promisor-remote: prevent lazy-fetch recursion in child fetch

fetch_objects() spawns a child `git fetch` to lazily fill in missing
objects. That child's index-pack, when it receives a thin pack
containing a REF_DELTA against a still-missing base, explicitly
calls promisor_remote_get_direct() — which is fetch_objects() again.
If the base is truly unavailable (e.g. because many refs in the
local store point at objects that have been garbage-collected on the
server), each recursive lazy-fetch can trigger another, leading to
unbounded recursion with runaway disk and process consumption.

The GIT_NO_LAZY_FETCH guard (introduced by e6d5479 (git: add
--no-lazy-fetch option, 2021-08-31)) already exists at the top of
fetch_objects(); the missing piece is propagating it into the child
fetch's environment. Add that propagation so the child's
index-pack, if it encounters a REF_DELTA against a missing base,
hits the guard and fails fast instead of recursing.

Depth-1 lazy fetch (the whole point of fetch_objects()) is
unaffected: only the child and its descendants see the variable.
With negotiationAlgorithm=noop the client advertises no "have"
lines, so a well-behaved server sends requested objects
un-deltified or deltified only against objects in the same pack;
the child's index-pack should never need a depth-2 fetch. If it
does, the server response was broken or the local store is already
corrupt, and further fetching would not help.

This is the same bug shape that 3a1ea94 (commit-graph.c: no lazy
fetch in lookup_commit_in_graph(), 2022-07-01) addressed at a
different entry point.

Add a test that verifies the child fetch environment contains
GIT_NO_LAZY_FETCH=1 via a reference-transaction hook.

Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ptarjan

promisor-remote.c

@@ -42,6 +42,13 @@ static int fetch_objects(struct repository *repo,
 	child.in = -1;
 	if (repo != the_repository)
 		prepare_other_repo_env(&child.env, repo->gitdir);
+	/*
+	 * Prevent the child's index-pack from recursing back into
+	 * fetch_objects() when resolving REF_DELTA bases it does not
+	 * have.  With noop negotiation the server should never need
+	 * to send such deltas, so a depth-2 fetch would not help.
+	 */
+	strvec_pushf(&child.env, "%s=1", NO_LAZY_FETCH_ENVIRONMENT);
 	strvec_pushl(&child.args, "-c", "fetch.negotiationAlgorithm=noop",
 		     "fetch", remote_name, "--no-tags",
 		     "--no-write-fetch-head", "--recurse-submodules=no",

t/t0411-clone-from-partial.sh

@@ -78,4 +78,30 @@ test_expect_success 'promisor lazy-fetching can be re-enabled' '
 	test_path_is_file script-executed
 '
 
+test_expect_success 'lazy-fetch child has GIT_NO_LAZY_FETCH=1' '
+	test_create_repo nolazy-server &&
+	test_commit -C nolazy-server foo &&
+	git -C nolazy-server repack -a -d --write-bitmap-index &&
+
+	git clone "file://$(pwd)/nolazy-server" nolazy-client &&
+	HASH=$(git -C nolazy-client rev-parse foo) &&
+	rm -rf nolazy-client/.git/objects/* &&
+
+	git -C nolazy-client config core.repositoryformatversion 1 &&
+	git -C nolazy-client config extensions.partialclone "origin" &&
+
+	# Install a reference-transaction hook to record the env var
+	# as seen by processes inside the child fetch.
+	test_hook -C nolazy-client reference-transaction <<-\EOF &&
+	echo "$GIT_NO_LAZY_FETCH" >>../env-in-child
+	EOF
+
+	rm -f env-in-child &&
+	git -C nolazy-client cat-file -p "$HASH" &&
+
+	# The hook runs inside the child fetch, which should have
+	# GIT_NO_LAZY_FETCH=1 in its environment.
+	grep "^1$" env-in-child
+'
+
 test_done