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Reftable bugfix.
* ps/reftable-compacted-tables-permission-fix:
reftable/stack: adjust permissions of compacted tables
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The write codepath for the reftable data learned to honor
core.fsync configuration.
* jc/reftable-core-fsync:
reftable/stack: fsync "tables.list" during compaction
reftable: honor core.fsync
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In 1df18a1c9a (reftable: honor core.fsync, 2024-01-23), we have added
code to fsync both newly written reftables as well as "tables.list" to
disk. But there are two code paths where "tables.list" is being written:
- When appending a new table due to a normal ref update.
- When compacting a range of tables during compaction.
We have only addressed the former code path, but do not yet sync the new
"tables.list" file in the latter. Fix this omission.
Note that we are not yet adding any tests. These tests will be added
once the "reftable" backend has been upstreamed.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Low-level I/O optimization for reftable.
* ps/reftable-optimize-io:
reftable/stack: fix race in up-to-date check
reftable/stack: unconditionally reload stack after commit
reftable/blocksource: use mmap to read tables
reftable/blocksource: refactor code to match our coding style
reftable/stack: use stat info to avoid re-reading stack list
reftable/stack: refactor reloading to use file descriptor
reftable/stack: refactor stack reloading to have common exit path
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When creating a new compacted table from a range of preexisting ones we
don't set the default permissions on the resulting table when specified
by the user. This has the effect that the "core.sharedRepository" config
will not be honored correctly.
Fix this bug and add a test to catch this issue. Note that we only test
on non-Windows platforms because Windows does not use POSIX permissions
natively.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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While the reffiles backend honors configured fsync settings, the
reftable backend does not. Address this by fsyncing reftable files using
the write-or-die api's fsync_component() in two places: when we
add additional entries into the table, and when we close the reftable
writer.
This commits adds a flush function pointer as a new member of
reftable_writer because we are not sure that the first argument to the
*write function pointer always contains a file descriptor. In the case of
strbuf_add_void, the first argument is a buffer. This way, we can pass
in a corresponding flush function that knows how to flush depending on
which writer is being used.
This patch does not contain tests as they will need to wait for another
patch to start to exercise the reftable backend. At that point, the
tests will be added to observe that fsyncs are happening when the
reftable is in use.
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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In 6fdfaf15a0 (reftable/stack: use stat info to avoid re-reading stack
list, 2024-01-11) we have introduced a new mechanism to avoid re-reading
the table list in case stat(3P) figures out that the stack didn't change
since the last time we read it.
While this change significantly improved performance when writing many
refs, it can unfortunately lead to false negatives in very specific
scenarios. Given two processes A and B, there is a feasible sequence of
events that cause us to accidentally treat the table list as up-to-date
even though it changed:
1. A reads the reftable stack and caches its stat info.
2. B updates the stack, appending a new table to "tables.list". This
will both use a new inode and result in a different file size, thus
invalidating A's cache in theory.
3. B decides to auto-compact the stack and merges two tables. The file
size now matches what A has cached again. Furthermore, the
filesystem may decide to recycle the inode number of the file we
have replaced in (2) because it is not in use anymore.
4. A reloads the reftable stack. Neither the inode number nor the
file size changed. If the timestamps did not change either then we
think the cached copy of our stack is up-to-date.
In fact, the commit introduced three related issues:
- Non-POSIX compliant systems may not report proper `st_dev` and
`st_ino` values in stat(3P), which made us rely solely on the
file's potentially coarse-grained mtime and ctime.
- `stat_validity_check()` and friends may end up not comparing
`st_dev` and `st_ino` depending on the "core.checkstat" config,
again reducing the signal to the mtime and ctime.
- `st_ino` can be recycled, rendering the check moot even on
POSIX-compliant systems.
Given that POSIX defines that "The st_ino and st_dev fields taken
together uniquely identify the file within the system", these issues led
to the most important signal to establish file identity to be ignored or
become useless in some cases.
Refactor the code to stop using `stat_validity_check()`. Instead, we
manually stat(3P) the file descriptors to make relevant information
available. On Windows and MSYS2 the result will have both `st_dev` and
`st_ino` set to 0, which allows us to address the first issue by not
using the stat-based cache in that case. It also allows us to make sure
that we always compare `st_dev` and `st_ino`, addressing the second
issue.
The third issue of inode recycling can be addressed by keeping the file
descriptor of "files.list" open during the lifetime of the reftable
stack. As the file will still exist on disk even though it has been
unlinked it is impossible for its inode to be recycled as long as the
file descriptor is still open.
This should address the race in a POSIX-compliant way. The only real
downside is that this mechanism cannot be used on non-POSIX-compliant
systems like Windows. But we at least have the second-level caching
mechanism in place that compares contents of "files.list" with the
currently loaded list of tables.
