Remove os.Exit callsites below the entrypoint: phase 1 (#2198)

* Rebuild docs: help-catalog index count drifted on main (666 -> 667)

Pre-existing drift from a recent catalog addition; surfaced by make dev.

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

* Remove os.Exit callsites below the entrypoint: phase 1 (plans/exit.md)

Phase 1 of plans/exit.md: the mechanical swaps, plus the single-exit-point
scaffolding.

- New sentinel lib.ExitRequest{Code} (io.EOF-style control flow): returned by
  code paths that have already printed what they wanted (--version, -h,
  'put --explain' on a valid expression, 'put -x') instead of exiting mid-stack.
- pkg/entrypoint: new exitOnError() maps errors to exit codes in one place:
  ErrHelpRequested -> 0, ErrUsagePrinted -> 1, ExitRequest -> its code,
  anything else printed (JSON if --errors-json) -> 1.
- pkg/climain: version/help/usage/validation exits become returned errors;
  the ErrHelpRequested/ErrUsagePrinted -> exit translation moves up to the
  entrypoint; loadMlrrcOrDie becomes error-returning loadMlrrcFiles.
- Transformer ParseCLI/constructor exits -> returned errors (cut,
  having_fields, nest, reorder, merge_fields, reshape, rename, split, tee,
  subs, put_or_filter). merge_fields' bad-regex message formerly mis-reported
  itself as coming from the cut verb; tee's unrecognized-option exit was
  formerly silent; split/tee constructor failures formerly exited without
  printing the constructor's error.
- YAML/JSON record-writer marshal errors -> returned through Write, matching
  the CSV writer's error path.
- lib.WriteTempFileOrDie -> WriteTempFile (string, error); its sole caller
  (regtest diff helper) degrades gracefully.

Stderr messages and exit codes are byte-identical for all regression-covered
paths (4779 cases pass); runtime Transform-path exits are phase 3.

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

* Add lib.NewExitZeroRequest() constructor for exit-0 sentinel returns

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

---------

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

pkg/climain/mlrcli_mlrrc.go

@@ -11,19 +11,7 @@ import (
 	"github.com/johnkerl/miller/v6/pkg/cli"
 )
 
-// loadMlrrcOrDie is a fatal-error wrapper around loadMlrrc.
-func loadMlrrcOrDie(
-	options *cli.TOptions,
-	profileName string,
-) {
-	err := loadMlrrc(options, profileName)
-	if err != nil {
-		fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-		os.Exit(1)
-	}
-}
-
-// loadMlrrc rule: If $MLRRC is set, use it and only it.  Otherwise try first
+// loadMlrrcFiles rule: If $MLRRC is set, use it and only it.  Otherwise try first
 // $HOME/.mlrrc and then ./.mlrrc but let them stack: e.g. $HOME/.mlrrc is
 // lots of settings and maybe in one subdir you want to override just a
 // setting or two.
@@ -34,7 +22,7 @@ func loadMlrrcOrDie(
 // skipped. Non-empty means global lines are applied first, then the lines in
 // any [profileName] section. It's a fatal error if a profile was requested
 // but no matching section exists in any .mlrrc file processed.
-func loadMlrrc(
+func loadMlrrcFiles(
 	options *cli.TOptions,
 	profileName string,
 ) error {
@@ -85,7 +73,7 @@ func loadMlrrc(
 	return checkMlrrcProfileWasFound(profileName, foundProfile, loadedPaths)
 }
 
-// checkMlrrcProfileWasFound is a helper function for loadMlrrc: if a profile
+// checkMlrrcProfileWasFound is a helper function for loadMlrrcFiles: if a profile
 // was requested via --profile {name} / -P {name}, there must be a matching
 // [name] section in at least one processed .mlrrc file.
 func checkMlrrcProfileWasFound(
@@ -108,7 +96,7 @@ func checkMlrrcProfileWasFound(
 	)
 }
 
-// tryLoadMlrrc is a helper function for loadMlrrc. The first return value is
+// tryLoadMlrrc is a helper function for loadMlrrcFiles. The first return value is
 // whether the file could be opened at all: an unopenable file is not an error
 // (that's the normal case when no .mlrrc file exists). The second is any
 // parse error within an opened file.

