t/lib-httpd: make http-429 first-request check atomic

http-429.sh records "already returned 429 once" with a "test -f"
followed by a "touch" of a shared state file. That check-then-act is not
atomic: Apache can run this CGI for several requests at once, and two of
them can both pass the "test -f" before either "touch"es, so both treat
themselves as the first request. The retry flow that drives this
endpoint is mostly sequential, so this has not been seen to fail, but
the race is latent.

Decide whether this is the first request with a single atomic mkdir,
which fails if the directory already exists, so exactly one of any
concurrent requests is rate-limited and the rest are forwarded.

Skipping state for "permanent" is required for correctness, not just an
optimization. The marker tells a later or concurrent request that a 429
has already been served, so that it forwards to git-http-backend instead
of rate-limiting. Since "permanent" must return 429 to every request,
that marker must never become visible to another such request.

The original did not achieve this by staying stateless: its "touch" of
the marker ran unconditionally, and the "permanent" case removed it
afterward with "rm -f". That create-then-remove leaves a window in which
a concurrent "permanent" request sees the marker and is forwarded. It is
the same class of check-then-act race this patch removes from the
first-request check, latent for the same reason: the flow is mostly
sequential. This version fuses the check and the mark into one atomic
mkdir and, rather than recreate the pattern as mkdir-then-rmdir, skips
the mkdir for "permanent" with a "!= permanent" guard. No marker is ever
created, so there is no window and every "permanent" request
rate-limits.

There is no accompanying regression test. The check and the set are
adjacent commands with no external step in between to synchronize on, so
the overlap cannot be forced deterministically, only reproduced
probabilistically; the fix is preventive.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-httpd/http-429.sh

@@ -26,14 +26,24 @@ repo_path="${remaining#*/}"  # Get rest (repo path)
 # The repo name is the first component before any "/"
 repo_name="${repo_path%%/*}"
 
-# Use current directory (HTTPD_ROOT_PATH) for state file
-# Create a safe filename from test_context, retry_after and repo_name
-# This ensures all requests for the same test context share the same state file
+# Use current directory (HTTPD_ROOT_PATH) for state.
+# Create a safe name from test_context, retry_after and repo_name so that all
+# requests for the same test context share the same state.
 safe_name=$(echo "${test_context}-${retry_after}-${repo_name}" | tr '/' '_' | tr -cd 'a-zA-Z0-9_-')
-state_file="http-429-state-${safe_name}"
+state="http-429-state-${safe_name}"
 
-# Check if this is the first call (no state file exists)
-if test -f "$state_file"
+# This endpoint returns 429 to the first request and forwards later ones to
+# git-http-backend, so the retry succeeds. Apache can run this CGI for several
+# requests at once, so a single atomic "mkdir" elects that first request: the
+# one whose mkdir succeeds returns 429 and leaves the directory behind as the
+# "already rate-limited" marker; every later request finds the directory (mkdir
+# fails) and is forwarded.
+#
+# "permanent" is the exception: it must return 429 to every request and never
+# succeed, so it skips the mkdir and records no state. A leftover directory
+# would make its own later requests find the marker and be forwarded, which is
+# exactly what "permanent" must not do.
+if test "$retry_after" != permanent && ! mkdir "$state" 2>/dev/null
 then
 	# Already returned 429 once, forward to git-http-backend
 	# Set PATH_INFO to just the repo path (without retry-after value)
@@ -52,9 +62,6 @@ then
 	exec "$GIT_EXEC_PATH/git-http-backend"
 fi
 
-# Mark that we've returned 429
-touch "$state_file"
-
 # Output HTTP 429 response
 printf "Status: 429 Too Many Requests\r\n"
 
@@ -67,8 +74,7 @@ case "$retry_after" in
 		printf "Retry-After: invalid-format-123abc\r\n"
 		;;
 	permanent)
-		# Always return 429, don't set state file for success
-		rm -f "$state_file"
+		# Always return 429
 		printf "Retry-After: 1\r\n"
 		printf "Content-Type: text/plain\r\n"
 		printf "\r\n"