Multiple build timeout by n_jobs

NEWS.md

@@ -1,5 +1,9 @@
 # cargo-mutants changelog
 
+## Unreleased
+
+- Fixed: The auto-set timeout for building mutants is now 2 times the baseline build time times the number of jobs, with a minimum of 20 seconds. This was changed because builds of mutants contend with each other for access to CPUs and may be slower than the baseline build.
+
 ## 24.5.0
 
 - Fixed: Follow `path` attributes on `mod` statements.

book/src/timeouts.md

@@ -24,9 +24,11 @@ build and test the unmodified tree (baseline).
 
 The default timeouts are:
 
-- `cargo build`/`cargo check`: 2 times the baseline build time (with a minimum of 20 seconds)
+- `cargo build`/`cargo check`: 2 times the baseline build time times the number of jobs (with a minimum of 20 seconds)
 - `cargo test`: 5 times baseline test time (with a minimum of 20 seconds)
 
+The build timeout scales with the number of jobs to reflect that cargo often spawns many jobs, and so builds run in parallel are likely to take longer than the baseline, which has no external parallelism.
+
 The minimum of 20 seconds for the test timeout can be overridden by the
 `--minimum-test-timeout` option or the `CARGO_MUTANTS_MINIMUM_TEST_TIMEOUT`
 environment variable, measured in seconds.

src/lab.rs

@@ -85,6 +85,7 @@ pub fn test_mutants(
         }
         BaselineStrategy::Skip => Timeouts::without_baseline(&options),
     };
+    debug!(?timeouts);
     let mut build_dirs = vec![build_dir];
     let jobs = max(1, min(options.jobs.unwrap_or(1), mutants.len()));
     for i in 1..jobs {

src/timeouts.rs

@@ -62,7 +62,6 @@ fn phase_timeout(
     if let Some(timeout) = explicit_timeout {
         return timeout;
     }
-
     match baseline_duration {
         Some(_) if options.in_place && phase != Phase::Test => {
             warn_fallback_timeout(phase.name(), "--in-place");
@@ -73,7 +72,6 @@ fn phase_timeout(
                 minimum,
                 Duration::from_secs((baseline_duration.as_secs_f64() * multiplier).ceil() as u64),
             );
-
             if options.show_times {
                 info!(
                     "Auto-set {} timeout to {}",
@@ -107,7 +105,9 @@ fn build_timeout(baseline_duration: Option<Duration>, options: &Options) -> Dura
         options.build_timeout,
         baseline_duration,
         Duration::from_secs(20),
-        options.build_timeout_multiplier.unwrap_or(2.0),
+        options
+            .build_timeout_multiplier
+            .unwrap_or(2.0 * options.jobs.unwrap_or(1) as f64),
         options,
     )
 }
@@ -157,6 +157,16 @@ mod test {
         );
     }
 
+    #[test]
+    fn mutant_build_timeout_with_multiple_jobs() {
+        let args = Args::parse_from(["mutants", "--jobs=4"]);
+        let options = Options::new(&args, &Config::default()).unwrap();
+        assert_eq!(
+            build_timeout(Some(Duration::from_secs(40)), &options),
+            Duration::from_secs(40 * 2 * 4),
+        );
+    }
+
     #[test]
     fn build_timeout_is_affected_by_in_place_build() {
         let args = Args::parse_from(["mutants", "--build-timeout-multiplier", "1.5", "--in-place"]);