git-prompt: stop manually parsing HEAD with unknown ref formats

We're manually parsing the HEAD reference in git-prompt to figure out
whether it is a symbolic or direct reference. This makes it intimately
tied to the on-disk format we use to store references and will stop
working once we gain additional reference backends in the Git project.

Ideally, we would refactor the code to exclusively use plumbing tools to
read refs such that we do not have to care about the on-disk format at
all. Unfortunately though, spawning processes can be quite expensive on
some systems like Windows. As the Git prompt logic may be executed quite
frequently we try very hard to spawn as few processes as possible. This
refactoring is thus out of question for now.

Instead, condition the logic on the repository's ref format: if the repo
uses the the "files" backend we can continue to use the old logic and
read the respective files from disk directly. If it's anything else,
then we use git-symbolic-ref(1) to read the value of HEAD.

This change makes the Git prompt compatible with the upcoming "reftable"
format.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

contrib/completion/git-prompt.sh

@@ -408,7 +408,7 @@ __git_ps1 ()
 
 	local repo_info rev_parse_exit_code
 	repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
-		--is-bare-repository --is-inside-work-tree \
+		--is-bare-repository --is-inside-work-tree --show-ref-format \
 		--short HEAD 2>/dev/null)"
 	rev_parse_exit_code="$?"
 
@@ -421,6 +421,8 @@ __git_ps1 ()
 		short_sha="${repo_info##*$'\n'}"
 		repo_info="${repo_info%$'\n'*}"
 	fi
+	local ref_format="${repo_info##*$'\n'}"
+	repo_info="${repo_info%$'\n'*}"
 	local inside_worktree="${repo_info##*$'\n'}"
 	repo_info="${repo_info%$'\n'*}"
 	local bare_repo="${repo_info##*$'\n'}"
@@ -479,12 +481,25 @@ __git_ps1 ()
 			b="$(git symbolic-ref HEAD 2>/dev/null)"
 		else
 			local head=""
-			if ! __git_eread "$g/HEAD" head; then
-				return $exit
-			fi
-			# is it a symbolic ref?
-			b="${head#ref: }"
-			if [ "$head" = "$b" ]; then
+
+			case "$ref_format" in
+			files)
+				if ! __git_eread "$g/HEAD" head; then
+					return $exit
+				fi
+
+				if [[ $head == "ref: "* ]]; then
+					head="${head#ref: }"
+				else
+					head=""
+				fi
+				;;
+			*)
+				head="$(git symbolic-ref HEAD 2>/dev/null)"
+				;;
+			esac
+
+			if test -z "$head"; then
 				detached=yes
 				b="$(
 				case "${GIT_PS1_DESCRIBE_STYLE-}" in
@@ -502,6 +517,8 @@ __git_ps1 ()
 
 				b="$short_sha..."
 				b="($b)"
+			else
+				b="$head"
 			fi
 		fi
 	fi