From 3704fed5eae8ca2fa20bcf6adb277ee83b012ce0 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Sun, 26 Mar 2023 22:45:40 +0000 Subject: [PATCH 1/4] split-index & fsmonitor: demonstrate a bug This commit adds a new test case that demonstrates a bug in the split-index code that is triggered under certain circumstances when the FSMonitor is enabled, and its symptom manifests in the form of one of the following error messages: BUG: fsmonitor.c:20: fsmonitor_dirty has more entries than the index (2 > 1) BUG: unpack-trees.c:776: pos doesn't point to the first entry of / in index error: invalid path '' error: The following untracked working tree files would be overwritten by reset: initial.t Which of these error messages appears depends on timing-dependent conditions. Technically the root cause lies with a bug in the split-index code that has nothing to do with FSMonitor, but for the sake of this new test case it was the easiest way to trigger the bug. The bug is this: Under specific conditions, Git needs to skip writing the "link" extension (which is the index extension containing the information pertaining to the split-index). To do that, the `base_oid` attribute of the `split_index` structure in the in-memory index is zeroed out, and `do_write_index()` specifically checks for a "null" `base_oid` to understand that the "link" extension should not be written. However, this violates the consistency of the in-memory index structure, but that does not cause problems in most cases because the process exits without using the in-memory index structure anymore, anyway. But: _When_ the in-memory index is still used (which is the case e.g. in `git rebase`), subsequent writes of `the_index` are at risk of writing out a bogus index file, one that _should_ have a "link" extension but does not. In many cases, the `SPLIT_INDEX_ORDERED` flag _happens_ to be set for subsequent writes, forcing the shared index to be written, which re-initializes `base_oid` to a non-bogus state, and all is good. When it is _not_ set, however, all kinds of mayhem ensue, resulting in above-mentioned error messages, and often enough putting worktrees in a totally broken state where the only recourse is to manually delete the `index` and the `index.lock` files and then call `git reset` manually. Not something to ask users to do. The reason why it is comparatively easy to trigger the bug with FSMonitor is that there is _another_ bug in the FSMonitor code: `mark_fsmonitor_valid()` sets `cache_changed` to 1, i.e. treating that variable as a Boolean. But it is a bit field, and 1 happens to be the `SOMETHING_CHANGED` bit that forces the "link" extension to be skipped when writing the index, among other things. "Comparatively easy" is a relative term in this context, for sure. The essence of how the new test case triggers the bug is as following: 1. The `git rebase` invocation will first reset the worktree to a commit that contains only the `one.t` file, and then execute a rebase script that starts with the following commands (commit hashes skipped): label onto reset initial pick two label two reset two pick three [...] 2. Before executing the `label` command, a split index is written, as well as the shared index. 3. The `reset initial` command in the rebase script writes out a new split index but skips writing the shared index, as intended. 4. The `pick two` command updates the worktree and refreshes the index, marking the `two.t` entry as valid via the FSMonitor, which sets the `SOMETHING_CHANGED` bit in `cache_changed`, which in turn causes the `base_oid` attribute to be zeroed out and a full (non-split) index to be written (making sure _not_ to write the "link" extension). 5. Now, the `reset two` command will leave the worktree alone, but still write out a new split index, not writing the shared index (because `base_oid` is still zeroed out, and there is no index entry update requiring it to be written, either). 