Fall back to direct cache write if tempfile creation on the same fs fails by myzhang1029 · Pull Request #2369 · mozilla/sccache

myzhang1029 · 2025-03-20T19:58:22Z

Please see #2288 for the discussion.

src/cache/cache.rs

codecov-commenter · 2025-04-15T07:02:30Z

Codecov Report

❌ Patch coverage is 97.02970% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.42%. Comparing base (2b65ac8) to head (9c3dc37).

Files with missing lines	Patch %	Lines
src/cache/cache_io.rs	97.02%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2369      +/-   ##
==========================================
+ Coverage   73.37%   73.42%   +0.05%     
==========================================
  Files          68       68              
  Lines       37337    37432      +95     
==========================================
+ Hits        27395    27486      +91     
- Misses       9942     9946       +4

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sylvestre · 2025-05-20T07:46:00Z

would it be possible to add a test to make sure we don't regress in the future?
thanks

myzhang1029 · 2025-05-26T14:26:05Z

@sylvestre
I am not familiar with a similar concept under Windows, so is it fine if I hardcode a test for something under /dev and make this test case cfg(unix)?

myzhang1029 · 2025-05-26T14:57:13Z

Rebased on main and added two tests. Both tests succeed on the branch and fail before the patch.

I am not entirely sure if the use of /dev/fd is portable enough to all the unices that Rust and sccache intend to support. Please feel free to skip that one if not.

sylvestre · 2025-05-26T15:16:03Z

could you please add high level tests in https://github.com/mozilla/sccache/blob/main/tests/system.rs too ? thanks

like

sccache/tests/system.rs

Line 1825 in 7025295

fn test_sccache_command(preprocessor_cache_mode: bool) {

myzhang1029 · 2025-06-16T02:30:08Z

@sylvestre Sorry for the delayed response. I am planning to test this by adding two functions that are similar to test_basic_compile in system.rs.

One of them should test /dev/null and the other for /dev/stdout (AssertCmd::assert seems to capture stdout already --- better approach maybe?) They would:

Compiles a file
Repeats the compilation command
Check that both commands succeed and that #miss and #hit are both 1.

myzhang1029 · 2025-06-16T03:20:10Z

On a second thought, I have a question:

I think I named the PR a bit too narrow. Maybe like

Fall back to direct cache write if parent directory is not writable

I was think of reframing the PR this way, because this way, we don't have to hardcode /dev anymore. We can instead:

run the first compilation command
chmod u-w the destination dir
run the second (cached) pass

src/cache/cache.rs

myzhang1029 · 2025-10-10T20:43:10Z

@AJIOB I have pushed the change. Sorry for the delay.

myzhang1029 · 2025-10-11T11:15:14Z

There is an issue with the tests: we use the same INPUT, so the cache stats depend on the previous tests. I feel like it would become a maintenance disaster to hardcode the expected stats into the tests in this case. Would it be acceptable to put a pair of start_local_daemon stop_local_daemon in each test?

myzhang1029 · 2025-10-13T22:05:32Z

CI should work now.

I have relaxed one more error handling: in the case that same-fs file creation fails, we also allow chmod to fail.

AJIOB · 2026-03-11T12:13:04Z

Hi @myzhang1029 ,

I face with that error
As a result, the OpenSSL Perl incorrectly generates assembly file.

Do you have some plans to actualize the PR?

myzhang1029 · 2026-03-11T12:17:19Z

Hello @AJIOB

Is my understanding correct that this patch would fix that error you mentioned?

Lots has happened in the repo and let me work on this today!

AJIOB · 2026-03-11T12:19:16Z

I hope it helps.

In the trace mode I have:

[2026-03-11T12:18:02Z DEBUG sccache::compiler::compiler] Found gcc
[2026-03-11T12:18:02Z TRACE sccache::server] Inserting POSSIBLY PROXIED cache map info for "/usr/bin/gcc"
[2026-03-11T12:18:02Z DEBUG sccache::server] check_compiler: Supported compiler
[2026-03-11T12:18:02Z DEBUG sccache::server] parse_arguments: Ok: ["-Wa,-v", "-c", "-o", "/dev/null", "-x", "assembler", "/dev/null"]
[2026-03-11T12:18:02Z DEBUG sccache::compiler::compiler] [null]: get_cached_or_compile: ["-Wa,-v", "-c", "-o", "/dev/null", "-x", "assembler", "/dev/null"]
[2026-03-11T12:18:02Z TRACE sccache::util] Hashed 0 files in 0.000 s
[2026-03-11T12:18:02Z DEBUG sccache::compiler::c] generate_hash_key: Disabling preprocessor cache because assembler language doesn't need C preprocessing
[2026-03-11T12:18:02Z DEBUG sccache::compiler::compiler] [null]: generate_hash_key took 0.000 s
[2026-03-11T12:18:02Z DEBUG sccache::compiler::compiler] [null]: Hash key: 11b765162fadb5068b7bb35ddec6a07e17f458f37cef7c0d884955a66981f15c
[2026-03-11T12:18:02Z TRACE sccache::cache::disk] DiskCache::get(11b765162fadb5068b7bb35ddec6a07e17f458f37cef7c0d884955a66981f15c)
[2026-03-11T12:18:02Z DEBUG sccache::compiler::compiler] [null]: Cache hit in 0.014 s
[2026-03-11T12:18:02Z ERROR sccache::server] ["null"] fatal error: Permission denied (os error 13) at path "/dev/.tmp9gyxnV"
[2026-03-11T12:18:02Z ERROR sccache::server] ["null"] 	Permission denied (os error 13) at path "/dev/.tmp9gyxnV"

