedOutputStream("(stdout)", os.Stdout)
 		return true
 	}
@@ -665,7 +670,9 @@ func handleRedirectWrite(repl *Repl, args []string) bool {
 	}
 	fmt.Printf("Redirecting record output to \"%s\"\n", filename)
 
-	repl.closeBufferedOutputStream()
+	if err := repl.closeBufferedOutputStream(); err != nil {
+		fmt.Printf("mlr %s: %v\n", repl.replName, err)
+	}
 	repl.setBufferedOutputStream(filename, handle)
 
 	return true
@@ -695,7 +702,9 @@ func handleRedirectAppend(repl *Repl, args []string) bool {
 	}
 	fmt.Printf("Redirecting record output to \"%s\"\n", filename)
 
-	repl.closeBufferedOutputStream()
+	if err := repl.closeBufferedOutputStream(); err != nil {
+		fmt.Printf("mlr %s: %v\n", repl.replName, err)
+	}
 	repl.setBufferedOutputStream(filename, handle)
 
 	return true
@@ -874,11 +883,7 @@ func handleHelp(repl *Repl, args []string) bool {
 }
 
 func handleHelpFindSingle(repl *Repl, arg string) {
-	foundAny := false
+	foundAny := cst.TryUsageForKeywordApproximate(arg)
-
-	if cst.TryUsageForKeywordApproximate(arg) {
-		foundAny = true
-	}
 
 	if cst.BuiltinFunctionManagerInstance.TryListBuiltinFunctionUsageApproximate(arg) {
 		foundAny = true

pkg/terminals/script/entry.go

@@ -123,8 +123,14 @@ func ScriptMain(args []string) int {
 		scriptUsage(scriptName, os.Stderr, 1)
 	}
 
-	cli.FinalizeReaderOptions(&options.ReaderOptions)
+	if err := cli.FinalizeReaderOptions(&options.ReaderOptions); err != nil {
-	cli.FinalizeWriterOptions(&options.WriterOptions)
+		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", scriptName, err)
+		return 1
+	}
+	if err := cli.FinalizeWriterOptions(&options.WriterOptions); err != nil {
+		fmt.Fprintf(os.Stderr, "mlr %s: %v\n", scriptName, err)
+		return 1
+	}
 	options.WriterOptions.AutoFlatten = cli.DecideFinalFlatten(&options.WriterOptions)
 	options.WriterOptions.AutoUnflatten = cli.DecideFinalUnflatten(options, [][]string{})
 
@@ -143,7 +149,11 @@ func ScriptMain(args []string) int {
 		os.Exit(1)
 	}
 
-	scr.bufferedRecordOutputStream.Flush()
+	err = scr.bufferedRecordOutputStream.Flush()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "mlr script: %v\n", err)
+		os.Exit(1)
+	}
 	err = scr.closeBufferedOutputStream()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "mlr script: %v\n", err)

pkg/terminals/script/runner.go

@@ -126,9 +126,10 @@ func (scr *Script) run() error {
 			}
 
 			if rac.Record == nil {
-				// Output string from print/dump etc
+				// Output string from print/dump etc; terminal output, nothing
-				bufferedOutput.WriteString(rac.OutputString)
+				// useful to do on write failure
-				bufferedOutput.Flush()
+				_, _ = bufferedOutput.WriteString(rac.OutputString)
+				_ = bufferedOutput.Flush()
 				continue
 			}
 

pkg/transformers/aaa_transformer_json.go

@@ -40,14 +40,14 @@ func captureUsageFunc(usageFunc TransformerUsageFunc) string {
 	done := make(chan string)
 	go func() {
 		var buf bytes.Buffer
-		io.Copy(&buf, r)
+		_, _ = io.Copy(&buf, r)
 		done <- buf.String()
 	}()
 
 	usageFunc(w)
-	w.Close()
+	_ = w.Close()
 	s := <-done
-	r.Close()
+	_ = r.Close()
 	return s
 }
 

pkg/transformers/join.go

@@ -253,7 +253,9 @@ func transformerJoinParseCLI(
 		}
 	}
 
-	cli.FinalizeReaderOptions(&opts.joinFlagOptions.ReaderOptions)
+	if err := cli.FinalizeReaderOptions(&opts.joinFlagOptions.ReaderOptions); err != nil {
+		return nil, cli.VerbErrorf(verb, "%v", err)
+	}
 
 	if opts.leftFileName == "" {
 		return nil, cli.VerbErrorf(verb, "need left file name")

pkg/transformers/put_or_filter.go

@@ -192,7 +192,7 @@ func transformerPutOrFilterParseCLI(
 	verb := args[argi]
 	argi++
 
