On Tue, Jun 16, 2020 at 07:20:41PM +0000, Han-Wen Nienhuys via GitGitGadget wrote:

* Add GIT_TEST_REFTABLE environment var to control default ref storage

* Add test_prerequisite REFTABLE.

* Skip some tests that are incompatible:

  * t9903-bash-prompt - The bash mode reads .git/HEAD directly

The patch below fixes this incompatibility, relying on the fact that
the reftable specs clearly specify what must be written to the
placeholder '.git/HEAD'.  You can queue this as a preparatory patch to
this series.

Note, however, that, as poined out in my other email, the current
reftable code doesn't follow the specs WRT what must be written to
'.git/HEAD'.  The patch below follows the code, not the specs, so it
will have to be updated as well.

  ---  >8  ---

Subject: [PATCH] git-prompt: prepare for reftable refs backend

In our git-prompt script we strive to use Bash builtins wherever
possible, because fork()-ing subshells for command substitutions and
fork()+exec()-ing Git commands are expensive on some platforms.  We
even read and parse '.git/HEAD' using Bash builtins to get the name of
the current branch [1].  However, the upcoming reftable refs backend
won't use '.git/HEAD' at all, but will write an invalid refname as
placeholder for backwards compatibility instead, which will break our
git-prompt script.

Update the git-prompt script to recognize the placeholder '.git/HEAD'
written by the reftable backend (its content is specified in the
reftable specs), and then fall back to use 'git symbolic-ref' to get
the name of the current branch.

[1] 3a43c4b5bd (bash prompt: use bash builtins to find out current
    branch, 2011-03-31)

Signed-off-by: SZEDER Gábor <redacted>
---
 contrib/completion/git-prompt.sh | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index 014cd7c3cf..708d0b4b95 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh

@@ -460,10 +460,15 @@ __git_ps1 ()
 			if ! __git_eread "$g/HEAD" head; then
 				return $exit
 			fi
-			# is it a symbolic ref?
 			b="${head#ref: }"
 			if [ "$head" = "$b" ]; then
 				detached=yes
+			elif [ "$b" = "refs/.invalid" ]; then
+				# Reftable
+				b="$(git symbolic-ref HEAD 2>/dev/null)" ||
+				detached=yes
+			fi
+			if [ "$detached" = yes ]; then
 				b="$(
 				case "${GIT_PS1_DESCRIBE_STYLE-}" in
 				(contains)

-- 
2.27.0.365.g477ada09ef