pkg/climain/mlrcli_mlrrc_test.go

@@ -39,7 +39,7 @@ func TestMlrrcGlobalOnlyBackCompat(t *testing.T) {
 	// .mlrrc format.
 	writeTempMlrrc(t, "icsv\nojson\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, ""); err != nil {
+	if err := loadMlrrcFiles(options, ""); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	if options.ReaderOptions.InputFileFormat != "csv" {
@@ -53,7 +53,7 @@ func TestMlrrcGlobalOnlyBackCompat(t *testing.T) {
 func TestMlrrcSectionsIgnoredWithoutProfile(t *testing.T) {
 	writeTempMlrrc(t, testMlrrcContents)
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, ""); err != nil {
+	if err := loadMlrrcFiles(options, ""); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	if options.ReaderOptions.InputFileFormat != "csv" {
@@ -68,7 +68,7 @@ func TestMlrrcSectionsIgnoredWithoutProfile(t *testing.T) {
 func TestMlrrcProfileSelection(t *testing.T) {
 	writeTempMlrrc(t, testMlrrcContents)
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err != nil {
+	if err := loadMlrrcFiles(options, "j"); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	// Global setting applies first ...
@@ -84,7 +84,7 @@ func TestMlrrcProfileSelection(t *testing.T) {
 func TestMlrrcRepeatedSectionsAccumulate(t *testing.T) {
 	writeTempMlrrc(t, testMlrrcContents)
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err != nil {
+	if err := loadMlrrcFiles(options, "j"); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	// no-auto-flatten is in the second [j] block.
@@ -96,7 +96,7 @@ func TestMlrrcRepeatedSectionsAccumulate(t *testing.T) {
 func TestMlrrcOtherProfileNotApplied(t *testing.T) {
 	writeTempMlrrc(t, testMlrrcContents)
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "p"); err != nil {
+	if err := loadMlrrcFiles(options, "p"); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	if options.WriterOptions.OutputFileFormat != "pprint" {
@@ -107,7 +107,7 @@ func TestMlrrcOtherProfileNotApplied(t *testing.T) {
 func TestMlrrcMissingProfileIsError(t *testing.T) {
 	writeTempMlrrc(t, testMlrrcContents)
 	options := cli.DefaultOptions()
-	err := loadMlrrc(options, "nonesuch")
+	err := loadMlrrcFiles(options, "nonesuch")
 	if err == nil {
 		t.Fatal("expected error for missing profile, got nil")
 	}
@@ -119,7 +119,7 @@ func TestMlrrcMissingProfileIsError(t *testing.T) {
 func TestMlrrcProfileWithMlrrcNoneIsError(t *testing.T) {
 	t.Setenv("MLRRC", "__none__")
 	options := cli.DefaultOptions()
-	err := loadMlrrc(options, "j")
+	err := loadMlrrcFiles(options, "j")
 	if err == nil {
 		t.Fatal("expected error for profile with MLRRC=__none__, got nil")
 	}
@@ -132,7 +132,7 @@ func TestMlrrcProfileWithNoMlrrcFileIsError(t *testing.T) {
 	t.Setenv("MLRRC", filepath.Join(t.TempDir(), "nonexistent"))
 	t.Setenv("HOME", t.TempDir())
 	options := cli.DefaultOptions()
-	err := loadMlrrc(options, "j")
+	err := loadMlrrcFiles(options, "j")
 	if err == nil {
 		t.Fatal("expected error for profile with no .mlrrc file, got nil")
 	}
@@ -144,7 +144,7 @@ func TestMlrrcProfileWithNoMlrrcFileIsError(t *testing.T) {
 func TestMlrrcWhitespaceAndCommentsAroundSectionHeaders(t *testing.T) {
 	writeTempMlrrc(t, "icsv\n\n  [ j ]  # a comment\nojson\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err != nil {
+	if err := loadMlrrcFiles(options, "j"); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	if options.WriterOptions.OutputFileFormat != "json" {
@@ -156,7 +156,7 @@ func TestMlrrcMalformedSectionHeadersAreErrors(t *testing.T) {
 	for _, header := range []string{"[j", "[]", "[ ]", "[a[b]"} {
 		writeTempMlrrc(t, header+"\n")
 		options := cli.DefaultOptions()
-		err := loadMlrrc(options, "")
+		err := loadMlrrcFiles(options, "")
 		if err == nil {
 			t.Errorf("expected parse error for header %q, got nil", header)
 		} else if !strings.Contains(err.Error(), "parse error") {
@@ -170,12 +170,12 @@ func TestMlrrcUnusedProfileLinesNotValidated(t *testing.T) {
 	// invocations of mlr.
 	writeTempMlrrc(t, "icsv\n[j]\nthis-is-not-a-flag\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, ""); err != nil {
+	if err := loadMlrrcFiles(options, ""); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
 	// But it is a parse error when the section is selected.
 	options = cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err == nil {
+	if err := loadMlrrcFiles(options, "j"); err == nil {
 		t.Fatal("expected parse error for selected profile with bad line, got nil")
 	}
 }
@@ -183,7 +183,7 @@ func TestMlrrcUnusedProfileLinesNotValidated(t *testing.T) {
 func TestMlrrcGlobalParseErrorsStillFatal(t *testing.T) {
 	writeTempMlrrc(t, "this-is-not-a-flag\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, ""); err == nil {
+	if err := loadMlrrcFiles(options, ""); err == nil {
 		t.Fatal("expected parse error, got nil")
 	}
 }
@@ -192,7 +192,7 @@ func TestMlrrcPrepipeStillDisallowed(t *testing.T) {
 	// Code-execution flags are disallowed in .mlrrc -- inside profiles too.
 	writeTempMlrrc(t, "[j]\nprepipe zcat\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err == nil {
+	if err := loadMlrrcFiles(options, "j"); err == nil {
 		t.Fatal("expected error for --prepipe within a profile, got nil")
 	}
 }
@@ -203,7 +203,7 @@ func TestMlrrcProfileFlagDisallowedWithinMlrrc(t *testing.T) {
 	for _, line := range []string{"profile j", "--profile j", "-P j"} {
 		writeTempMlrrc(t, line+"\n")
 		options := cli.DefaultOptions()
-		err := loadMlrrc(options, "")
+		err := loadMlrrcFiles(options, "")
 		if err == nil {
 			t.Errorf("expected parse error for %q within .mlrrc, got nil", line)
 		} else if !strings.Contains(err.Error(), "parse error") {
@@ -213,7 +213,7 @@ func TestMlrrcProfileFlagDisallowedWithinMlrrc(t *testing.T) {
 	// Inside a selected profile section, too.
 	writeTempMlrrc(t, "[j]\nprofile p\n")
 	options := cli.DefaultOptions()
-	if err := loadMlrrc(options, "j"); err == nil {
+	if err := loadMlrrcFiles(options, "j"); err == nil {
 		t.Fatal("expected parse error for --profile within a profile section, got nil")
 	}
 }

