Avoid double-caching when ccache is installed in PATH #2524

avikivity · 2025-12-20T20:12:31Z

On Linux, ccache is typically installed in $PATH as /usr/lib64/ccache/g++ or similar. If we keep it there, all compilations will be cached by both ccache and sccache.

While the user could easily disable ccache with CCACHE_DISABLE, it's reasonable to assume many will forget and will have their disk space doubly consumed by both caches. Better to recognize this and disable ccache under sccache.

This patch does this by removing the ccache binary paths from $PATH.

Fixes #2519

codecov-commenter · 2025-12-20T20:16:43Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.14%. Comparing base (2b92721) to head (ce9f95b).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2524      +/-   ##
==========================================
- Coverage   71.24%   71.14%   -0.10%     
==========================================
  Files          64       65       +1     
  Lines       35593    35833     +240     
==========================================
+ Hits        25357    25495     +138     
- Misses      10236    10338     +102

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avikivity · 2025-12-20T21:00:10Z

update: fix rustfmt error

avikivity · 2026-01-07T12:07:06Z

@sylvestre please review

codspeed-hq · 2026-01-10T14:06:20Z

CodSpeed Performance Report

Merging this PR will degrade performance by 5.29%

_{Comparing avikivity:disable-ccache (ce9f95b) with main (82bfa28)}

Summary

❌ 1 regressed benchmark
✅ 53 untouched benchmarks
⏩ 2 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
❌	Simulation	`time_macro_finder_chunked`	971.9 µs	1,026.1 µs	-5.29%

2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

avikivity · 2026-01-20T13:50:22Z

@sylvestre can I help in any way to push this along?

sylvestre · 2026-01-22T13:02:10Z

src/util.rs

+    use std::env;
+    use std::path::Path;
+
+    const CCACHE_DIRS: &[&str] = &["/usr/lib/ccache", "/usr/lib64/ccache"];


The CCACHE_DIRS list only contains Linux paths. Consider adding macOS Homebrew paths (/usr/local/opt/ccache/libexec, /opt/homebrew/opt/ccache/libexec) or adding a comment explaining why they're omitted

also, it would be nice to be able to configure things somewhere

We can generalize it to any path that contains a component ccache.

About configuration, is it still needed if we just filter out directories with ccache components?

sylvestre · 2026-01-22T13:02:58Z

src/util.rs

+/// This prevents double-caching when ccache is installed and has
+/// /usr/lib/ccache or /usr/lib64/ccache in PATH, since those directories
+/// contain wrapper scripts that would call ccache.
+pub fn filter_ccache_from_path(env_vars: Vec<(OsString, OsString)>) -> Vec<(OsString, OsString)> {


Function allocates a new Vec even when no ccache dirs are present in PATH. Consider short-circuiting to return the input Vec unchanged when filtering is not needed

sylvestre · 2026-01-22T13:26:04Z

src/compiler/compiler.rs

        } = self;
+        // Filter out ccache directories from PATH to avoid double-caching
+        // when ccache is also installed on the system.
+        let env_vars = filter_ccache_from_path(env_vars.to_vec());


Filter is only applied to SingleCompileCommand but NvccCompileCommand (in nvcc.rs) also implements CompileCommandImpl and may need the same treatment it is intentional ?

Not intentional, I'll take a look.

sylvestre · 2026-01-22T13:26:28Z

sorry for the latency

avikivity · 2026-01-22T15:37:55Z

v2:

filter out any directory with a ccache component, so it also works on MacOS
avoid allocations if no ccache directories are present
apply to detect_c_compiler
apply to nvcc

avikivity · 2026-01-22T15:40:02Z

v3:

apply rustfmt

On Linux, ccache is typically installed in $PATH as /usr/lib64/ccache/g++ or similar. If we keep it there, all compilations will be cached by both ccache and sccache. While the user could easily disable ccache with CCACHE_DISABLE, it's reasonable to assume many will forget and will have their disk space doubly consumed by both caches. Better to recognize this and disable ccache under sccache. This patch does this by removing the ccache binary paths from $PATH. The check is fuzzy - any directory that has a `ccache` component is removed. It's assumed that people won't have directories with such names in $PATH that are not ccache masquarade directories. Fixes mozilla#2519

avikivity · 2026-01-22T16:42:31Z

v4:

use slice instead of Vec

avikivity force-pushed the disable-ccache branch from ec91704 to 608d5c6 Compare December 20, 2025 20:59

sylvestre force-pushed the disable-ccache branch 2 times, most recently from 1fe7bb9 to 4bb5579 Compare January 13, 2026 15:33

sylvestre reviewed Jan 22, 2026

View reviewed changes

avikivity force-pushed the disable-ccache branch from 4bb5579 to 2a7f25c Compare January 22, 2026 15:36

avikivity force-pushed the disable-ccache branch from 2a7f25c to ecce16b Compare January 22, 2026 15:39

avikivity force-pushed the disable-ccache branch from ecce16b to ce9f95b Compare January 22, 2026 16:42

Avoid double-caching when ccache is installed in PATH #2524

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Avoid double-caching when ccache is installed in PATH #2524

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Merging this PR will degrade performance by 5.29%

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