Preserve discovery failures for damaged checkouts

internal/git/discovery_test.go

@@ -55,7 +55,7 @@ func TestDiscoverRepositoryRoot(t *testing.T) {
 	}
 }
 
-func TestNoRepositoryDiagnostic(t *testing.T) {
+func TestNoRepositoryBoundary(t *testing.T) {
 	for _, tc := range []struct {
 		message string
 		want    bool
@@ -65,12 +65,63 @@ func TestNoRepositoryDiagnostic(t *testing.T) {
 		{"fatal: not a git repository: '/bad'", false},
 		{"fatal: detected dubious ownership in repository", false},
 	} {
-		if got := noRepositoryDiagnostic(tc.message); got != tc.want {
+		if _, got := noRepositoryBoundary(tc.message); got != tc.want {
 			t.Errorf("diagnostic %q = %t", tc.message, got)
 		}
 	}
 }
 
+func TestDiscoverRepositoryRootHonorsDiscoveryBoundaries(t *testing.T) {
+	repo := newRemoteFixture(t)
+	child := filepath.Join(repo.work, "child")
+	if err := os.Mkdir(child, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	alias := filepath.Join(t.TempDir(), "alias")
+	if err := os.Symlink(repo.work, alias); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(filepath.Join(repo.work, ".git", "HEAD")); err != nil {
+		t.Fatal(err)
+	}
+	for _, tc := range []struct {
+		name, directory, ceiling string
+		standalone               bool
+	}{
+		{"damaged ancestor", child, "", false},
+		{"excluded ancestor", child, repo.work, true},
+		{"physical ceiling alias", child, alias, true},
+		{"literal ceiling alias", child, string(os.PathListSeparator) + alias, false},
+		{"ceiling includes current directory", repo.work, repo.work, false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("GIT_CEILING_DIRECTORIES", tc.ceiling)
+			_, found, err := DiscoverRepositoryRoot(t.Context(), tc.directory)
+			if tc.standalone {
+				if err != nil || found {
+					t.Fatalf("excluded checkout: found %t, error %v", found, err)
+				}
+			} else {
+				var exitErr *exec.ExitError
+				if found || !errors.As(err, &exitErr) || exitErr.ExitCode() != 128 {
+					t.Fatalf("damaged checkout: found %t, error %v", found, err)
+				}
+			}
+		})
+	}
+	bin := t.TempDir()
+	script := "#!/bin/sh\nprintf 'fatal: not a git repository (or any parent up to mount point %s)\\nStopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).\\n' \"$DISCOVERY_BOUNDARY\" >&2\nexit 128\n"
+	if err := os.WriteFile(filepath.Join(bin, "git"), []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin)
+	t.Setenv("GIT_CEILING_DIRECTORIES", "")
+	t.Setenv("DISCOVERY_BOUNDARY", repo.work)
+	if _, found, err := DiscoverRepositoryRoot(t.Context(), child); err != nil || found {
+		t.Fatalf("filesystem boundary: found %t, error %v", found, err)
+	}
+}
+
 func TestDiscoverRepositoryRootFailures(t *testing.T) {
 	dir := t.TempDir()
 	bin := t.TempDir()