pkg/climain/mlrcli_parse.go

@@ -151,18 +151,23 @@ func parseCommandLinePassOne(
 
 		if args[argi][0] == '-' {
 			if args[argi] == registry.VersionFlag {
-				// Exiting flag: handle it immediately.
+				// Exiting flag: print immediately, then request exit 0 from the
+				// entrypoint.
 				fmt.Printf("mlr %s\n", version.STRING)
-				os.Exit(0)
+				parseErr = lib.NewExitZeroRequest()
+				return
 			} else if args[argi] == registry.BareVersionFlag {
-				// Exiting flag: handle it immediately.
+				// Exiting flag: print immediately, then request exit 0 from the
+				// entrypoint.
 				fmt.Printf("%s\n", version.STRING)
-				os.Exit(0)
+				parseErr = lib.NewExitZeroRequest()
+				return
 			} else if help.ParseTerminalUsage(args[argi]) {
-				// Exiting flag: handle it immediately.
+				// Exiting flag: the usage text has been printed.
 				// Most help is in the 'mlr help' terminal but there are a few
 				// shorthands like 'mlr -h' and 'mlr -F'.
-				os.Exit(0)
+				parseErr = lib.NewExitZeroRequest()
+				return
 
 			} else if args[argi] == "--norc" {
 				argi += 1
@@ -196,13 +201,15 @@ func parseCommandLinePassOne(
 			// The first verb in the then-chain can *optionally* be preceded by
 			// 'then'.  The others one *must* be.
 			if args[argi] == "then" || args[argi] == "+" {
-				cli.CheckArgCount(args, argi, argc, 1)
 				oargi++
 				argi++
 			}
 			if argi >= argc {
-				fmt.Fprintln(os.Stderr, "mlr: 'then' must have a verb after it.")
-				os.Exit(1)
+				parseErr = &CLIError{
+					Kind: "generic",
+					Msg:  "mlr: 'then' must have a verb after it.",
+				}
+				return
 			}
 			verb := args[argi]
 			onFirst = false
@@ -226,11 +233,11 @@ func parseCommandLinePassOne(
 				false, // false for first pass of CLI-parse, true for second pass -- this is the first pass
 			)
 			if err != nil {
-				if errors.Is(err, cli.ErrHelpRequested) {
-					os.Exit(0)
-				}
-				if errors.Is(err, cli.ErrUsagePrinted) {
-					os.Exit(1)
+				if errors.Is(err, cli.ErrHelpRequested) || errors.Is(err, cli.ErrUsagePrinted) {
+					// Sentinel errors: any output has already been printed; the
+					// entrypoint maps these to their exit codes.
+					parseErr = err
+					return
 				}
 				// Return a structured error so the caller can emit JSON or
 				// prose depending on --errors-json.
@@ -268,7 +275,8 @@ func parseCommandLinePassOne(
 			// piped stdin) defaults to 'cat'.
 			if argc == 1 {
 				help.MainUsage(os.Stderr)
-				os.Exit(1)
+				parseErr = cli.ErrUsagePrinted
+				return
 			}
 			verbSequences = append(verbSequences, []string{"cat"})
 		}
@@ -326,13 +334,17 @@ func parseCommandLinePassTwo(
 		}
 	}
 	if loadMlrrc {
-		loadMlrrcOrDie(options, mlrrcProfileName)
+		if err = loadMlrrcFiles(options, mlrrcProfileName); err != nil {
+			return nil, nil, err
+		}
 	} else if mlrrcProfileName != "" {
-		fmt.Fprintf(os.Stderr,
