Auto merge of #18853 - asajeffrey:constellation-chaos-monkey-dont-kill-pending-pipelines, r=jdm

Random pipeline closure is less likely to kill pending pipelines.

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Make it less likely for pending pipelines to be randomly killed. Pending pipelines send more messages to the constellation, so at the moment we're skewing to killing pipelines early in their lifetimes.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #18852
- [X] These changes do not require tests because it's fixing test infrastructure

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components/constellation/constellation.rs

@@ -2902,12 +2902,21 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // In order to get repeatability, we sort the pipeline ids.
         let mut pipeline_ids: Vec<&PipelineId> = self.pipelines.keys().collect();
         pipeline_ids.sort();
-        if let Some((ref mut rng, _)) = self.random_pipeline_closure {
+        if let Some((ref mut rng, probability)) = self.random_pipeline_closure {
             if let Some(pipeline_id) = rng.choose(&*pipeline_ids) {
                 if let Some(pipeline) = self.pipelines.get(pipeline_id) {
                     // Don't kill the mozbrowser pipeline
                     if PREFS.is_mozbrowser_enabled() && pipeline.parent_info.is_none() {
                         info!("Not closing mozbrowser pipeline {}.", pipeline_id);
+                    } else if
+                        self.pending_changes.iter().any(|change| change.new_pipeline_id == pipeline.id) &&
+                        probability <= rng.gen::<f32>()
+                    {
+                        // We tend not to close pending pipelines, as that almost always
+                        // results in pipelines being closed early in their lifecycle,
+                        // and not stressing the constellation as much.
+                        // https://github.com/servo/servo/issues/18852
+                        info!("Not closing pending pipeline {}.", pipeline_id);
                     } else {
                         // Note that we deliberately do not do any of the tidying up
                         // associated with closing a pipeline. The constellation should cope!