6. When it is turn to run `pick three`, the index is read, but it is too short: It only contains a single entry when there should be two, because the "link" extension is missing from the written-out index file. There are three bugs at play, actually, which will be fixed over the course of the next commits: - The `base_oid` attribute should not be zeroed out to indicate when the "link" extension should not be written, as it puts the in-memory index structure into an inconsistent state. - The FSMonitor should not overwrite bits in `cache_changed`. - The `unpack_trees()` function tries to reuse the `split_index` structure from the source index, if any, but does not propagate the `SPLIT_INDEX_ORDERED` flag. While a fix for the second bug would let this test case pass, there are other conditions where the `SOMETHING_CHANGED` bit is set. Therefore, the bug that most crucially needs to be fixed is the first one. Signed-off-by: Johannes Schindelin Signed-off-by: Junio C Hamano --- t/t7527-builtin-fsmonitor.sh | 37 ++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) diff --git a/t/t7527-builtin-fsmonitor.sh b/t/t7527-builtin-fsmonitor.sh index d419085379..cbafdd6960 100755 --- a/t/t7527-builtin-fsmonitor.sh +++ b/t/t7527-builtin-fsmonitor.sh @@ -1003,4 +1003,41 @@ test_expect_success !UNICODE_COMPOSITION_SENSITIVE 'Unicode nfc/nfd' ' egrep "^event: nfd/d_${utf8_nfc}/?$" ./unicode.trace ' +test_expect_failure 'split-index and FSMonitor work well together' ' + git init split-index && + test_when_finished "git -C \"$PWD/split-index\" \ + fsmonitor--daemon stop" && + ( + cd split-index && + git config core.splitIndex true && + # force split-index in most cases + git config splitIndex.maxPercentChange 99 && + git config core.fsmonitor true && + + # Create the following commit topology: + # + # * merge three + # |\ + # | * three + # * | merge two + # |\| + # | * two + # * | one + # |/ + # * 5a5efd7 initial + + test_commit initial && + test_commit two && + test_commit three && + git reset --hard initial && + test_commit one && + test_tick && + git merge two && + test_tick && + git merge three && + + git rebase --force-rebase -r one + ) +' + test_done From 3b7a4475b0912d6d78c5e2354599dbc8f1ade479 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Sun, 26 Mar 2023 22:45:41 +0000 Subject: [PATCH 2/4] split-index; stop abusing the `base_oid` to strip the "link" extension When a split-index is in effect, the `$GIT_DIR/index` file needs to contain a "link" extension that contains all the information about the split-index, including the information about the shared index. However, in some cases Git needs to suppress writing that "link" extension (i.e. to fall back to writing a full index) even if the in-memory index structure _has_ a `split_index` configured. This is the case e.g. when "too many not shared" index entries exist. In such instances, the current code sets the `base_oid` field of said `split_index` structure to all-zero to indicate that `do_write_index()` should skip writing the "link" extension. This can lead to problems later on, when the in-memory index is still used to perform other operations and eventually wants to write a split-index, detects the presence of the `split_index` and reuses that, too (under the assumption that it has been initialized correctly and still has a non-null `base_oid`). Let's stop zeroing out the `base_oid` to indicate that the "link" extension should not be written. One might be tempted to simply call `discard_split_index()` instead, under the assumption that Git decided to write a non-split index and therefore the `split_index` structure might no longer be wanted. However, that is not possible because that would release index entries in `split_index->base` that are likely to still be in use. Therefore we cannot do that. The next best thing we _can_ do is to introduce a bit field to indicate specifically which index extensions (not) to write. So that's what we do here. Signed-off-by: Johannes Schindelin Signed-off-by: Junio C Hamano --- read-cache.c | 49 +++++++++++++++++++++++------------- t/t7527-builtin-fsmonitor.sh | 2 +- 2 files changed, 33 insertions(+), 18 deletions(-) diff --git a/read-cache.c b/read-cache.c index b09128b188..7c9a0eeeac 100644 --- a/read-cache.c +++ b/read-cache.c @@ -2868,6 +2868,16 @@ static int