-			"mlr: --profile \"%s\" was specified along with --norc, which disables .mlrrc processing.\n",
-			mlrrcProfileName,
-		)
-		os.Exit(1)
+		return nil, nil, &CLIError{
+			Kind: "generic",
+			Msg: fmt.Sprintf(
+				"mlr: --profile \"%s\" was specified along with --norc, which disables .mlrrc processing.",
+				mlrrcProfileName,
+			),
+		}
 	}
 
 	// Process the flag-sequences in order from pass one. We assume all the
@@ -402,12 +414,8 @@ func parseCommandLinePassTwo(
 			true, // false for first pass of CLI-parse, true for second pass -- this is pass two
 		)
 		if err != nil {
-			if errors.Is(err, cli.ErrHelpRequested) {
-				os.Exit(0)
-			}
-			if errors.Is(err, cli.ErrUsagePrinted) {
-				os.Exit(1)
-			}
+			// Sentinel errors (ErrHelpRequested, ErrUsagePrinted) pass through
+			// here too; the entrypoint maps them to their exit codes.
 			return nil, nil, err
 		}
 		// Unparsable verb-setups should have been found in pass one.
@@ -455,8 +463,10 @@ func parseCommandLinePassTwo(
 	}
 
 	if options.DoInPlace && len(options.FileNames) == 0 {
-		fmt.Fprintf(os.Stderr, "%s: -I option (in-place operation) requires input files.\n", "mlr")
-		os.Exit(1)
+		return nil, nil, &CLIError{
+			Kind: "generic",
+			Msg:  "mlr: -I option (in-place operation) requires input files.",
+		}
 	}
 
 	if options.HaveRandSeed {

pkg/entrypoint/entrypoint.go

@@ -4,6 +4,7 @@
 package entrypoint
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"path"
@@ -48,12 +49,7 @@ func Main() MainReturn {
 
 	options, recordTransformers, err := climain.ParseCommandLine(os.Args)
 	if err != nil {
-		if wantJSON {
-			climain.EmitStructuredError(err)
-		} else {
-			printError(err)
-		}
-		os.Exit(1)
+		exitOnError(err, wantJSON)
 	}
 
 	if !options.DoInPlace {
@@ -62,8 +58,7 @@ func Main() MainReturn {
 		err = processFilesInPlace(options)
 	}
 	if err != nil {
-		printError(err)
-		os.Exit(1)
+		exitOnError(err, false)
 	}
 
 	return MainReturn{
@@ -71,6 +66,29 @@ func Main() MainReturn {
 	}
 }
 
+// exitOnError terminates the process for a non-nil error from command-line
+// parsing or stream processing. Sentinel errors carry their own exit codes
+// and have already produced whatever output they wanted; any other error is
+// printed (structured if --errors-json is active) and exits 1. This is
+// intended to be the single os.Exit point below main; see plans/exit.md.
+func exitOnError(err error, wantJSON bool) {
+	var exitRequest *lib.ExitRequest
+	switch {
+	case errors.Is(err, cli.ErrHelpRequested):
+		os.Exit(0)
+	case errors.Is(err, cli.ErrUsagePrinted):
+		os.Exit(1)
+	case errors.As(err, &exitRequest):
+		os.Exit(exitRequest.Code)
+	case wantJSON:
+		climain.EmitStructuredError(err)
+		os.Exit(1)
+	default:
+		printError(err)
+		os.Exit(1)
+	}
+}
+
 // printError prints err to stderr. Errors that already carry an "mlr" prefix
 // (e.g. "mlr stats1: ..." or "mlr: verb not found") are printed as-is to
 // avoid double-prefixing.