This new mechanism performs roughly the same as the previous one that
relied on `stat_validity_check()`:
Benchmark 1: update-ref: create many refs (HEAD~)
Time (mean ± σ): 4.754 s ± 0.026 s [User: 2.204 s, System: 2.549 s]
Range (min … max): 4.694 s … 4.802 s 20 runs
Benchmark 2: update-ref: create many refs (HEAD)
Time (mean ± σ): 4.721 s ± 0.020 s [User: 2.194 s, System: 2.527 s]
Range (min … max): 4.691 s … 4.753 s 20 runs
Summary
update-ref: create many refs (HEAD~) ran
1.01 ± 0.01 times faster than update-ref: create many refs (HEAD)
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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After we have committed an addition to the reftable stack we call
`reftable_stack_reload()` to reload the stack and thus reflect the
changes that were just added. This function will only conditionally
reload the stack in case `stack_uptodate()` tells us that the stack
needs reloading. This check is wasteful though because we already know
that the stack needs reloading.
Call `reftable_stack_reload_maybe_reuse()` instead, which will
unconditionally reload the stack. This is merely a conceptual fix, the
code in question was not found to cause any problems in practice.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Whenever we call into the refs interfaces we potentially have to reload
refs in case they have been concurrently modified, either in-process or
externally. While this happens somewhat automatically for loose refs
because we simply try to re-read the files, the "packed" backend will
reload its snapshot of the packed-refs file in case its stat info has
changed since last reading it.
In the reftable backend we have a similar mechanism that is provided by
`reftable_stack_reload()`. This function will read the list of stacks
from "tables.list" and, if they have changed from the currently stored
list, reload the stacks. This is heavily inefficient though, as we have
to check whether the stack is up-to-date on basically every read and
thus keep on re-reading the file all the time even if it didn't change
at all.
We can do better and use the same stat(3P)-based mechanism that the
"packed" backend uses. Instead of reading the file, we will only open
the file descriptor, fstat(3P) it, and then compare the info against the
cached value from the last time we have updated the stack. This should
always work alright because "tables.list" is updated atomically via a
rename, so even if the ctime or mtime wasn't granular enough to identify
a change, at least the inode number or file size should have changed.
This change significantly speeds up operations where many refs are read,
like when using git-update-ref(1). The following benchmark creates N
refs in an otherwise-empty repository via `git update-ref --stdin`:
Benchmark 1: update-ref: create many refs (refcount = 1, revision = HEAD~)
Time (mean ± σ): 5.1 ms ± 0.2 ms [User: 2.4 ms, System: 2.6 ms]
Range (min … max): 4.8 ms … 7.2 ms 109 runs
Benchmark 2: update-ref: create many refs (refcount = 100, revision = HEAD~)
Time (mean ± σ): 19.1 ms ± 0.9 ms [User: 8.9 ms, System: 9.9 ms]
Range (min … max): 18.4 ms … 26.7 ms 72 runs
Benchmark 3: update-ref: create many refs (refcount = 10000, revision = HEAD~)
Time (mean ± σ): 1.336 s ± 0.018 s [User: 0.590 s, System: 0.724 s]
Range (min … max): 1.314 s … 1.373 s 10 runs
Benchmark 4: update-ref: create many refs (refcount = 1, revision = HEAD)
Time (mean ± σ): 5.1 ms ± 0.2 ms [User: 2.4 ms, System: 2.6 ms]
Range (min … max): 4.8 ms … 7.2 ms 109 runs
Benchmark 5: update-ref: create many refs (refcount = 100, revision = HEAD)
Time (mean ± σ): 14.8 ms ± 0.2 ms [User: 7.1 ms, System: 7.5 ms]
Range (min … max): 14.2 ms … 15.2 ms 82 runs
Benchmark 6: update-ref: create many refs (refcount = 10000, revision = HEAD)
Time (mean ± σ): 927.6 ms ± 5.3 ms [User: 437.8 ms, System: 489.5 ms]
Range (min … max): 919.4 ms … 936.4 ms 10 runs
Summary
update-ref: create many refs (refcount = 1, revision = HEAD) ran
1.00 ± 0.07 times faster than update-ref: create many refs (refcount = 1, revision = HEAD~)
2.89 ± 0.14 times faster than update-ref: create many refs (refcount = 100, revision = HEAD)
3.74 ± 0.25 times faster than update-ref: create many refs (refcount = 100, revision = HEAD~)
181.26 ± 8.30 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD)
261.01 ± 12.35 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD~)
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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We're about to introduce a stat(3P)-based caching mechanism to reload
the list of stacks only when it has changed. In order to avoid race
conditions this requires us to have a file descriptor available that we
can use to call fstat(3P) on.
Prepare for this by converting the code to use `fd_read_lines()` so that
we have the file descriptor readily available.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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The `reftable_stack_reload_maybe_reuse()` function is responsible for
reloading the reftable list from disk. The function is quite hard to
follow though because it has a bunch of different exit paths, many of
which have to free the same set of resources.