record_ieot(void) return !git_config_get_index_threads(&val) && val != 1; } +enum write_extensions { + WRITE_NO_EXTENSION = 0, + WRITE_SPLIT_INDEX_EXTENSION = 1<<0, + WRITE_CACHE_TREE_EXTENSION = 1<<1, + WRITE_RESOLVE_UNDO_EXTENSION = 1<<2, + WRITE_UNTRACKED_CACHE_EXTENSION = 1<<3, + WRITE_FSMONITOR_EXTENSION = 1<<4, +}; +#define WRITE_ALL_EXTENSIONS ((enum write_extensions)-1) + /* * On success, `tempfile` is closed. If it is the temporary file * of a `struct lock_file`, we will therefore effectively perform @@ -2876,7 +2886,7 @@ static int record_ieot(void) * rely on it. */ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, - int strip_extensions, unsigned flags) + enum write_extensions write_extensions, unsigned flags) { uint64_t start = getnanotime(); struct hashfile *f; @@ -3045,8 +3055,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, return -1; } - if (!strip_extensions && istate->split_index && - !is_null_oid(&istate->split_index->base_oid)) { + if (write_extensions & WRITE_SPLIT_INDEX_EXTENSION && + istate->split_index) { struct strbuf sb = STRBUF_INIT; if (istate->sparse_index) @@ -3060,7 +3070,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, if (err) return -1; } - if (!strip_extensions && !drop_cache_tree && istate->cache_tree) { + if (write_extensions & WRITE_CACHE_TREE_EXTENSION && + !drop_cache_tree && istate->cache_tree) { struct strbuf sb = STRBUF_INIT; cache_tree_write(&sb, istate->cache_tree); @@ -3070,7 +3081,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, if (err) return -1; } - if (!strip_extensions && istate->resolve_undo) { + if (write_extensions & WRITE_RESOLVE_UNDO_EXTENSION && + istate->resolve_undo) { struct strbuf sb = STRBUF_INIT; resolve_undo_write(&sb, istate->resolve_undo); @@ -3081,7 +3093,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, if (err) return -1; } - if (!strip_extensions && istate->untracked) { + if (write_extensions & WRITE_UNTRACKED_CACHE_EXTENSION && + istate->untracked) { struct strbuf sb = STRBUF_INIT; write_untracked_extension(&sb, istate->untracked); @@ -3092,7 +3105,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, if (err) return -1; } - if (!strip_extensions && istate->fsmonitor_last_update) { + if (write_extensions & WRITE_FSMONITOR_EXTENSION && + istate->fsmonitor_last_update) { struct strbuf sb = STRBUF_INIT; write_fsmonitor_extension(&sb, istate); @@ -3166,8 +3180,10 @@ static int commit_locked_index(struct lock_file *lk) return commit_lock_file(lk); } -static int do_write_locked_index(struct index_state *istate, struct lock_file *lock, - unsigned flags) +static int do_write_locked_index(struct index_state *istate, + struct lock_file *lock, + unsigned flags, + enum write_extensions write_extensions) { int ret; int was_full = istate->sparse_index == INDEX_EXPANDED; @@ -3185,7 +3201,7 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l */ trace2_region_enter_printf("index", "do_write_index", the_repository, "%s", get_lock_file_path(lock)); - ret = do_write_index(istate, lock->tempfile, 0, flags); + ret = do_write_index(istate, lock->tempfile, write_extensions, flags); trace2_region_leave_printf("index", "do_write_index", the_repository, "%s", get_lock_file_path(lock)); @@ -3214,7 +3230,7 @@ static int write_split_index(struct index_state *istate, { int ret; prepare_to_write_split_index(istate); - ret = do_write_locked_index(istate, lock, flags); + ret = do_write_locked_index(istate, lock, flags, WRITE_ALL_EXTENSIONS); finish_writing_split_index(istate); return ret; } @@ -3289,7 +3305,7 @@ static int write_shared_index(struct index_state *istate, trace2_region_enter_printf("index", "shared/do_write_index", the_repository, "%s", get_tempfile_path(*temp)); - ret = do_write_index(si->base, *temp, 1, flags); + ret = do_write_index(si->base, *temp, WRITE_NO_EXTENSION, flags); trace2_region_leave_printf("index", "shared/do_write_index", the_repository, "%s", get_tempfile_path(*temp)); @@ -3366,9 +3382,8 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock, if ((!si && !test_split_index_env) || alternate_index_output || (istate->cache_changed & ~EXTMASK)) { - if (si) - oidclr(&si->base_oid); - ret = do_write_locked_index(istate, lock, flags); + ret = do_write_locked_index(istate, lock, flags, + ~WRITE_SPLIT_INDEX_EXTENSION); goto out; } @@ -3394,8 +3409,8 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock, /* Same initial permissions as the main .git/index file */ temp = mks_tempfile_sm(git_path("sharedindex_XXXXXX"), 0, 0666); if (!temp) { - oidclr(&si->base_oid); - ret = do_write_locked_index(istate, lock, flags); + ret = do_write_locked_index(istate, lock, flags, + ~WRITE_SPLIT_INDEX_EXTENSION); goto out; } ret = write_shared_index(istate, &temp, flags); diff --git a/t/t7527-builtin-fsmonitor.sh b/t/t7527-builtin-fsmonitor.sh index cbafdd6960..9fab9a2ab3 100755 --- a/t/t7527-builtin-fsmonitor.sh +++ b/t/t7527-builtin-fsmonitor.sh @@ -1003,7 +1003,7 @@ test_expect_success !UNICODE_COMPOSITION_SENSITIVE 'Unicode nfc/nfd' ' egrep "^event: nfd/d_${utf8_nfc}/?$" ./unicode.trace ' -test_expect_failure 'split-index and FSMonitor work well together' ' +test_expect_success 'split-index and FSMonitor work well together' ' git init split-index && test_when_finished "git -C \"$PWD/split-index\" \ fsmonitor--daemon stop" && From be6b65b91bb48c9399c8a6a358dd29b198f2bd79 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Sun, 26 Mar 2023 22:45:42 +0000 Subject: [PATCH 3/4] fsmonitor: avoid overriding `cache_changed` bits As of e636a7b4d030 (read-cache: be specific what part of the index has changed, 2014-06-13), the paradigm `cache_changed = 1` fell out of fashion and it became a bit field instead. This is important because some bits have specific meaning and should not be unset without care, e.g. `SPLIT_INDEX_ORDERED`. However, b5a816975206 (mark_fsmonitor_valid(): mark the index as changed if needed, 2019-05-24) did use the `cache_changed` attribute as if it were a Boolean instead of a bit field. That not only would override the `SPLIT_INDEX_ORDERED` bit when marking index entries as valid via the FSMonitor, but worse: it would set the `SOMETHING_OTHER` bit (whose value is 1). This means that Git would unnecessarily force a full index to be written out when a split index was asked for. Let's instead use the bit that is specifically intended to indicate FSMonitor-triggered changes, allowing the split-index feature to work as designed. Noticed-by: Jeff Hostetler Signed-off-by: Johannes Schindelin Signed-off-by: Junio C Hamano --- fsmonitor.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fsmonitor.h b/fsmonitor.h index edf7ce5203..778707b131 100644 --- a/fsmonitor.h +++ b/fsmonitor.h @@ -86,7 +86,7 @@ static inline void mark_fsmonitor_valid(struct index_state *istate, struct cache !(ce->ce_flags & CE_FSMONITOR_VALID)) { if (S_ISGITLINK(ce->ce_mode)) return; - istate->cache_changed = 1; + istate->cache_changed |= FSMONITOR_CHANGED; ce->ce_flags |= CE_FSMONITOR_VALID; trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_clean '%s'", ce->name); } From 061dd722dcc7a0e1f506c426ff6591b0ca8f14b6 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Sun, 26 Mar 2023 22:45:43 +0000 Subject: [PATCH 4/4] unpack-trees: take care to propagate the split-index flag When copying the `split_index` structure from one index structure to another, we need to propagate the `SPLIT_INDEX_ORDERED` flag, too, if it is set, otherwise Git might forget to write the shared index when that is actually needed. It just so _happens_ that in many instances when `unpack_trees()` is called, the result causes the shared index to be written anyway, but there are edge cases when that is not so. Signed-off-by: Johannes Schindelin Signed-off-by: Junio C Hamano --- unpack-trees.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/unpack-trees.c b/unpack-trees.c index 90b92114be..ca5e47c77c 100644 --- a/unpack-trees.c +++ b/unpack-trees.c @@ -1916,6 +1916,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options * create a new one. */ o->result.split_index = o->src_index->split_index; + if (o->src_index->cache_changed & SPLIT_INDEX_ORDERED) + o->result.cache_changed |= SPLIT_INDEX_ORDERED; o->result.split_index->refcount++; } else { o->result.split_index = init_split_index(&o->result);