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/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 {