Refactor the function to have a common exit path. While at it, touch up
the style of this function a bit to match our usual coding style better.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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In 5c086453ff (reftable/stack: perform auto-compaction with
transactional interface, 2023-12-11), we fixed a bug where the
transactional interface to add changes to a reftable stack did not
perform auto-compaction by calling `reftable_stack_auto_compact()` in
`reftable_stack_addition_commit()`. While correct, this change may now
cause us to perform auto-compaction twice in the non-transactional
interface `reftable_stack_add()`:
- It performs auto-compaction by itself.
- It now transitively performs auto-compaction via the transactional
interface.
Remove the first instance so that we only end up doing auto-compaction
once.
Reported-by: Han-Wen Nienhuys <hanwenn@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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In order to compact multiple stacks we iterate through the merged ref
and log records. When there is any error either when reading the records
from the old merged table or when writing the records to the new table
then we break out of the respective loops. When breaking out of the loop
for the ref records though the error code will be overwritten, which may
cause us to inadvertently skip over bad ref records. In the worst case,
this can lead to a compacted stack that is missing records.
Fix the code by using `goto done` instead so that any potential error
codes are properly returned to the caller.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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When writing a new reftable stack, Git will first create the stack with
a random suffix so that concurrent updates will not try to write to the
same file. This random suffix is computed via a call to rand(3P). But we
never seed the function via srand(3P), which means that the suffix is in
fact always the same.
Fix this bug by using `git_rand()` instead, which does not need to be
initialized. While this function is likely going to be slower depending
on the platform, this slowness should not matter in practice as we only
use it when writing a new reftable stack.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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When starting a transaction via `reftable_stack_init_addition()`, we
create a lockfile for the reftable stack itself which we'll write the
new list of tables to. But if we terminate abnormally e.g. via a call to
`die()`, then we do not remove the lockfile. Subsequent executions of
Git which try to modify references will thus fail with an out-of-date
error.
Fix this bug by registering the lock as a `struct tempfile`, which
ensures automatic cleanup for us.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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In `reftable_stack_reload_once()` we iterate over all the tables added
to the stack in order to figure out whether any of the tables needs to
be reloaded. We use a set of buffers in this context to compute the
paths of these tables, but discard those buffers on every iteration.
This is quite wasteful given that we do not need to transfer ownership
of the allocated buffer outside of the loop.
Refactor the code to instead reuse the buffers to reduce the number of
allocations we need to do. Note that we do not have to manually reset
the buffer because `stack_filename()` does this for us already.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Whenever updating references or reflog entries in the reftable stack, we
need to add a new table to the stack, thus growing the stack's length by
one. The stack can grow to become quite long rather quickly, leading to
performance issues when trying to read records. But besides performance
issues, this can also lead to exhaustion of file descriptors very
rapidly as every single table requires a separate descriptor when
opening the stack.
While git-pack-refs(1) fixes this issue for us by merging the tables, it
runs too irregularly to keep the length of the stack within reasonable
limits. This is why the reftable stack has an auto-compaction mechanism:
`reftable_stack_add()` will call `reftable_stack_auto_compact()` after
its added the new table, which will auto-compact the stack as required.
But while this logic works alright for `reftable_stack_add()`, we do not
do the same in `reftable_addition_commit()`, which is the transactional
equivalent to the former function that allows us to write multiple
updates to the stack atomically. Consequentially, we will easily run
into file descriptor exhaustion in code paths that use many separate
transactions like e.g. non-atomic fetches.
Fix this issue by calling `reftable_stack_auto_compact()`.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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There are calls to write(3P) where we don't properly handle interrupts.
Convert them to use `write_in_full()`.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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There are calls to pread(3P) and read(3P) where we don't properly handle
interrupts. Convert them to use `pread_in_full()` and `read_in_full()`,
respectively.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Problems identified by Coverity in the reftable code have been
corrected.
* hn/reftable-coverity-fixes:
reftable: add print functions to the record types
reftable: make reftable_record a tagged union
reftable: remove outdated file reftable.c
reftable: implement record equality generically
reftable: make reftable-record.h function signatures const correct
reftable: handle null refnames in reftable_ref_record_equal
reftable: drop stray printf in readwrite_test
reftable: order unittests by complexity
reftable: all xxx_free() functions accept NULL arguments
reftable: fix resource warning
reftable: ignore remove() return value in stack_test.c
reftable: check reftable_stack_auto_compact() return value
reftable: fix resource leak blocksource.c
reftable: fix resource leak in block.c error path
reftable: fix OOB stack write in print functions
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This would trigger in the unlikely event that we are compacting, and
the next available file handle is 0.
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Create files with mode 0666, so umask works as intended. Provides an override,
which is useful to support shared repos (test t1301-shared-repo.sh).
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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