Make --error-file allow abs paths

This also enforces th previously unenforced "must not start with a
period" rule.

README.md

@@ -146,9 +146,10 @@ OPTIONS
             full history of the repo.
 
     --error-file <string>, $GIT_SYNC_ERROR_FILE
-            The name of an optional file (under --root) into which errors will
-            be written.  This must be a filename, not a path, and may not start
-            with a period.
+            The path to an optional file into which errors will be written.
+            This may be an absolute path or a relative path, in which case it
+            is relative to --root.  If it is relative to --root, the first path
+            element may not start with a period.
 
     --exechook-backoff <duration>, $GIT_SYNC_EXECHOOK_BACKOFF
             The time to wait before retrying a failed --exechook-command.  If

cmd/git-sync/main.go

@@ -71,7 +71,7 @@ var flRoot = pflag.String("root", envString("GIT_SYNC_ROOT", ""),
 var flLink = pflag.String("link", envString("GIT_SYNC_LINK", ""),
 	"the path (absolute or relative to --root) at which to create a symlink to the directory holding the checked-out files (defaults to the leaf dir of --repo)")
 var flErrorFile = pflag.String("error-file", envString("GIT_SYNC_ERROR_FILE", ""),
-	"an optional file into which errors will be written under --root (defaults to disabled)")
+	"the path (absolute or relative to --root) to an optional file into which errors will be written (defaults to disabled)")
 var flPeriod = pflag.Duration("period", envDuration("GIT_SYNC_PERIOD", 10*time.Second),
 	"how long to wait between syncs, must be >= 10ms; --wait overrides this")
 var flSyncTimeout = pflag.Duration("sync-timeout", envDuration("GIT_SYNC_SYNC_TIMEOUT", 120*time.Second),
@@ -326,7 +326,14 @@ func main() {
 	pflag.Parse()
 
 	// Needs to happen very early for errors to be written to a file.
-	log := logging.New(*flRoot, *flErrorFile, *flVerbose)
+	log := func() *logging.Logger {
+		if strings.HasPrefix(*flErrorFile, ".") {
+			fmt.Fprintf(os.Stderr, "ERROR: --error-file may not start with '.'")
+			os.Exit(1)
+		}
+		dir, file := filepath.Split(makeAbsPath(*flErrorFile, *flRoot))
+		return logging.New(dir, file, *flVerbose)
+	}()
 	cmdRunner := cmd.NewRunner(log)
 
 	if *flVersion {
@@ -754,6 +761,20 @@ func main() {
 	}
 }
 
+// makeAbsPath makes an absolute path from a path which might be absolute
+// or relative.  If the path is already absolute, it will be used.  If it is
+// not absolute, it will be joined with the provided root. If the path is
+// empty, the result will be empty.
+func makeAbsPath(path, root string) string {
+	if path == "" {
+		return ""
+	}
+	if filepath.IsAbs(path) {
+		return path
+	}
+	return filepath.Join(root, path)
+}
+
 const redactedString = "REDACTED"
 
 func redactURL(urlstr string) string {
@@ -1844,9 +1865,10 @@ OPTIONS
             full history of the repo.
 
     --error-file <string>, $GIT_SYNC_ERROR_FILE
-            The name of an optional file (under --root) into which errors will
-            be written.  This must be a filename, not a path, and may not start
-            with a period.
+            The path to an optional file into which errors will be written.
+            This may be an absolute path or a relative path, in which case it
+            is relative to --root.  If it is relative to --root, the first path
+            element may not start with a period.
 
     --exechook-backoff <duration>, $GIT_SYNC_EXECHOOK_BACKOFF
             The time to wait before retrying a failed --exechook-command.  If

cmd/git-sync/main_test.go

@@ -148,6 +148,33 @@ func TestEnvDuration(t *testing.T) {
 	}
 }
 
+func TestMakeAbsPath(t *testing.T) {
+	cases := []struct {
+		path string
+		root string
+		exp  string
+	}{{
+		path: "", root: "", exp: "",
+	}, {
+		path: "", root: "/root", exp: "",
+	}, {
+		path: "path", root: "/root", exp: "/root/path",
+	}, {
+		path: "p/a/t/h", root: "/root", exp: "/root/p/a/t/h",
+	}, {
+		path: "/path", root: "/root", exp: "/path",
+	}, {
+		path: "/p/a/t/h", root: "/root", exp: "/p/a/t/h",
+	}}
+
+	for _, tc := range cases {
+		res := makeAbsPath(tc.path, tc.root)
+		if res != tc.exp {
+			t.Errorf("expected: %q, got: %q", tc.exp, res)
+		}
+	}
+}
+
 func TestManualHasNoTabs(t *testing.T) {
 	if strings.Contains(manual, "\t") {
 		t.Fatal("the manual text contains a tab")

test_e2e.sh

@@ -2149,6 +2149,58 @@ function e2e::export_error() {
     assert_file_absent "$ROOT"/error.json
 }
 
+##############################################
+# Test export-error with an absolute path
+##############################################
+function e2e::export_error_abs_path() {
+    echo "$FUNCNAME" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME"
+
+    (
+        set +o errexit
+        GIT_SYNC \
+            --repo="file://$REPO" \
+            --branch=does-not-exit \
+            --root="$ROOT" \
+            --link="link" \
+            --error-file="$ROOT/dir/error.json" \
+            >> "$1" 2>&1
+        RET=$?
+        if [[ "$RET" != 1 ]]; then
+            fail "expected exit code 1, got $RET"
+        fi
+        assert_file_absent "$ROOT"/link
+        assert_file_absent "$ROOT"/link/file
+        assert_file_contains "$ROOT"/dir/error.json "Remote branch does-not-exit not found in upstream origin"
+    )
+}
+
+##############################################
+# Test export-error with invalid path
+##############################################
+function e2e::export_error_invalid_file() {
+    echo "$FUNCNAME" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME"
+
+    (
+        set +o errexit
+        GIT_SYNC \
+            --repo="file://$REPO" \
+            --branch="$MAIN_BRANCH" \
+            --root="$ROOT" \
+            --link="link" \
+            --error-file=".error.json" \
+            >> "$1" 2>&1
+        RET=$?
+        if [[ "$RET" != 1 ]]; then
+            fail "expected exit code 1, got $RET"
+        fi
+        assert_file_absent "$ROOT"/link
+        assert_file_absent "$ROOT"/link/file
+        assert_file_absent "$ROOT"/.error.json
+    )
+}
+
 ##############################################
 # Test github HTTPS
 # TODO: it would be better if we set up a local HTTPS server