git: Implement HEAD guessing logicbackup_set_default_branch

2 files changed, 121 insertions, 0 deletions

diff --git a/internal/git/reference.go b/internal/git/reference.go
index 96a32190d..5966cc5c5 100644
--- a/internal/git/reference.go
+++ b/internal/git/reference.go
@@ -3,6 +3,7 @@ package git
 import (
 	"bytes"
 	"context"
+	"errors"
 	"strings"
 )
 
@@ -120,3 +121,50 @@ func CheckRefFormat(ctx context.Context, gitCmdFactory CommandFactory, refName s
 
 	return true, nil
 }
+
+// GuessHead tries to guess what branch HEAD would be pointing at given a list
+// of references. The references are assumed to be ordered by name.
+//
+// This function should match the corresponding function in git:
+// https://github.com/git/git/blob/2a97289ad8b103625d3a1a12f66c27f50df822ce/remote.c#L2198
+func GuessHead(refs []Reference) (ReferenceName, error) {
+	head := findRefByName(refs, "HEAD")
+	if head == nil {
+		return "", errors.New("missing HEAD ref")
+	}
+	if head.IsSymbolic {
+		return ReferenceName(head.Target), nil
+	}
+
+	main := findRefByName(refs, ReferenceName(DefaultRef))
+	if main != nil && head.Target == main.Target {
+		return main.Name, nil
+	}
+
+	master := findRefByName(refs, ReferenceName(LegacyDefaultRef))
+	if master != nil && head.Target == master.Target {
+		return master.Name, nil
+	}
+
+	for i := range refs {
+		if _, ok := refs[i].Name.Branch(); !ok {
+			continue
+		}
+		if refs[i].Target == head.Target {
+			return refs[i].Name, nil
+		}
+	}
+
+	return "", errors.New("no matching ref")
+}
+
+// findRefByName filters through refs and returns the first ref where name
+// matches exactly. nil is returned when no match is found.
+func findRefByName(refs []Reference, name ReferenceName) *Reference {
+	for i := range refs {
+		if refs[i].Name == name {
+			return &refs[i]
+		}
+	}
+	return nil
+}
diff --git a/internal/git/reference_test.go b/internal/git/reference_test.go
index 745b182da..05da8bf48 100644
--- a/internal/git/reference_test.go
+++ b/internal/git/reference_test.go
@@ -131,3 +131,76 @@ func TestReferenceName_Branch(t *testing.T) {
 		})
 	}
 }
+
+func TestGuessHead(t *testing.T) {
+	for _, tc := range []struct {
+		desc         string
+		refs         []git.Reference
+		expectedErr  string
+		expectedHead git.ReferenceName
+	}{
+		{
+			desc:        "no refs",
+			expectedErr: "missing HEAD ref",
+		},
+		{
+			desc: "symbolic HEAD in refs",
+			refs: []git.Reference{
+				git.NewSymbolicReference("HEAD", "refs/heads/banana"),
+				git.NewReference("refs/heads/master", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+			},
+			expectedHead: "refs/heads/banana",
+		},
+		{
+			desc: "matching default branch",
+			refs: []git.Reference{
+				git.NewReference("HEAD", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/apple", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/banana", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/main", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+			},
+			expectedHead: "refs/heads/main",
+		},
+		{
+			desc: "matching legacy default branch",
+			refs: []git.Reference{
+				git.NewReference("HEAD", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/apple", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/banana", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/master", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+			},
+			expectedHead: "refs/heads/master",
+		},
+		{
+			desc: "other matches",
+			refs: []git.Reference{
+				git.NewReference("HEAD", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/apple", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/banana", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/carrot", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/main", "5da601ef10e314884bbade9d5b063be37579ccf0"),
+			},
+			expectedHead: "refs/heads/apple",
+		},
+		{
+			desc: "no matches",
+			refs: []git.Reference{
+				git.NewReference("HEAD", "5da601ef10e314884bbade9d5b063be37579ccf9"),
+				git.NewReference("refs/heads/apple", "5da601ef10e314884bbade9d5b063be37579ccf0"),
+				git.NewReference("refs/heads/banana", "5da601ef10e314884bbade9d5b063be37579ccf0"),
+				git.NewReference("refs/heads/carrot", "5da601ef10e314884bbade9d5b063be37579ccf0"),
+			},
+			expectedErr: "no matching ref",
+		},
+	} {
+		t.Run(tc.desc, func(t *testing.T) {
+			head, err := git.GuessHead(tc.refs)
+			if tc.expectedErr == "" {
+				require.NoError(t, err)
+			} else {
+				require.EqualError(t, err, tc.expectedErr)
+			}
+			require.Equal(t, tc.expectedHead, head)
+		})
+	}
+}