objectsigner: Add context to Sign and KeyID to ssh signer

plugin/objectsigner/gpg/example_test.go

@@ -1,6 +1,7 @@
 package gpg_test
 
 import (
+	"context"
 	"fmt"
 	"strings"
 
@@ -21,7 +22,7 @@ func ExampleFromKey() {
 		panic(err)
 	}
 
-	sig, err := signer.Sign(strings.NewReader("signed commit message\n"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("signed commit message\n"))
 	if err != nil {
 		panic(err)
 	}

plugin/objectsigner/gpg/gpg.go

@@ -4,6 +4,7 @@ package gpg
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -36,8 +37,10 @@ type signer struct {
 }
 
 // Sign reads message and returns an ASCII-armored detached GPG
-// signature created with the signer's OpenPGP key.
-func (s *signer) Sign(message io.Reader) ([]byte, error) {
+// signature created with the signer's OpenPGP key. The context is accepted
+// for interface uniformity across signers; native OpenPGP signing is purely
+// local and does not consult it.
+func (s *signer) Sign(_ context.Context, message io.Reader) ([]byte, error) {
 	if message == nil {
 		return nil, ErrNilMessage
 	}

plugin/objectsigner/gpg/gpg_test.go

@@ -2,6 +2,7 @@ package gpg_test
 
 import (
 	"bytes"
+	"context"
 	"io"
 	"strings"
 	"testing"
@@ -62,7 +63,7 @@ func TestSign(t *testing.T) {
 			signer, err := gpg.FromKey(generateTestKey(t))
 			require.NoError(t, err)
 
-			sig, err := signer.Sign(test.message)
+			sig, err := signer.Sign(context.Background(), test.message)
 			if test.wantErr != "" {
 				require.ErrorContains(t, err, test.wantErr)
 				require.Nil(t, sig)
@@ -85,7 +86,7 @@ func TestSignVerifyRoundTrip(t *testing.T) {
 
 	message := "signed commit message\n"
 
-	sig, err := signer.Sign(strings.NewReader(message))
+	sig, err := signer.Sign(context.Background(), strings.NewReader(message))
 	require.NoError(t, err)
 
 	keyring := openpgp.EntityList{key}
@@ -106,10 +107,10 @@ func TestSignDifferentMessagesProduceDifferentSignatures(t *testing.T) {
 	signer, err := gpg.FromKey(key)
 	require.NoError(t, err)
 
-	sig1, err := signer.Sign(strings.NewReader("message one"))
+	sig1, err := signer.Sign(context.Background(), strings.NewReader("message one"))
 	require.NoError(t, err)
 
-	sig2, err := signer.Sign(strings.NewReader("message two"))
+	sig2, err := signer.Sign(context.Background(), strings.NewReader("message two"))
 	require.NoError(t, err)
 
 	assert.NotEqual(t, sig1, sig2, "different messages produced identical signatures")

plugin/objectsigner/program/program.go

@@ -150,17 +150,17 @@ func resolveProgram(program string, lookPath func(string) (string, error)) (stri
 }
 
 // Sign reads message and returns the signature produced by the external
-// binary.
-func (s *signer) Sign(message io.Reader) ([]byte, error) {
+// 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(context.Background(), message)
+		return s.signStdio(ctx, message)
 	case FormatSSH:
-		return s.signSSH(context.Background(), message)
+		return s.signSSH(ctx, message)
 	default:
 		return nil, fmt.Errorf("%w: %q", ErrUnsupportedFormat, s.format)
 	}

plugin/objectsigner/program/program_test.go

@@ -135,11 +135,61 @@ func TestSign_NilMessage(t *testing.T) {
 
 	signer, _ := newTestSigner(t, FormatOpenPGP, "ABC", nil)
 
-	sig, err := signer.Sign(nil)
+	sig, err := signer.Sign(context.Background(), nil)
 	require.ErrorIs(t, err, ErrNilMessage)
 	require.Nil(t, sig)
 }
 