-	var dslStrings []string = []string{}
+	dslStrings := []string{}
 	haveDSLStringsHere := false
 	echoDSLString := false
 	printASTAsTree := false
@@ -340,7 +340,9 @@ func transformerPutOrFilterParseCLI(
 		}
 	}
 
-	cli.FinalizeWriterOptions(&options.WriterOptions)
+	if err := cli.FinalizeWriterOptions(&options.WriterOptions); err != nil {
+		return nil, cli.VerbErrorf(verb, "%v", err)
+	}
 
 	// If they've used either of 'mlr put -f {filename}' or 'mlr put -e
 	// {expression}' then that specifies their DSL expression. But if they've

pkg/transformers/split.go

@@ -88,17 +88,17 @@ func transformerSplitParseCLI(
 	argi++
 
 	var n int64 = 0
-	var doMod bool = false
+	doMod := false
-	var doSize bool = false
+	doSize := false
 	var groupByFieldNames []string = nil
-	var emitDownstream bool = false
+	emitDownstream := false
-	var escapeFileNameCharacters bool = true
+	escapeFileNameCharacters := true
-	var fileNamePartJoiner string = splitDefaultFileNamePartJoiner
+	fileNamePartJoiner := splitDefaultFileNamePartJoiner
-	var doAppend bool = false
+	doAppend := false
-	var outputFileNamePrefix string = splitDefaultOutputFileNamePrefix
+	outputFileNamePrefix := splitDefaultOutputFileNamePrefix
-	var outputFileNameSuffix string = "uninit"
+	outputFileNameSuffix := "uninit"
 	haveOutputFileNameSuffix := false
-	var outputFolder string = ""
+	outputFolder := ""
 
 	var localOptions *cli.TOptions = nil
 	if mainOptions != nil {
@@ -200,7 +200,9 @@ func transformerSplitParseCLI(
 		return nil, cli.VerbErrorf(verb, "-n, -g, and -s are mutually exclusive")
 	}
 
-	cli.FinalizeWriterOptions(&localOptions.WriterOptions)
+	if err := cli.FinalizeWriterOptions(&localOptions.WriterOptions); err != nil {
+		return nil, cli.VerbErrorf(verb, "%v", err)
+	}
 	if !haveOutputFileNameSuffix {
 		outputFileNameSuffix = localOptions.WriterOptions.OutputFileFormat
 	}

pkg/transformers/tee.go

@@ -98,7 +98,9 @@ func transformerTeeParseCLI(
 		}
 	}
 
-	cli.FinalizeWriterOptions(&localOptions.WriterOptions)
+	if err := cli.FinalizeWriterOptions(&localOptions.WriterOptions); err != nil {
+		return nil, cli.VerbErrorf(verbNameTee, "%v", err)
+	}
 
 	// Get the filename/command from the command line, after the flags
 	if argi >= argc {

plans/lintfixes.md

@@ -87,6 +87,118 @@ nil check before len), 3 QF1007 (merge conditional assignment), 3 QF1006 (lift i
    - `os.Setenv`/`os.Unsetenv` — low-risk but cheap to fix
 2. `staticcheck` style — 34 findings, mostly mechanical (type inference, nil-check cleanup, etc.)
 
