Re: Optimizing writes to unchanged files during merges?

From

Junio C Hamano <gitster@pobox.com>

Date

Apr 12, 2018, 23:17 UTC

Message-ID

<xmqqtvsg10bu.fsf@gitster-ct.c.googlers.com>

In-Reply-To

<xmqq604w2j4u.fsf@gitster-ct.c.googlers.com>

Junio C Hamano <gitster@pobox.com> writes:

> Linus Torvalds <torvalds@linux-foundation.org> writes:
>
>> Now, the reason it was marked as changed is that the xfs branch _had_
>> in fact changed it, but the changes were already upstream and got
>> merged away. But the file still got written out (with the same
>> contents it had before the merge), and 'make' obviously only looks at
>> modification time, so make rebuilt everything.
>
> Thanks for a clear description of the issue.  It does sound
> interesting.

Here is a fix.

 git-merge-one-file.sh | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
index 9879c59395..aa7392f7ff 100755
--- a/git-merge-one-file.sh
+++ b/git-merge-one-file.sh
@@ -137,11 +137,21 @@ case "${1:-.}${2:-.}${3:-.}" in
 		ret=1
 	fi
 
+	# Does the merge result happen to be identical to what we
+	# already have?  Then do not cause unnecessary recompilation.
+	# But don't bother the optimization if we need to chmod
+	if cmp -s "$4" "$src1" && test "$6" = "$7"
+	then
+		:; # happy
+	else
+
 	# Create the working tree file, using "our tree" version from the
 	# index, and then store the result of the merge.
 	git checkout-index -f --stage=2 -- "$4" && cat "$src1" >"$4" || exit 1
 	rm -f -- "$orig" "$src1" "$src2"
 
+	fi
+
 	if test "$6" != "$7"
 	then
 		if test -n "$msg"