+// TestSign_ThreadsContext asserts Sign threads the caller's context into the
+// command invocation for every format, rather than building the command with a
+// fresh context.Background(). The proof is propagation: cancelling the context
+// passed to Sign is observable through the context the command was created
+// with, which is only possible if it is the same context.
+func TestSign_ThreadsContext(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		format     Format
+		signingKey string
+	}{
+		{name: "openpgp", format: FormatOpenPGP, signingKey: "KEYID"},
+		{name: "x509", format: FormatX509, signingKey: "KEYID"},
+		{name: "ssh", format: FormatSSH, signingKey: "/path/to/key"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			signer, calls := newTestSigner(t, test.format, test.signingKey, func(cmd *mockCommand) error {
+				if test.format == FormatSSH {
+					return writeSignatureFile(cmd.args[len(cmd.args)-1])
+				}
+
+				_, err := io.WriteString(cmd.stdout, "SIG\n")
+				require.NoError(t, err)
+
+				return nil
+			})
+
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+
+			_, err := signer.Sign(ctx, strings.NewReader("body"))
+			require.NoError(t, err)
+
+			require.Len(t, calls(), 1)
+			got := calls()[0].ctx
+			require.NotNil(t, got)
+
+			require.NoError(t, got.Err())
+			cancel()
+			assert.ErrorIs(t, got.Err(), context.Canceled)
+		})
+	}
+}
+
 func TestSign_StdioFormats(t *testing.T) {
 	t.Parallel()
 
@@ -163,7 +213,7 @@ func TestSign_StdioFormats(t *testing.T) {
 				return nil
 			})
 
-			sig, err := signer.Sign(strings.NewReader("commit body\n"))
+			sig, err := signer.Sign(context.Background(), strings.NewReader("commit body\n"))
 			require.NoError(t, err)
 			assert.Equal(t, "STDIO-SIG\n", string(sig))
 			assert.Equal(t, "commit body\n", stdin)
@@ -184,7 +234,7 @@ func TestSign_StdioFailure(t *testing.T) {
 		return errStdioExit
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "stdio failed")
 	require.Nil(t, sig)
@@ -206,7 +256,7 @@ func TestSign_SSH(t *testing.T) {
 		return writeSignatureFile(bufferFile)
 	})
 
-	sig, err := signer.Sign(strings.NewReader("commit body\n"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("commit body\n"))
 	require.NoError(t, err)
 	assert.Equal(t, "SSH-SIG\n", string(sig))
 	assert.Equal(t, "commit body\n", buffer)
@@ -234,7 +284,7 @@ func TestSign_SSHExpandsHomePath(t *testing.T) {
 		return writeSignatureFile(bufferFile)
 	})
 
-	sig, err := signer.Sign(strings.NewReader("commit body\n"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("commit body\n"))
 	require.NoError(t, err)
 	assert.Equal(t, "SSH-SIG\n", string(sig))
 
@@ -290,7 +340,7 @@ func TestSign_SSHLiteralKey(t *testing.T) {
 				return writeSignatureFile(bufferFile)
 			})
 
-			sig, err := signer.Sign(strings.NewReader("commit body\n"))
+			sig, err := signer.Sign(context.Background(), strings.NewReader("commit body\n"))
 			require.NoError(t, err)
 			assert.Equal(t, "SSH-SIG\n", string(sig))
 			assert.Equal(t, "commit body\n", buffer)
@@ -320,7 +370,7 @@ func TestSign_SSHFailure(t *testing.T) {
 		return errSSHExit
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "ssh failed")
 	require.Nil(t, sig)
@@ -335,7 +385,7 @@ func TestSign_SSHPathPrefixedSshDash(t *testing.T) {
 		return writeSignatureFile(bufferFile)
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.NoError(t, err)
 	require.NotNil(t, sig)
 
@@ -359,7 +409,7 @@ func TestSign_StdioOutputTooLarge(t *testing.T) {
 		return nil
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.ErrorIs(t, err, ErrOutputLimitExceeded)
 	assert.Contains(t, err.Error(), "stdout")
 	require.Nil(t, sig)
@@ -379,7 +429,7 @@ func TestSign_StderrTooLarge(t *testing.T) {
 		return nil
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.ErrorIs(t, err, ErrOutputLimitExceeded)
 	assert.Contains(t, err.Error(), "stderr")
 	require.Nil(t, sig)
@@ -401,12 +451,13 @@ func TestSign_SSHSignatureTooLarge(t *testing.T) {
 		return nil
 	})
 
-	sig, err := signer.Sign(strings.NewReader("body"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("body"))
 	require.ErrorIs(t, err, ErrSignatureTooLarge)
 	require.Nil(t, sig)
 }
 
 type mockCommand struct {
+	ctx     context.Context //nolint:containedctx // captured to assert Sign threads its context into the command.
 	run     func(*mockCommand) error
 	stdin   io.Reader
 	stdout  io.Writer
@@ -457,8 +508,9 @@ func stubCommand(run func(*mockCommand) error) (
 ) {
 	calls := make([]*mockCommand, 0, 1)
 