pkg/lib/exit.go

@@ -0,0 +1,24 @@
+package lib
+
+import "fmt"
+
+// ExitRequest is a sentinel error requesting process termination with the
+// given exit code. Code paths that have already produced all the output they
+// want (e.g. --version, or 'mlr put --explain' on a valid expression) return
+// an ExitRequest instead of calling os.Exit mid-stack; the entrypoint unwraps
+// it with errors.As and exits with the requested code. This is control flow
+// in the io.EOF style: an "error" that isn't a failure. See plans/exit.md.
+type ExitRequest struct {
+	Code int
+}
+
+func (e *ExitRequest) Error() string {
+	return fmt.Sprintf("exit status %d", e.Code)
+}
+
+// NewExitZeroRequest is for the common case of requesting a normal, exit-0
+// termination: e.g. --version or -h, where the requested output has been
+// printed and nothing further should run.
+func NewExitZeroRequest() *ExitRequest {
+	return &ExitRequest{Code: 0}
+}

pkg/lib/util.go

@@ -158,27 +158,24 @@ func GetArrayKeysSorted(input map[string]string) []string {
 
 // WriteTempFile places the contents string into a temp file, which the caller
 // must remove.
-func WriteTempFileOrDie(contents string) string {
+func WriteTempFile(contents string) (string, error) {
 	// Use "" as first argument to os.CreateTemp to use default directory.
 	// Nominally "/tmp" or somesuch on all unix-like systems, but not for Windows.
 	handle, err := os.CreateTemp("", "mlr-temp")
 	if err != nil {
-		fmt.Printf("mlr: could not create temp file.\n")
-		os.Exit(1)
+		return "", fmt.Errorf("could not create temp file: %w", err)
 	}
 
 	_, err = handle.WriteString(contents)
 	if err != nil {
-		fmt.Printf("mlr: could not populate temp file.\n")
-		os.Exit(1)
+		return "", fmt.Errorf("could not populate temp file: %w", err)
 	}
 
 	err = handle.Close()
 	if err != nil {
-		fmt.Printf("mlr: could not finish write of  temp file.\n")
-		os.Exit(1)
+		return "", fmt.Errorf("could not finish write of temp file: %w", err)
 	}
-	return handle.Name()
+	return handle.Name(), nil
 }
 
 func StripEmpties(input []string) []string {

pkg/output/record_writer_json_jsonl.go

@@ -2,8 +2,6 @@ package output
 
 import (
 	"bufio"
-	"fmt"
-	"os"
 
 	"github.com/johnkerl/miller/v6/pkg/cli"
 	"github.com/johnkerl/miller/v6/pkg/mlrval"
@@ -56,11 +54,9 @@ func (writer *RecordWriterJSON) Write(
 	}
 
 	if writer.writerOptions.WrapJSONOutputInOuterList {
-		writer.writeWithListWrap(outrec, context, bufferedOutputStream, outputIsStdout)
-	} else {
-		writer.writeWithoutListWrap(outrec, context, bufferedOutputStream, outputIsStdout)
+		return writer.writeWithListWrap(outrec, context, bufferedOutputStream, outputIsStdout)
 	}
-	return nil
+	return writer.writeWithoutListWrap(outrec, context, bufferedOutputStream, outputIsStdout)
 }
 
 func (writer *RecordWriterJSON) writeWithListWrap(
@@ -68,7 +64,7 @@ func (writer *RecordWriterJSON) writeWithListWrap(
 	context *types.Context,
 	bufferedOutputStream *bufio.Writer,
 	outputIsStdout bool,
-) {
+) error {
 	if outrec != nil { // Not end of record stream
 		if !writer.wroteAnyRecords {
 			bufferedOutputStream.WriteString("[\n")
@@ -78,8 +74,7 @@ func (writer *RecordWriterJSON) writeWithListWrap(
 		// can place it neatly with commas here (if the user requested them).
 		s, err := outrec.FormatAsJSON(writer.jsonFormatting, outputIsStdout)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-			os.Exit(1)
+			return err
 		}
 
 		if writer.wroteAnyRecords {
@@ -102,6 +97,7 @@ func (writer *RecordWriterJSON) writeWithListWrap(
 		}
 
