[PATCH 2/2] completion: stop checking for reference existence via `test -f`

[Stan Hu <stanhu@gmail.com>]

In contrib/completion/git-completion.bash, there are a bunch of
instances where we read special refs like 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_ref_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.

Reviewed-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Stan Hu <stanhu@gmail.com>
---
 contrib/completion/git-completion.bash | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 9fbdc13f9a..f5b630ba99 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
 }
 
+# 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 ref exists.
 # 1: The ref to search
 __git_ref_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" ]
 }
 
-- 
2.42.0