git: add support for linking a parent tag to a commit

Add a new struct `AnnotatedTag` that represents a Git annotated tag and
add it as a new field `Commit.Parent`, allowing for a commit to contain
information about its parent tag. Add support for verifying the parent
tag as well.

Signed-off-by: Sanskar Jaiswal <[email protected]>

git/git.go

@@ -87,6 +87,8 @@ type Commit struct {
 	Encoded []byte
 	// Message is the commit message, containing arbitrary text.
 	Message string
+	// Parent is the parent tag, that points to this commit.
+	Parent *AnnotatedTag
 }
 
 // String returns a string representation of the Commit, composed
@@ -115,22 +117,7 @@ func (c *Commit) AbsoluteReference() string {
 // It returns the fingerprint of the key the signature was verified
 // with, or an error.
 func (c *Commit) Verify(keyRing ...string) (string, error) {
-	if c.Signature == "" {
-		return "", fmt.Errorf("commit does not have a PGP signature")
-	}
-
-	for _, r := range keyRing {
-		reader := strings.NewReader(r)
-		keyring, err := openpgp.ReadArmoredKeyRing(reader)
-		if err != nil {
-			return "", fmt.Errorf("unable to read armored key ring: %w", err)
-		}
-		signer, err := openpgp.CheckArmoredDetachedSignature(keyring, bytes.NewBuffer(c.Encoded), bytes.NewBufferString(c.Signature), nil)
-		if err == nil {
-			return signer.PrimaryKey.KeyIdString(), nil
-		}
-	}
-	return "", fmt.Errorf("unable to verify commit with any of the given key rings")
+	return verifySignature(keyRing, c.Signature, c.Encoded)
 }
 
 // ShortMessage returns the first 50 characters of a commit subject.
@@ -143,6 +130,35 @@ func (c *Commit) ShortMessage() string {
 	return subject
 }
 
+// AnnotatedTag represents a Git annotated tag.
+type AnnotatedTag struct {
+	// Hash is the hash of the tag.
+	Hash Hash
+	// Name is the name of the tag.
+	Name string
+	// Author is the original author of the tag.
+	Author Signature
+	// Signature is the PGP signature of the tag.
+	Signature string
+	// Encoded is the encoded tag, without any signature.
+	Encoded []byte
+	// Message is the tagmessage, containing arbitrary text.
+	Message string
+}
+
+// Verify the Signature of the tag with the given key rings.
+// It returns the fingerprint of the key the signature was verified
+// with, or an error.
+func (t *AnnotatedTag) Verify(keyRing ...string) (string, error) {
+	return verifySignature(keyRing, t.Signature, t.Encoded)
+}
+
+// String returns a short string representation of the tag in the format
+// of <name@hash>, for eg: <1.0.0@a0c14dc8580a23f79bc654faa79c4f62b46c2c22>
+func (t *AnnotatedTag) String() string {
+	return fmt.Sprintf("%s@%s", t.Name, t.Hash.String())
+}
+
 // ErrRepositoryNotFound indicates that the repository (or the ref in
 // question) does not exist at the given URL.
 type ErrRepositoryNotFound struct {
@@ -168,3 +184,22 @@ func IsConcreteCommit(c Commit) bool {
 	}
 	return false
 }
+
+func verifySignature(keyRing ...string, sig string, payload []byte) (string, error) {
+	if sig == "" {
+		return "", fmt.Errorf("commit does not have a PGP signature")
+	}
+
+	for _, r := range keyRing {
+		reader := strings.NewReader(r)
+		keyring, err := openpgp.ReadArmoredKeyRing(reader)
+		if err != nil {
+			return "", fmt.Errorf("unable to read armored key ring: %w", err)
+		}
+		signer, err := openpgp.CheckArmoredDetachedSignature(keyring, bytes.NewBuffer(payload), bytes.NewBufferString(sig), nil)
+		if err == nil {
+			return signer.PrimaryKey.KeyIdString(), nil
+		}
+	}
+	return "", fmt.Errorf("unable to verify commit with any of the given key rings")
+}