 	}
+	return nil
 }
 
 func (writer *RecordWriterJSON) writeWithoutListWrap(
@@ -109,20 +105,20 @@ func (writer *RecordWriterJSON) writeWithoutListWrap(
 	_ *types.Context,
 	bufferedOutputStream *bufio.Writer,
 	outputIsStdout bool,
-) {
+) error {
 	if outrec == nil {
 		// End of record stream
-		return
+		return nil
 	}
 
 	// The Mlrmap FormatAsJSON doesn't include the final newline, so that we
 	// can place it neatly with commas here (if the user requested them).
 	s, err := outrec.FormatAsJSON(writer.jsonFormatting, outputIsStdout)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-		os.Exit(1)
+		return err
 	}
 
 	bufferedOutputStream.WriteString(s)
 	bufferedOutputStream.WriteString("\n")
+	return nil
 }

pkg/output/record_writer_yaml.go

@@ -2,8 +2,6 @@ package output
 
 import (
 	"bufio"
-	"fmt"
-	"os"
 
 	"gopkg.in/yaml.v3"
 
@@ -33,25 +31,22 @@ func (writer *RecordWriterYAML) Write(
 	outputIsStdout bool,
 ) error {
 	if writer.writerOptions.WrapYAMLOutputInOuterList {
-		writer.writeWithListWrap(outrec, bufferedOutputStream)
-	} else {
-		writer.writeWithoutListWrap(outrec, bufferedOutputStream)
+		return writer.writeWithListWrap(outrec, bufferedOutputStream)
 	}
-	return nil
+	return writer.writeWithoutListWrap(outrec, bufferedOutputStream)
 }
 
 func (writer *RecordWriterYAML) writeWithListWrap(
 	outrec *mlrval.Mlrmap,
 	bufferedOutputStream *bufio.Writer,
-) {
+) error {
 	if outrec != nil {
 		if writer.bufferedRecords == nil {
 			writer.bufferedRecords = []*yaml.Node{}
 		}
 		native, err := mlrval.MlrmapToYAMLNative(outrec)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-			os.Exit(1)
+			return err
 		}
 		writer.bufferedRecords = append(writer.bufferedRecords, native)
 	} else {
@@ -60,34 +55,33 @@ func (writer *RecordWriterYAML) writeWithListWrap(
 		seqNode.Content = writer.bufferedRecords
 		out, err := yaml.Marshal(seqNode)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-			os.Exit(1)
+			return err
 		}
 		bufferedOutputStream.Write(out)
 		writer.bufferedRecords = nil
 	}
+	return nil
 }
 
 func (writer *RecordWriterYAML) writeWithoutListWrap(
 	outrec *mlrval.Mlrmap,
 	bufferedOutputStream *bufio.Writer,
-) {
+) error {
 	if outrec == nil {
-		return
+		return nil
 	}
 	if writer.wroteAnyRecords {
 		bufferedOutputStream.WriteString("---\n")
 	}
 	native, err := mlrval.MlrmapToYAMLNative(outrec)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-		os.Exit(1)
+		return err
 	}
 	out, err := yaml.Marshal(native)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "mlr: %v\n", err)
-		os.Exit(1)
+		return err
 	}
 	bufferedOutputStream.Write(out)
 	writer.wroteAnyRecords = true
+	return nil
 }

pkg/terminals/regtest/invoker.go

@@ -2,6 +2,7 @@ package regtest
 
 import (
 	"bytes"
+	"fmt"
 	"os"
 	"os/exec"
 	"strings"
@@ -70,9 +71,17 @@ func RunDiffCommandOnStrings(
 ) (
 	diffOutput string,
 ) {
-	actualOutputFileName := lib.WriteTempFileOrDie(actualOutput)
-	expectedOutputFileName := lib.WriteTempFileOrDie(expectedOutput)
+	// The diff output is best-effort debugging help (see below); if we can't
+	// write the temp files for it, say so in its place rather than aborting.
+	actualOutputFileName, err := lib.WriteTempFile(actualOutput)
+	if err != nil {
+		return fmt.Sprintf("(diff unavailable: %v)", err)
+	}
 	defer func() { _ = os.Remove(actualOutputFileName) }()
+	expectedOutputFileName, err := lib.WriteTempFile(expectedOutput)
+	if err != nil {
+		return fmt.Sprintf("(diff unavailable: %v)", err)
+	}
 	defer func() { _ = os.Remove(expectedOutputFileName) }()
 
