[PATCH] http-backend: drop strbuf pre-sizing from get_info_packs()

[PATCH] http-backend: drop strbuf pre-sizing from get_info_packs()

[Yusuke Hayashi via B4 Relay]

From: Yusuke Hayashi <yusuke8h@gmail.com>

get_info_packs() iterates over all packs in the repository to count
local packs, calls `strbuf_grow(&buf, cnt * 53 + 2)`, and then iterates
over the packs a second time to format each entry with `strbuf_addf()`.

This pre-sizing dates back to 2f4038ab33 (Git-aware CGI to provide dumb
HTTP transport, 2009-10-30) and is no longer worth keeping: `53` assumes
the length of a SHA-1 "P pack-<hex>.pack\n" line (under-counting the
77-byte lines in SHA-256 repositories), `cnt * 53 + 2` uses unchecked
arithmetic, and `strbuf_addf()` / `strbuf_addch()` already grow the
buffer automatically without needing an extra pass over the pack list.
Similar pre-sizing was recently removed in a9ce8526dc (pretty: drop
strbuf pre-sizing from add_rfc2047(), 2026-05-12).

Drop `cnt`, the counting loop, and the `strbuf_grow()` call.

Signed-off-by: Yusuke Hayashi <yusuke8h@gmail.com>
---
Inspired by https://lore.kernel.org/git/20260513185408.GA147423@coredump.intra.peff.net/
---
 http-backend.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/http-backend.c b/http-backend.c
index c7566b1d12..da9516a4c1 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -612,15 +612,8 @@ static void get_info_packs(struct strbuf *hdr, char *arg UNUSED)
 	size_t objdirlen = strlen(repo_get_object_directory(the_repository));
 	struct strbuf buf = STRBUF_INIT;
 	struct packed_git *p;
-	size_t cnt = 0;
 
 	select_getanyfile(hdr);
-	repo_for_each_pack(the_repository, p) {
-		if (p->pack_local)
-			cnt++;
-	}
-
-	strbuf_grow(&buf, cnt * 53 + 2);
 	repo_for_each_pack(the_repository, p) {
 		if (p->pack_local)
 			strbuf_addf(&buf, "P %s\n", p->pack_name + objdirlen + 6);

---
base-commit: 9354c5f04055932909b660a237905b360cba195a
change-id: 20261113-http-backend-drop-strbuf-presizing-6f496552d0ca

Re: [PATCH] http-backend: drop strbuf pre-sizing from get_info_packs()

[Patrick Steinhardt]

On Fri, Nov 13, 2026 at 02:17:58AM +0000, Yusuke Hayashi via B4 Relay wrote:
> From: Yusuke Hayashi <yusuke8h@gmail.com>
> 
> get_info_packs() iterates over all packs in the repository to count
> local packs, calls `strbuf_grow(&buf, cnt * 53 + 2)`, and then iterates
> over the packs a second time to format each entry with `strbuf_addf()`.
> 
> This pre-sizing dates back to 2f4038ab33 (Git-aware CGI to provide dumb
> HTTP transport, 2009-10-30) and is no longer worth keeping: `53` assumes
> the length of a SHA-1 "P pack-<hex>.pack\n" line (under-counting the
> 77-byte lines in SHA-256 repositories), `cnt * 53 + 2` uses unchecked
> arithmetic, and `strbuf_addf()` / `strbuf_addch()` already grow the
> buffer automatically without needing an extra pass over the pack list.
> Similar pre-sizing was recently removed in a9ce8526dc (pretty: drop
> strbuf pre-sizing from add_rfc2047(), 2026-05-12).

So this explains that the behaviour is buggy, but that may lead the
reader to wonder why we don't just fix the bug. And sure, we know to
grow the buffer automatically, but the hint is intended as a performance
improvement so that we know to allocate the buffer properly right from
the start.

Note that I'm not saying that this change doesn't make sense. But I
think the argument that "others do it" is rather weak. The more
important question to answer is why it's fine to skip it in this
particular function.

Thanks!

Patrick