completion: support pseudoref existence checks for reftables

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 8edd002eed07c17871e2e7eee2fdad56d288a43e..e21a39b406fb2e262a5277807385e4c14d17063f 100644 (file)
--- 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 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" ]
 }