+## IMPORTANT: issue counts before round 5 were capped
+
+All counts above (the "134 baseline", the "84 remaining") were taken with golangci-lint's
+**default `max-same-issues=3`**, which reports at most 3 findings per identical message text.
+Running with `--max-same-issues=0 --max-issues-per-linter=0` on the same tree shows the true
+backlog before round 5 was **1271 issues: 1202 errcheck, 69 staticcheck** (not 84/50/34).
+CI shares this cap, so fixing the visible 3 of a kind just surfaces the next 3 — whack-a-mole.
+All counts from here on are uncapped.
+
+## Round 5 (branch `johnkerl/lint5`, 2026-07-03): staticcheck batch — DONE
+
+Fixed all 69 staticcheck findings; **staticcheck is now at zero** (uncapped). `make check`
+passes (4676 regression cases). Defers errcheck to rounds 6+; this inverts the priority order
+above, deliberately: the staticcheck fixes are low-risk and retire an entire linter, while the
+errcheck sites need per-site judgment and a few of them (see rounds 6+ below) may be real
+user-facing bugs deserving focused PRs.
+
+What was fixed (uncapped counts):
+
+1. **ST1023 + QF1011 — omit explicit type (37):** `pkg/bifs/arithmetic.go` (20: all min/max
+   helpers), `pkg/bifs/bits.go` (2), `pkg/dsl/cst/udf.go`, `pkg/dsl/cst/uds.go`,
+   `pkg/input/record_reader_dkvp_nidx.go`, `pkg/output/record_writer_csv.go`,
+   `pkg/output/record_writer_tsv.go`, `pkg/transformers/put_or_filter.go`,
+   `pkg/transformers/split.go` (9, whole option-declaration block for consistency).
+2. **S1009 + S1031 — redundant nil checks (14+1):** `pkg/dsl/cst/{blocks,evaluable,leaves,lvalues,udf,uds}.go`,
+   `pkg/mlrval/mlrval_yaml.go`. Also fixed two unflagged-but-identical adjacent patterns
+   (`udf.go`/`uds.go` inner `typeNode.Children` check, `leaves.go:23`) for consistency.
+3. **QF1007 — merge conditional assignment into declaration (3):**
+   `found := false; if cond { found = true }` → `found := cond` in
+   `pkg/terminals/help/entry.go` (×2) and `pkg/terminals/repl/verbs.go`. Only the first
+   conditional merges; the subsequent `if ... { found = true }` blocks must stay as-is since
+   each callee is invoked for its help-printing side effect (no `||` short-circuiting).
+4. **QF1006 — lift break into loop condition (3):** `pkg/bifs/relative_time.go` (×2) →
+   `for remainingInput != "" {`; `pkg/mlrval/mlrmap_accessors.go` (`Mlrmap.Label`) →
+   `for i < numNewNames {` (interior `pe == nil` break stays).
+5. **QF1001 — De Morgan (3):** `pkg/dsl/cst/builtin_functions.go:878,960` →
+   `btype != mlrval.MT_ABSENT && btype != mlrval.MT_BOOL`.
+   `pkg/terminals/help/entry_which.go:167`: mechanical De Morgan reads worse, and staticcheck
+   also flags a merely-outermost negation `!((...) || (...))`; settled on naming the predicate
+   (`isWordRune := ...; return !isWordRune`), which staticcheck accepts and reads best.
+6. **SA9003 — empty branches (9):** all were no-op branches existing only to hold a comment;
+   deleted the branch, kept the comment adjacent. `pkg/dsl/cst/for.go` (×4 data-heterogeneity
+   no-ops), `pkg/dsl/cst/lvalues.go` (×2 TODO-comment else branches), `pkg/dsl/cst/root.go`
+   (nil-udf explainer), `pkg/input/record_reader_csv.go` (SkipComments else),
+   `pkg/terminals/repl/verbs.go` (zero-filenames-is-stdin).
+
+After this round: **1202 issues remain, all errcheck** (uncapped).
+
+## Round 6 (branch `johnkerl/lint5`, 2026-07-03): errcheck — DONE
+
+**golangci-lint now reports 0 issues** (uncapped) on `./cmd/mlr ./pkg/...`. `make check` passes.
+Done in the same PR as round 5. True pre-round breakdown by callee: 893 `Fprintf` + 31
+`Fprintln` + 25 `Fprint` (= 949 `fmt.Fprint*`, of which ~894 were `pkg/transformers` usage
+printers), 140 `WriteString`, 28 `Close`, 12 `Set`, 10 `Remove`, 10 `Flush`, 9 `RemoveIndexed`,
+8 `Setenv`, 5 `Write`, 9 `Finalize{Reader,Writer}Options`, 3 `PutIndexed`, plus a small tail.
+