 	// This is diff or fc

pkg/transformers/cut.go

@@ -162,12 +162,7 @@ func NewTransformerCut(
 			// Handles "a.*b"i Miller case-insensitive-regex specification
 			regex, err := lib.CompileMillerRegex(regexString)
 			if err != nil {
-				fmt.Fprintf(
-					os.Stderr,
-					"%s %s: cannot compile regex [%s]\n",
-					"mlr", verbNameCut, regexString,
-				)
-				os.Exit(1)
+				return nil, cli.VerbErrorf(verbNameCut, "cannot compile regex [%s]", regexString)
 			}
 			tr.regexes[i] = regex
 		}

pkg/transformers/having_fields.go

@@ -207,15 +207,7 @@ func NewTransformerHavingFields(
 		// Strip off the leading " and trailing " or "i.
 		regex, err := lib.CompileMillerRegex(regexString)
 		if err != nil {
-			fmt.Fprintf(
-				os.Stderr,
-				"%s %s: cannot compile regex \"%s\"\n",
-				"mlr",
-				verbNameHavingFields,
-				regexString,
-			)
-			os.Exit(1)
-			// return nil, err
+			return nil, cli.VerbErrorf(verbNameHavingFields, "cannot compile regex \"%s\"", regexString)
 		}
 		tr.regex = regex
 

pkg/transformers/merge_fields.go

@@ -282,12 +282,8 @@ func NewTransformerMergeFields(
 		// Handles "a.*b"i Miller case-insensitive-regex specification
 		regex, err := lib.CompileMillerRegex(regexString)
 		if err != nil {
-			fmt.Fprintf(
-				os.Stderr,
-				"%s %s: cannot compile regex [%s]\n",
-				"mlr", verbNameCut, regexString,
-			)
-			os.Exit(1)
+			// (This formerly mis-reported itself as coming from the cut verb.)
+			return nil, cli.VerbErrorf(verbNameMergeFields, "cannot compile regex [%s]", regexString)
 		}
 		tr.valueFieldNameRegexes[i] = regex
 	}

pkg/transformers/nest.go

@@ -313,12 +313,7 @@ func NewTransformerNest(
 	if doRegexes {
 		fieldRegex, err := lib.CompileMillerRegex(fieldName)
 		if err != nil {
-			fmt.Fprintf(
-				os.Stderr,
-				"%s %s: cannot compile regex [%s]\n",
-				"mlr", verbNameNest, fieldName,
-			)
-			os.Exit(1)
+			return nil, cli.VerbErrorf(verbNameNest, "cannot compile regex [%s]", fieldName)
 		}
 		tr.fieldRegex = fieldRegex
 		// implode uses fieldRegex directly when doRegexes
@@ -327,12 +322,7 @@ func NewTransformerNest(
 		regexString := "^" + fieldName + "_[0-9]+$"
 		regex, err := lib.CompileMillerRegex(regexString)
 		if err != nil {
-			fmt.Fprintf(
-				os.Stderr,
-				"%s %s: cannot compile regex [%s]\n",
-				"mlr", verbNameNest, regexString,
-			)
-			os.Exit(1)
+			return nil, cli.VerbErrorf(verbNameNest, "cannot compile regex [%s]", regexString)
 		}
 		tr.regex = regex
 	}

pkg/transformers/put_or_filter.go

@@ -459,7 +459,8 @@ func NewTransformerPut(
 	// --explain is a validate/dry-run: report whether the DSL parsed and
 	// type-checked, then exit without reading the input stream. A parse/build
 	// error is returned so it flows through the normal error path (including
-	// --errors-json); a valid expression prints a confirmation and exits 0.
+	// --errors-json); a valid expression prints a confirmation and requests
+	// exit 0 via the ExitRequest sentinel.
 	if doExplain {
 		if err != nil {
 			return nil, err
@@ -469,11 +470,10 @@ func NewTransformerPut(
 			verbName = "filter"
 		}
 		if warningsAreFatal && hadWarnings {
-			fmt.Fprintf(os.Stderr, "mlr %s: DSL expression has warnings treated as fatal.\n", verbName)
-			os.Exit(1)
+			return nil, cli.VerbErrorf(verbName, "DSL expression has warnings treated as fatal.")
 		}
 		fmt.Printf("mlr %s: DSL expression is valid.\n", verbName)
-		os.Exit(0)
+		return nil, lib.NewExitZeroRequest()
 	}
 
