[PATCH v2 2/2] completion: support pseudoref existence checks for reftables

[Stan Hu <stanhu@gmail.com>]

In contrib/completion/git-completion.bash, there are a bunch of
instances where we read pseudorefs, such as HEAD, MERGE_HEAD,
REVERT_HEAD, and others via the filesystem. However, the upcoming
reftable refs backend won't use '.git/HEAD' at all but instead will
write an invalid refname as placeholder for backwards compatibility,
which will break the git-completion script.

Update the '__git_pseudoref_exists' function to:

1. Recognize the placeholder '.git/HEAD' written by the reftable
   backend (its content is specified in the reftable specs).
2. If reftable is in use, use 'git rev-parse' to determine whether the
    given ref exists.
3. Otherwise, continue to use 'test -f' to check for the ref's filename.

Signed-off-by: Stan Hu <stanhu@gmail.com>
---
 contrib/completion/git-completion.bash | 39 ++++++++++++++++++++------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 9fbdc13f9a..b2e407c7e7 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -122,12 +122,35 @@ __git ()
 		${__git_dir:+--git-dir="$__git_dir"} "$@" 2>/dev/null
 }
 
-# Runs git in $__git_repo_path to determine whether a ref exists.
-# 1: The ref to search
-__git_ref_exists ()
+# Helper function to read the first line of a file into a variable.
+# __git_eread requires 2 arguments, the file path and the name of the
+# variable, in that order.
+#
+# This is taken from git-prompt.sh.
+__git_eread ()
+{
+	test -r "$1" && IFS=$'\r\n' read -r "$2" <"$1"
+}
+
+# Runs git in $__git_repo_path to determine whether a pseudoref exists.
+# 1: The pseudo-ref to search
+__git_pseudoref_exists()
 {
 	local ref=$1
 
+	# If the reftable is in use, we have to shell out to 'git rev-parse'
+	# to determine whether the ref exists instead of looking directly in
+	# the filesystem to determine whether the ref exists. Otherwise, use
+	# Bash builtins since executing Git commands are expensive on some
+	# platforms.
+	if __git_eread "$__git_repo_path/HEAD" head; then
+		b="${head#ref: }"
+		if [ "$b" == "refs/heads/.invalid" ]; then
+			__git -C "$__git_repo_path" rev-parse --verify --quiet "$ref" 2>/dev/null
+			return $?
+		fi
+	fi
+
 	[ -f "$__git_repo_path/$ref" ]
 }
 
@@ -1634,7 +1657,7 @@ __git_cherry_pick_inprogress_options=$__git_sequencer_inprogress_options
 _git_cherry_pick ()
 {
 	__git_find_repo_path
-	if __git_ref_exists CHERRY_PICK_HEAD; then
+	if __git_pseudoref_exists CHERRY_PICK_HEAD; then
 		__gitcomp "$__git_cherry_pick_inprogress_options"
 		return
 	fi
@@ -2076,7 +2099,7 @@ _git_log ()
 	__git_find_repo_path
 
 	local merge=""
-	if __git_ref_exists MERGE_HEAD; then
+	if __git_pseudoref_exists MERGE_HEAD; then
 		merge="--merge"
 	fi
 	case "$prev,$cur" in
@@ -2962,7 +2985,7 @@ _git_restore ()
 		__gitcomp_builtin restore
 		;;
 	*)
-		if __git_ref_exists HEAD; then
+		if __git_pseudoref_exists HEAD; then
 			__git_complete_index_file "--modified"
 		fi
 	esac
@@ -2973,7 +2996,7 @@ __git_revert_inprogress_options=$__git_sequencer_inprogress_options
 _git_revert ()
 {
 	__git_find_repo_path
-	if __git_ref_exists REVERT_HEAD; then
+	if __git_pseudoref_exists REVERT_HEAD; then
 		__gitcomp "$__git_revert_inprogress_options"
 		return
 	fi
@@ -3602,7 +3625,7 @@ __gitk_main ()
 	__git_find_repo_path
 
 	local merge=""
-	if __git_ref_exists MERGE_HEAD; then
+	if __git_pseudoref_exists MERGE_HEAD; then
 		merge="--merge"
 	fi
 	case "$cur" in
-- 
2.42.0