-	commandContext := func(_ context.Context, binary string, args ...string) command {
+	commandContext := func(ctx context.Context, binary string, args ...string) command {
 		cmd := &mockCommand{
+			ctx:     ctx,
 			run:     run,
 			stdin:   nil,
 			stdout:  nil,

plugin/objectsigner/ssh/example_test.go

@@ -1,6 +1,7 @@
 package ssh_test
 
 import (
+	"context"
 	"crypto/ed25519"
 	"crypto/rand"
 	"fmt"
@@ -29,7 +30,7 @@ func ExampleFromKey() {
 		panic(err)
 	}
 
-	sig, err := signer.Sign(strings.NewReader("signed commit message\n"))
+	sig, err := signer.Sign(context.Background(), strings.NewReader("signed commit message\n"))
 	if err != nil {
 		panic(err)
 	}

plugin/objectsigner/ssh/ssh.go

@@ -4,6 +4,7 @@
 package ssh
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -69,8 +70,10 @@ type signer struct {
 
 // Sign reads the message from r and returns an armored SSH signature created
 // with the signer's SSH key and hash algorithm.
-// The signature uses the "git" namespace.
-func (s *signer) Sign(message io.Reader) ([]byte, error) {
+// The signature uses the "git" namespace. The context is accepted for
+// interface uniformity across signers; SSH signing is purely local and does
+// not consult it.
+func (s *signer) Sign(_ context.Context, message io.Reader) ([]byte, error) {
 	if message == nil {
 		return nil, ErrNilMessage
 	}
@@ -82,3 +85,9 @@ func (s *signer) Sign(message io.Reader) ([]byte, error) {
 
 	return sshsig.Armor(sig), nil
 }
+
+// KeyID returns the SHA256 fingerprint of the signer's SSH public key,
+// in the form "SHA256:...".
+func (s *signer) KeyID() string {
+	return gossh.FingerprintSHA256(s.signer.PublicKey())
+}

plugin/objectsigner/ssh/ssh_test.go

@@ -1,6 +1,7 @@
 package ssh_test
 
 import (
+	"context"
 	"crypto/ed25519"
 	"crypto/rand"
 	"io"
@@ -71,7 +72,7 @@ func TestSign(t *testing.T) {
 			signer, err := ssh.FromKey(sshSigner, ssh.WithHashAlgorithm(test.algo))
 			require.NoError(t, err)
 
-			sig, err := signer.Sign(test.message)
+			sig, err := signer.Sign(context.Background(), test.message)
 			if test.wantErr == "" {
 				require.NoError(t, err)
 				assert.NotEmpty(t, sig)
@@ -95,7 +96,7 @@ func TestSignVerifyRoundTrip(t *testing.T) {
 
 	message := "signed commit message\n"
 
-	sig, err := signer.Sign(strings.NewReader(message))
+	sig, err := signer.Sign(context.Background(), strings.NewReader(message))
 	require.NoError(t, err)
 
 	ssig, err := sshsig.Unarmor(sig)
@@ -113,15 +114,26 @@ func TestSignDifferentMessagesProduceDifferentSignatures(t *testing.T) {
 	signer, err := ssh.FromKey(sshSigner)
 	require.NoError(t, err)
 
-	sig1, err := signer.Sign(strings.NewReader("message one"))
+	sig1, err := signer.Sign(context.Background(), strings.NewReader("message one"))
 	require.NoError(t, err)
 
-	sig2, err := signer.Sign(strings.NewReader("message two"))
+	sig2, err := signer.Sign(context.Background(), strings.NewReader("message two"))
 	require.NoError(t, err)
 
 	assert.NotEqual(t, sig1, sig2, "different messages produced identical signatures")
 }
 
+func TestKeyID(t *testing.T) {
+	t.Parallel()
+
+	key := generateTestSigner(t)
+	signer, err := ssh.FromKey(key)
+	require.NoError(t, err)
+
+	assert.Equal(t, gossh.FingerprintSHA256(key.PublicKey()), signer.KeyID())
+	assert.Contains(t, signer.KeyID(), "SHA256:")
+}
+
 //nolint:ireturn // gossh.NewSignerFromKey returns gossh.Signer (interface); no concrete type is accessible
 func generateTestSigner(t *testing.T) gossh.Signer {
 	t.Helper()