golang/go · error

writing metafiles file

Error message

writing metafiles file: %v

What it means

After serializing the coverage meta-files collection, WriteCoverMetaFilesFile creates the action's objdir and writes the JSON to <objdir>/<MetaFilesFileName> via sh.writeFile. If the write fails (disk full, permission denied, missing parent dir, path too long), the error is wrapped and returned.

Solutions

1. Check disk space and write permissions on TMPDIR and GOCACHE: 'df -h', 'touch <tmp>/x'.
2. Move GOCACHE to a larger, writable local directory.
3. Run 'go clean -cache' to reset a possibly corrupted objdir tree.
4. On Windows, shorten module paths or enable long-path support.

Example fix

// before
$ go test -coverpkg=./... ./...
// error: writing metafiles file: open /tmp/.../b001/metafiles: no space left on device

// after
$ export GOCACHE=$HOME/.cache/go-build
$ export TMPDIR=$HOME/tmp
$ go test -coverpkg=./... ./...

// Pre-flight: confirm TMPDIR/GOCACHE are writable with free space.
func preflightCache() error {
    for _, d := range []string{os.Getenv("GOCACHE"), os.Getenv("TMPDIR")} {
        if d == "" { continue }
        var stat syscall.Statfs_t
        if err := syscall.Statfs(d, &stat); err != nil { return err }
        if stat.Bavail*uint64(stat.Bsize) < 1<<30 {
            return fmt.Errorf("%s has < 1 GiB free", d)
        }
    }
    return nil
}

When it happens

Trigger: Running 'go test -coverpkg' where the build objdir lives on a full or read-only filesystem, or where permission to create files is missing. The earlier sh.Mkdir succeeded, so the failure is specifically in the file write step.

Common situations: TMPDIR/GOCACHE on a small tmpfs; CI container with read-only mounted volume; SELinux denying writes; concurrent build evicting the objdir mid-write; path length limits on Windows.

Related errors

• error adding target to cache
• error opening target for caching
• finding
• internal error: unable to locate build action for package
• loading cached file

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/2c9445262e13e572. Report an issue: GitHub.

Appendix: source

Thrown at src/cmd/go/internal/work/cover.go:168

		collection.MetaFileFragments = append(collection.MetaFileFragments, metaFilesFile)
	}

	// Serialize it.
	data, err := json.Marshal(collection)
	if err != nil {
		return fmt.Errorf("marshal MetaFileCollection: %v", err)
	}
	data = append(data, '\n') // makes -x output more readable

	// Create the directory for this action's objdir and
	// then write out the serialized collection
	// to a file in the directory.
	if err := sh.Mkdir(a.Objdir); err != nil {
		return err
	}
	mfpath := a.Objdir + coverage.MetaFilesFileName
	if err := sh.writeFile(mfpath, data); err != nil {
		return fmt.Errorf("writing metafiles file: %v", err)
	}

	// We're done.
	return nil
}

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