gitutil: find default remote by tracking branch

util/gitutil/gitutil.go

@@ -78,7 +78,13 @@ func (c *Git) GitDir() (string, error) {
 }
 
 func (c *Git) RemoteURL() (string, error) {
-	// Try to get the remote URL from the origin remote first
+	// Try default remote based on remote tracking branch
+	if remote, err := c.currentRemote(); err == nil && remote != "" {
+		if ru, err := c.clean(c.run("remote", "get-url", remote)); err == nil && ru != "" {
+			return stripCredentials(ru), nil
+		}
+	}
+	// Next try to get the remote URL from the origin remote first
 	if ru, err := c.clean(c.run("remote", "get-url", "origin")); err == nil && ru != "" {
 		return stripCredentials(ru), nil
 	}
@@ -149,6 +155,22 @@ func (c *Git) clean(out string, err error) (string, error) {
 	return out, err
 }
 
+func (c *Git) currentRemote() (string, error) {
+	symref, err := c.clean(c.run("symbolic-ref", "-q", "HEAD"))
+	if err != nil {
+		return "", err
+	}
+	if symref == "" {
+		return "", nil
+	}
+	// git for-each-ref --format='%(upstream:remotename)'
+	remote, err := c.clean(c.run("for-each-ref", "--format=%(upstream:remotename)", symref))
+	if err != nil {
+		return "", err
+	}
+	return remote, nil
+}
+
 func IsUnknownRevision(err error) bool {
 	if err == nil {
 		return false

util/gitutil/gitutil_test.go

@@ -106,8 +106,9 @@ func TestGitDescribeTags(t *testing.T) {
 
 func TestGitRemoteURL(t *testing.T) {
 	type remote struct {
-		name string
-		url  string
+		name     string
+		url      string
+		tracking string
 	}
 
 	cases := []struct {
@@ -165,6 +166,36 @@ func TestGitRemoteURL(t *testing.T) {
 			},
 			fail: true,
 		},
+		{
+			name: "single tracking branch",
+			remotes: []remote{
+				{
+					name:     "foo",
+					url:      "https://github.com/tonistiigi/buildx.git",
+					tracking: "master",
+				},
+			},
+			expected: "https://github.com/tonistiigi/buildx.git",
+		},
+		{
+			name: "fork tracking branch",
+			remotes: []remote{
+				{
+					name: "origin",
+					url:  "git@github.com:crazy-max/buildx.git",
+				},
+				{
+					name:     "foobranch",
+					url:      "https://github.com/tonistiigi/buildx.git",
+					tracking: "master",
+				},
+				{
+					name: "crazymax",
+					url:  "git@github.com:crazy-max/buildx.git",
+				},
+			},
+			expected: "https://github.com/tonistiigi/buildx.git",
+		},
 	}
 	for _, tt := range cases {
 		tt := tt
@@ -177,6 +208,9 @@ func TestGitRemoteURL(t *testing.T) {
 			GitCommit(c, t, "initial commit")
 			for _, r := range tt.remotes {
 				GitSetRemote(c, t, r.name, r.url)
+				if r.tracking != "" {
+					GitSetMainUpstream(c, t, r.name, r.tracking)
+				}
 			}
 
 			ru, err := c.RemoteURL()

util/gitutil/testutil.go

@@ -52,6 +52,15 @@ func GitSetRemote(c *Git, tb testing.TB, name string, url string) {
 	require.NoError(tb, err)
 }
 
+func GitSetMainUpstream(c *Git, tb testing.TB, remote, target string) {
+	tb.Helper()
+	_, err := fakeGit(c, "fetch", "--depth", "1", remote, target)
+	require.NoError(tb, err)
+
+	_, err = fakeGit(c, "branch", "--set-upstream-to", remote+"/"+target, "main")
+	require.NoError(tb, err)
+}
+
 func Mktmp(tb testing.TB) string {
 	tb.Helper()
 	folder := tb.TempDir()