completion: don't disambiguate short refs

szeder · gitster · commit b2b68114519b · 2017-03-23T11:18:22.000-07:00
When the completion script lists short refs it does so using the 'git for-each-ref' format 'refname:short', which makes sure that all listed refs are unambiguous. While disambiguating refs is technically correct in this case, as opposed to the cases discussed in the previous patch, this disambiguation involves several stat() syscalls for each ref, thus, unfortunately, comes at a steep cost especially on Windows and/or when there are a lot of refs to be listed. A user of Git for Windows reported[1] 'git checkout <TAB>' taking ~11 seconds in a repository with just about 4000 refs. However, it's questionable whether ambiguous refs are really that bad to justify that much extra cost: - Ambiguous refs are not that common, - even if a repository contains ambiguous refs, they only hurt when the user actually happens to want to do something with one of the ambiguous refs, and - the issue can be easily circumvented by renaming those ambiguous refs. - On the other hand, apparently not that many refs are needed to make refs completion unacceptably slow on Windows, - and this slowness bites each and every time the user attempts refs completion, even when the repository doesn't contain any ambiguous refs. - Furthermore, circumventing the issue might not be possible or might be considerably more difficult and requires various trade-offs (e.g. working in a repository with only a few selected important refs while keeping a separate repository with all refs for reference). Arguably, in this case the benefits of technical correctness are rather minor compared to the price we pay for it, and we are better off opting for performance over correctness. Use the 'git for-each-ref' format 'refname:strip=2' to list short refs to spare the substantial cost of disambiguating. This speeds up refs completion considerably. Uniquely completing a branch in a repository with 100k local branches, all packed, best of five: On Linux, before: $ time __git_complete_refs --cur=maste real 0m1.662s user 0m1.368s sys 0m0.296s After: real 0m0.831s user 0m0.808s sys 0m0.028s On Windows, before: real 0m12.457s user 0m1.016s sys 0m0.092s After: real 0m1.480s user 0m1.031s sys 0m0.060s [1] - git-for-windows#524 Signed-off-by: SZEDER Gábor <szeder.dev@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
@@ -401,7 +401,7 @@ __git_refs ()
 			for i in HEAD FETCH_HEAD ORIG_HEAD MERGE_HEAD; do
 				if [ -e "$dir/$i" ]; then echo $pfx$i; fi
 			done
-			format="refname:short"
+			format="refname:strip=2"
 			refs="refs/tags refs/heads refs/remotes"
 			;;
 		esac
@@ -412,7 +412,7 @@ __git_refs ()
 			# Try to find a remote branch that matches the completion word
 			# but only output if the branch name is unique
 			local ref entry
-			__git for-each-ref --shell --format="ref=%(refname:short)" \
+			__git for-each-ref --shell --format="ref=%(refname:strip=2)" \
 				"refs/remotes/" | \
 			while read -r entry; do
 				eval "$entry"
@@ -437,7 +437,7 @@ __git_refs ()
 	*)
 		if [ "$list_refs_from" = remote ]; then
 			echo "HEAD"
-			__git for-each-ref --format="%(refname:short)" \
+			__git for-each-ref --format="%(refname:strip=2)" \
 				"refs/remotes/$remote/" | sed -e "s#^$remote/##"
 		else
 			__git ls-remote "$remote" HEAD \
