Output & Exit Codes

stdout is the product

A command writes its product — Markdown or JSON — to App.Stdout as-is. Everything else goes through output.Writer on stderr:

The writer is one concrete type with no colour and no JSON mode. labelsync has both because it reports on a run; here the product is the output, and the few lines around it do not earn a styling dependency. Writes are best-effort: the writer has no error channel, which is why .golangci.yml excludes fmt.Fprint and fmt.Fprintln from errcheck.

The slog boundary

log/slog is a debug-only channel on stderr, silent unless --debug, and never used for reporting. The default level is output.LevelSilent, above every level slog defines. If a user is supposed to see it, it is a Writer call; if it only helps someone debugging, it is slog.

App

cmd.App holds Stdout, the writer, the slog level, and the resolved persistent flags (Repo, PreviousTag, SinceFinal, Debug). Every command is built with it and closes over it. The root’s PersistentPreRunE rebuilds the streams over cmd.OutOrStdout() / cmd.ErrOrStderr() — never os.Stdout / os.Stderr — so a test drives the whole tree with buffers and asserts on the stdout/stderr split directly. -R is checked for shape there; whether the repository exists is GitHub’s answer.

TTY detection

output.IsTTY(stream) backs the non-interactive guard: the tag picker is only shown when stdin and stdout are terminals, and otherwise a missing tag fails with ErrInteractiveRequired. The picker itself draws on stderr (App.Stderr) and reads App.Stdin, both rebuilt from the command’s own streams like Stdout. It uses the isatty ioctl (golang.org/x/term), not the file mode — /dev/null is a character device too.

Exit codes

internal/util/exit holds the codes; see Usage › Exit codes for the table.

• Commands never call os.Exit, which skips deferred cleanup. They return an error; main maps it with exit.Of — 0 for nil, the carried code for an *exit.Err, 1 for anything else.
• An outcome that is not a failure (2, mismatch, or 4, skipped) travels on an *exit.Err with a nil Err field. main prints nothing for it: the command already reported the mismatch on stdout, or warned about what it skipped.
• A carrier holding a real failure never exits 0, even with its code left unset.
• 1 is exclusive and never combined with an outcome bit. The outcome bits are labelsync’s: 2 is its drift, 4 its skipped.