AJIOB · 2026-03-11T13:08:31Z

P.S. Just an easy-to-reproduce test on the latest sccache 0.14.0:

$ gcc -Wa,-v -c -o /dev/null -x assembler /dev/null
GNU assembler version 2.42 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.42
$ sccache gcc -Wa,-v -c -o /dev/null -x assembler /dev/null
sccache: encountered fatal error
sccache: error: Permission denied (os error 13) at path "/dev/.tmpNJtyLa"
sccache: caused by: Permission denied (os error 13) at path "/dev/.tmpNJtyLa"

AJIOB · 2026-03-13T06:03:26Z

Hi @myzhang1029 ,

Do you have any updates?

myzhang1029 · 2026-03-14T03:43:41Z

Sorry for the delay as my area was hit by some severe weather.

This time is largely just rebasing the code and resolving conflicts, but this version should already fix the openssl building error you mentioned. What is left is to figure out the tests properly.

AJIOB · 2026-03-15T13:45:26Z

Hi @sylvestre,

Is this PR OK to you?

myzhang1029 · 2026-03-15T14:39:09Z

Moved the unit tests to cache_io.rs since it uses a private interface.
I've moved the integration tests to branch pr2369-tests since I realized that the expected constants (counts) depend on the platform.

Edit: The integration tests have been merged back.

myzhang1029 · 2026-03-15T18:05:17Z

@AJIOB I believe everything that has been discussed so far is addressed now. Please kindly review!

AJIOB

All looks fine for me

AJIOB · 2026-03-20T11:33:57Z

CC @sylvestre ,

Can we merge it, please?

Signed-off-by: Zhang Maiyun <me@maiyun.me>

Fixes mozilla#2288 The new logic tries to write to a temp file and atomically move it as before, but if a temp file cannot be created, it falls back to writing directly to the final location. In the latter case, we also ignore errors from `set_file_mode`. Signed-off-by: Zhang Maiyun <me@maiyun.me>

Co-authored-by: Alex Overchenko <aleksandr9809@gmail.com>

In addition, when `/dev/fd/{fd}` doesn't actually exist (e.g. FreeBSD without `fdescfs`), skip the test to extract to `/dev/fd/{fd}`. Signed-off-by: Zhang Maiyun <me@maiyun.me>

myzhang1029 · 2026-04-04T01:44:38Z

Just gonna rebase onto main to see if the new tests still work :)

AJIOB reviewed Apr 10, 2025

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sylvestre force-pushed the main branch from 55ca695 to 1b2d572 Compare April 25, 2025 09:18

myzhang1029 force-pushed the main branch from 1b2d572 to 6014307 Compare May 26, 2025 14:56

AJIOB reviewed Jun 16, 2025

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sylvestre force-pushed the main branch from 1d71caa to 4306c7c Compare June 20, 2025 16:22

myzhang1029 force-pushed the main branch from 4306c7c to c80179a Compare October 10, 2025 20:35

myzhang1029 force-pushed the main branch from e199f50 to 0f0ed67 Compare October 13, 2025 22:02

myzhang1029 force-pushed the main branch from 0f0ed67 to 0d08664 Compare January 24, 2026 03:28

myzhang1029 force-pushed the main branch from 0d08664 to adc03c5 Compare March 14, 2026 03:33

myzhang1029 force-pushed the main branch 2 times, most recently from eac34f9 to 1116960 Compare March 15, 2026 14:37

myzhang1029 force-pushed the main branch from 1116960 to 9c3dc37 Compare March 15, 2026 14:57

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AJIOB approved these changes Mar 15, 2026

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myzhang1029 and others added 7 commits April 3, 2026 21:43

Skip cache output to /dev/null

044c7a9

Signed-off-by: Zhang Maiyun <me@maiyun.me>

Fix typo

1064126

Co-authored-by: Alex Overchenko <aleksandr9809@gmail.com>

Test for cache extraction to /dev/(null|fd)

24d5832

Skip output to NUL on Windows targets

37c0f9e

Portable unit test: create ro dir and write there

c6c69f4

In addition, when `/dev/fd/{fd}` doesn't actually exist (e.g. FreeBSD without `fdescfs`), skip the test to extract to `/dev/fd/{fd}`. Signed-off-by: Zhang Maiyun <me@maiyun.me>

High-level tests into /dev/(null|stdout)

c3b28d2

myzhang1029 force-pushed the main branch from bc9ec0c to c3b28d2 Compare April 4, 2026 01:44

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