Allow --dest to be an absolute path

This means it might live outside of --root.

cmd/git-sync/main.go

@@ -67,7 +67,7 @@ var flSubmodules = pflag.String("submodules", envString("GIT_SYNC_SUBMODULES", "
 var flRoot = pflag.String("root", envString("GIT_SYNC_ROOT", ""),
 	"the root directory for git-sync operations, under which --link will be created")
 var flLink = pflag.String("link", envString("GIT_SYNC_LINK", ""),
-	"the name of a symlink, under --root, which points to a directory in which --repo is checked out (defaults to the leaf dir of --repo)")
+	"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", ""),
 	"the name of a file into which errors will be written under --root (defaults to \"\", disabling error reporting)")
 var flPeriod = pflag.Duration("period", envDuration("GIT_SYNC_PERIOD", 10*time.Second),
@@ -328,10 +328,10 @@ func main() {
 		parts := strings.Split(strings.Trim(*flRepo, "/"), "/")
 		*flLink = parts[len(parts)-1]
 	}
-	if strings.Contains(*flLink, "/") {
-		handleError(log, true, "ERROR: --link must not contain '/'")
+	if !filepath.IsAbs(*flLink) {
+		*flLink = filepath.Join(*flRoot, *flLink)
 	}
-	if strings.HasPrefix(*flLink, ".") {
+	if strings.HasPrefix(filepath.Base(*flLink), ".") {
 		handleError(log, true, "ERROR: --link must not start with '.'")
 	}
 
@@ -829,28 +829,36 @@ func addUser() error {
 // directory and cleans up the previous worktree.  If there was a previous
 // worktree, this returns the path to it.
 func (git *repoSync) UpdateSymlink(ctx context.Context, newDir string) (string, error) {
+	linkDir, linkFile := filepath.Split(git.link)
+
+	// Make sure the link directory exists. We do this here, rather than at
+	// startup because it might be under --root and that gets wiped in some
+	// circumstances.
+	if err := os.MkdirAll(filepath.Dir(linkDir), os.FileMode(int(0755))); err != nil {
+		return "", fmt.Errorf("error making symlink dir: %v", err)
+	}
+
 	// Get currently-linked repo directory (to be removed), unless it doesn't exist
-	linkPath := filepath.Join(git.root, git.link)
-	oldWorktreePath, err := filepath.EvalSymlinks(linkPath)
+	oldWorktreePath, err := filepath.EvalSymlinks(git.link)
 	if err != nil && !os.IsNotExist(err) {
 		return "", fmt.Errorf("error accessing current worktree: %v", err)
 	}
 
 	// newDir is absolute, so we need to change it to a relative path.  This is
 	// so it can be volume-mounted at another path and the symlink still works.
-	newDirRelative, err := filepath.Rel(git.root, newDir)
+	newDirRelative, err := filepath.Rel(linkDir, newDir)
 	if err != nil {
 		return "", fmt.Errorf("error converting to relative path: %v", err)
 	}
 
 	const tmplink = "tmp-link"
-	git.log.V(1).Info("creating tmp symlink", "root", git.root, "dst", newDirRelative, "src", tmplink)
-	if _, err := git.run.Run(ctx, git.root, "ln", "-snf", newDirRelative, tmplink); err != nil {
+	git.log.V(1).Info("creating tmp symlink", "root", linkDir, "dst", newDirRelative, "src", tmplink)
+	if _, err := git.run.Run(ctx, linkDir, "ln", "-snf", newDirRelative, tmplink); err != nil {
 		return "", fmt.Errorf("error creating symlink: %v", err)
 	}
 
-	git.log.V(1).Info("renaming symlink", "root", git.root, "oldName", tmplink, "newName", git.link)
-	if _, err := git.run.Run(ctx, git.root, "mv", "-T", tmplink, git.link); err != nil {
+	git.log.V(1).Info("renaming symlink", "root", linkDir, "oldName", tmplink, "newName", linkFile)
+	if _, err := git.run.Run(ctx, linkDir, "mv", "-T", tmplink, linkFile); err != nil {
 		return "", fmt.Errorf("error replacing symlink: %v", err)
 	}
 
@@ -1188,8 +1196,7 @@ func (git *repoSync) SyncRepo(ctx context.Context) (bool, string, error) {
 		askpassCount.WithLabelValues(metricKeySuccess).Inc()
 	}
 
-	target := filepath.Join(git.root, git.link)
-	gitRepoPath := filepath.Join(target, ".git")
+	gitRepoPath := filepath.Join(git.root, ".git")
 	var hash string
 	_, err := os.Stat(gitRepoPath)
 	switch {
@@ -1646,11 +1653,14 @@ OPTIONS
             --http-bind).  (default: false)
 
     --link <string>, $GIT_SYNC_LINK
-            The name of the final symlink (under --root) which will point to the
-            current git worktree. This must be a filename, not a path, and may
-            not start with a period. The destination of this link (i.e.
-            readlink()) is the currently checked out SHA. (default: the leaf
-            dir of --repo)
+            The path to at which to create a symlink which points to the
+            current git directory, at the currently synced SHA.  This may be an
+            absolute path or a relative path, in which case it is relative to
+            --root.  The last path element is the name of the link and must not
+            start with a period.  Consumers of the synced files should always
+            use this link.  It is updated atomically and should always be
+            valid.  The basename of the target of the link is the current SHA).
+            (default: the leaf dir of --repo)
 
     --man
             Print this manual and exit.

test_e2e.sh

@@ -351,6 +351,48 @@ function e2e::head_once_root_exists_but_fails_sanity() {
 ## FIXME: test when repo is valid git, but not ar ref we need
 ## FIXME: test when repo is valid git, and is already correct
 
+##############################################
+# Test HEAD one-time with an absolute-path link
+##############################################
+function e2e::absolute_link() {
+    echo "$FUNCNAME" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME"
+
+    GIT_SYNC \
+        --one-time \
+        --repo="file://$REPO" \
+        --branch="$MAIN_BRANCH" \
+        --rev="HEAD" \
+        --root="$ROOT/root" \
+        --link="$ROOT/other/dir/link" \
+        >> "$1" 2>&1
+    assert_file_absent "$ROOT"/root/link
+    assert_link_exists "$ROOT"/other/dir/link
+    assert_file_exists "$ROOT"/other/dir/link/file
+    assert_file_eq "$ROOT"/other/dir/link/file "$FUNCNAME"
+}
+
+##############################################
+# Test HEAD one-time with a subdir-path link
+##############################################
+function e2e::subdir_link() {
+    echo "$FUNCNAME" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME"
+
+    GIT_SYNC \
+        --one-time \
+        --repo="file://$REPO" \
+        --branch="$MAIN_BRANCH" \
+        --rev="HEAD" \
+        --root="$ROOT" \
+        --link="other/dir/link" \
+        >> "$1" 2>&1
+    assert_file_absent "$ROOT"/link
+    assert_link_exists "$ROOT"/other/dir/link
+    assert_file_exists "$ROOT"/other/dir/link/file
+    assert_file_eq "$ROOT"/other/dir/link/file "$FUNCNAME"
+}
+
 ##############################################
 # Test default-branch syncing
 ##############################################