 	if warningsAreFatal && hadWarnings {
@@ -481,11 +481,11 @@ func NewTransformerPut(
 			"%s: Exiting due to warnings treated as fatal.\n",
 			"mlr",
 		)
-		os.Exit(1)
+		return nil, &lib.ExitRequest{Code: 1}
 	}
 
 	if exitAfterParse {
-		os.Exit(0)
+		return nil, lib.NewExitZeroRequest()
 	}
 
 	if err != nil {

pkg/transformers/rename.go

@@ -164,7 +164,10 @@ func NewTransformerRename(
 		tr.regexesAndReplacements = []*tRegexAndReplacement{}
 		for pe := oldToNewNames.Head; pe != nil; pe = pe.Next {
 			regexString := pe.Key
-			regex := lib.CompileMillerRegexOrDie(regexString)
+			regex, err := lib.CompileMillerRegex(regexString)
+			if err != nil {
+				return nil, err
+			}
 			replacement := pe.Value
 			_, replacementCaptureMatrix := lib.ReplacementHasCaptures(replacement)
 			regexAndReplacement := tRegexAndReplacement{

pkg/transformers/reorder.go

@@ -196,12 +196,7 @@ func NewTransformerReorder(
 			// Handles "a.*b"i Miller case-insensitive-regex specification
 			regex, err := lib.CompileMillerRegex(regexString)
 			if err != nil {
-				fmt.Fprintf(
-					os.Stderr,
-					"%s %s: cannot compile regex [%s]\n",
-					"mlr", verbNameReorder, regexString,
-				)
-				os.Exit(1)
+				return nil, cli.VerbErrorf(verbNameReorder, "cannot compile regex [%s]", regexString)
 			}
 			tr.regexes[i] = regex
 		}

pkg/transformers/reshape.go

@@ -276,12 +276,7 @@ func NewTransformerReshape(
 		for i, inputFieldRegexString := range inputFieldRegexStrings {
 			regex, err := lib.CompileMillerRegex(inputFieldRegexString)
 			if err != nil {
-				fmt.Fprintf(
-					os.Stderr,
-					"%s %s: cannot compile regex [%s]\n",
-					"mlr", verbNameReshape, inputFieldRegexString,
-				)
-				os.Exit(1)
+				return nil, cli.VerbErrorf(verbNameReshape, "cannot compile regex [%s]", inputFieldRegexString)
 			}
 			tr.inputFieldRegexes[i] = regex
 		}

pkg/transformers/split.go

@@ -230,8 +230,7 @@ func transformerSplitParseCLI(
 		&localOptions.WriterOptions,
 	)
 	if err != nil {
-		// Error message already printed out
-		os.Exit(1)
+		return nil, err
 	}
 
 	return transformer, nil

pkg/transformers/subs.go

@@ -177,7 +177,7 @@ func transformerSubsParseCLI(
 		switch opt {
 		case "-h", "--help":
 			usageFunc(os.Stdout)
-			os.Exit(0)
+			return nil, cli.ErrHelpRequested
 
 		case "-a":
 			doAllFieldNames = true
@@ -195,19 +195,19 @@ func transformerSubsParseCLI(
 			doAllFieldNames = false
 		default:
 			usageFunc(os.Stderr)
-			os.Exit(1)
+			return nil, cli.ErrUsagePrinted
 		}
 	}
 
 	if fieldNames == nil && !doAllFieldNames {
 		usageFunc(os.Stderr)
-		os.Exit(1)
+		return nil, cli.ErrUsagePrinted
 	}
 
 	// Get the old and new text from the command line
 	if (argc - argi) < 2 {
 		usageFunc(os.Stderr)
-		os.Exit(1)
+		return nil, cli.ErrUsagePrinted
 	}
 	oldText = args[argi]
 	newText = args[argi+1]

pkg/transformers/tee.go

@@ -95,7 +95,7 @@ func transformerTeeParseCLI(
 				// Nothing else to handle here.
 				argi = largi
 			} else {
-				os.Exit(1)
+				return nil, cli.VerbErrorf(verbNameTee, "option \"%s\" not recognized", opt)
 			}
 		}
 	}
@@ -123,8 +123,7 @@ func transformerTeeParseCLI(
 		&localOptions.WriterOptions,
 	)
 	if err != nil {
-		// Error message already printed out
-		os.Exit(1)
+		return nil, err
 	}
 
 	return transformer, nil