From: Paul Tarjan <github@paulisageek.com>
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 e6d5479e7a (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 3a1ea94a49 (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, and that only one fetch subprocess is spawned.
Cc: Jonathan Tan <jonathantanmy@google.com>
Cc: Han Xin <hanxin.hx@bytedance.com>
Cc: Jeff Hostetler <jeffhostetler@github.com>
Cc: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Paul Tarjan <github@paulisageek.com>
---
promisor-remote: prevent recursive lazy-fetch during index-pack
fetch_objects() in promisor-remote.c spawns a child git fetch to lazily
fill missing objects. That child's index-pack --fix-thin, when it hits a
REF_DELTA against a still-missing base, calls
promisor_remote_get_direct() — which is fetch_objects() again. Unbounded
recursion.
We hit this in production: 276 GB of promisor packs written in 90
minutes against a 100 GB monorepo with ~61K stale prefetch refs pointing
at GC'd commits. Every thin pack picked a bad delta base, and the
recursion fanned out until the mount filled.
The fix is one line: propagate GIT_NO_LAZY_FETCH=1 into the child
fetch's environment. The guard already exists at the top of
fetch_objects() (added by e6d5479e7a, 2021); nothing was setting it in
the child. This is the same bug shape that Han Xin's 3a1ea94a49 (2022)
closed at lookup_commit_in_graph().
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 objects un-deltified or deltified only against
objects in the same pack. A depth-2 fetch would not help; if the server
sends broken thin packs, recursing just makes it worse.Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2224%2Fptarjan%2Fclaude%2Ffix-lazy-fetch-recursion-KP9Hl-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2224/ptarjan/claude/fix-lazy-fetch-recursion-KP9Hl-v1 Pull-Request: https://github.com/git/git/pull/2224
promisor-remote.c | 7 +++ t/meson.build | 1 + t/t0412-promisor-no-lazy-fetch-recursion.sh | 49 +++++++++++++++++++++ 3 files changed, 57 insertions(+) create mode 100755 t/t0412-promisor-no-lazy-fetch-recursion.sh
Show changes to 3 files +57 −0
promisor-remote.c, t/meson.build, t/t0412-promisor-no-lazy-fetch-recursion.sh
diff --git a/promisor-remote.c b/promisor-remote.c index 96fa215b06..35c7aab93d 100644 --- a/promisor-remote.c +++ b/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", diff --git a/t/meson.build b/t/meson.build index e5174ee575..0499533dff 100644 --- a/t/meson.build +++ b/t/meson.build @@ -141,6 +141,7 @@ integration_tests = [ 't0303-credential-external.sh', 't0410-partial-clone.sh', 't0411-clone-from-partial.sh', + 't0412-promisor-no-lazy-fetch-recursion.sh', 't0450-txt-doc-vs-help.sh', 't0500-progress-display.sh', 't0600-reffiles-backend.sh', diff --git a/t/t0412-promisor-no-lazy-fetch-recursion.sh b/t/t0412-promisor-no-lazy-fetch-recursion.sh new file mode 100755 index 0000000000..ec203543d4 --- /dev/null +++ b/t/t0412-promisor-no-lazy-fetch-recursion.sh @@ -0,0 +1,49 @@ +#!/bin/sh + +test_description='promisor-remote: no recursive lazy-fetch + +Verify that fetch_objects() sets GIT_NO_LAZY_FETCH=1 in the child +fetch environment, so that index-pack cannot recurse back into +fetch_objects() when resolving REF_DELTA bases. +' + +. ./test-lib.sh + +test_expect_success 'setup' ' + test_create_repo server && + test_commit -C server foo && + git -C server repack -a -d --write-bitmap-index && + + git clone "file://$(pwd)/server" client && + HASH=$(git -C client rev-parse foo) && + rm -rf client/.git/objects/* && + + git -C client config core.repositoryformatversion 1 && + git -C client config extensions.partialclone "origin" +' + +test_expect_success 'lazy-fetch spawns only one fetch subprocess' ' + GIT_TRACE="$(pwd)/trace" git -C client cat-file -p "$HASH" && + + grep "git fetch" trace >fetches && + test_line_count = 1 fetches +' + +test_expect_success 'child of lazy-fetch has GIT_NO_LAZY_FETCH=1' ' + rm -rf client/.git/objects/* && + + # Install a reference-transaction hook to record the env var + # as seen by processes inside the child fetch. + test_hook -C client reference-transaction <<-\EOF && + echo "$GIT_NO_LAZY_FETCH" >>../env-in-child + EOF + + rm -f env-in-child && + git -C client cat-file -p "$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 base-commit: 7b2bccb0d58d4f24705bf985de1f4612e4cf06e5
-- gitgitgadget