objectsigner: Add context to Sign and KeyID to ssh signer

[pjbgf]

Evolve the per-signer API so signing can be cancelled and so callers can identify the signing key.

• The ssh, gpg and program signers' Sign method now takes a context.Context. The program signer threads it into the external binary invocation; the ssh and gpg signers sign locally and accept the context for interface uniformity.
• The ssh signer gains a KeyID method returning the SHA256 fingerprint of its public key.

Tests and examples are updated for the new signature. The auto plugin, which depends on the ssh and gpg modules, will be updated once these are released.

[Copilot]

Pull request overview

Updates the object signer plugins’ APIs to support context-aware signing (for cancellation/timeout propagation) and adds an SSH key identifier helper for callers.

Changes:

• Extend Sign for the SSH/GPG/program signers to accept context.Context.
• Thread context.Context through the program signer’s external command execution.
• Add KeyID() to the SSH signer (SHA256 public-key fingerprint) and update tests/examples accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
plugin/objectsigner/ssh/ssh.go	Add context.Context to Sign and introduce KeyID() for SSH key fingerprinting.
plugin/objectsigner/ssh/ssh_test.go	Update signing calls for new Sign(ctx, r) signature; add KeyID test.
plugin/objectsigner/ssh/example_test.go	Update example to pass a context into Sign.
plugin/objectsigner/program/program.go	Update Sign to accept/propagate context into external invocation.
plugin/objectsigner/program/program_test.go	Update tests to call Sign with a context.
plugin/objectsigner/gpg/gpg.go	Add context.Context to Sign (accepted for API uniformity).
plugin/objectsigner/gpg/gpg_test.go	Update signing calls for new Sign(ctx, r) signature.
plugin/objectsigner/gpg/example_test.go	Update example to pass a context into Sign.

Comments suppressed due to low confidence (1)

plugin/objectsigner/program/program.go:167

• Sign now takes a context.Context, but if a caller passes nil this will eventually reach exec.CommandContext (via newExecCommand) and panic. To keep the method safe and maintain the pre-context behavior, default a nil context to context.Background() (or return an error) before invoking commandContext.

// Sign reads message and returns the signature produced by the external
// binary. The context cancels the external program invocation.
func (s *signer) Sign(ctx context.Context, message io.Reader) ([]byte, error) {
	if message == nil {
		return nil, ErrNilMessage
	}

	switch s.format {
	case FormatOpenPGP, FormatX509:
		return s.signStdio(ctx, message)
	case FormatSSH:
		return s.signSSH(ctx, message)
	default:
		return nil, fmt.Errorf("%w: %q", ErrUnsupportedFormat, s.format)
	}
}

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