+### 6a — `.golangci.yml` with errcheck exclusions (~1090 findings)
+
+Added `.golangci.yml` (picked up automatically by the CI action) with
+`errcheck.exclude-functions` for `fmt.Fprint/Fprintf/Fprintln` (usage/error printers),
+`(*bufio.Writer).Write/WriteString` (sticky errors, surface at the now-checked final Flush),
+and `(*strings.Builder).WriteString` (documented never to fail). Also pinned
+`max-issues-per-linter: 0` and `max-same-issues: 0` so CI reports true counts from now on.
+Caveat found: the bufio exclusion doesn't match calls through struct fields
+(e.g. `repl.bufferedRecordOutputStream.WriteString`) — those got explicit `_ =` instead.
+
+### 6b — propagated: real error paths (~30 findings)
+
+- `cli.Finalize{Reader,Writer}Options` (9): join/put-or-filter/split/tee verb constructors now
+  return the error (`mlr join -i badformat` now exits 1 with "unrecognized input format"
+  instead of silently proceeding with wrong separators — verified end-to-end); repl/script
+  entry points print to stderr and exit; the unit-test site asserts nil.
+- `pkg/stream/stream.go` final `Flush` → propagated into `retval` (full disk / closed pipe no
+  longer exits 0 silently).
+- DSL `emit`/`print`/`dump` redirect writes (11): recursive emit calls, `printToRedirectFunc`,
+  `dumpToRedirectFunc`, and both `outputHandlerManager.WriteString` sites now propagate through
+  `Execute`, matching their sibling branches which already did.
+- `pkg/output/record_writer_csv.go` `WriteCSVRecordMaybeColorized` (2) → propagated through
+  `IRecordWriter.Write`, whose error the channel writer already reports.
+- `pkg/output/file_output_handlers.go` close-time `Flush` → propagated.
+- `pkg/runtime/stack.go` `PutIndexed` on fresh map → propagated (setIndexed returns error).
+- `ENV[...]` assignment `os.Setenv` → propagated through `Assign`.
+- REPL `writeRecord`: `recordWriter.Write` error now printed to the terminal (e.g. CSV
+  schema-change errors were silently swallowed in the REPL); `closeBufferedOutputStream` on
+  `:>` / `:>>` redirect switches now prints on error. Flush-before-close at repl/script exit
+  now checked like the close next to it.
+- `pkg/auxents/termcvt.go`: write-side `ostream.Close()` before the rename-over-original now
+  checked (had a `TODO: check return status`); `ostream.Write` in termcvt/unhex now exits on
+  error like the surrounding error handling.
+
+### 6c — explicit `_ =` ignores (~65 findings)
+
+- Unset-style DSL/runtime paths (9 `RemoveIndexed`, 1 `Unsetenv`, in `lvalues.go`/`stack.go`):
+  unset of a non-existent path is a no-op by design; the enclosing `Unassign` API has no error
+  return. Commented at each site.
+- `Mlrmap` unflatten `PutIndexed` (2): best-effort API with no error return; commented.
+- Mid-stream `FlushOnEveryRecord` flushes (pprint, channel writer): bufio errors are sticky and
+  the final Flush in `pkg/stream` is checked; commented.
+- Read-side `Close` (input readers ×10, auxents ×4, readfiles ×2, halfpipe, mlrrc, option_parse
+  `--load`, CPU-profile handle), `go process.Wait()` in halfpipe.
+- Init-time strftime `ss.Set` registrations (12) in `pkg/bifs/datetime.go` (constant specs).
+- In-memory usage-capture pipes (`io.Copy`/`Close`) in `keyword_usage_json.go` and
+  `aaa_transformer_json.go`.
+- regtest harness `os.Setenv`/`Unsetenv`/`Remove` (14) and `entrypoint.go` temp-file `os.Remove`
+  on error paths (5).
+- REPL/script terminal `WriteString`/`Flush` where the config exclusion can't reach (field
+  access); commented.
+
+Also fixed in passing: staticcheck QF1012 in `record_writer_pprint.go` (surfaced once the
+errcheck finding on the same line was excluded): `WriteString(fmt.Sprint(x))` where `x` is
+already a string → `WriteString(x)`; dropped the then-unused `fmt` import.
+
 ## Bug noticed in passing (fixed on `johnkerl/lint4`)
 
 In `pkg/mlrval/mlrval_collections.go`, `removeIndexedOnArray` with a single in-bounds index removed