Fix a source of contention in LegacyIncludeScanner.lookup(). This is another
instance where CompactHashMap's computeIfAbsent() is used with a computation
that is not short and simple, leading to contention.
RELNOTES: None.
PiperOrigin-RevId: 346276142
diff --git a/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java b/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
index 9a689db..c6239dd 100644
--- a/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
+++ b/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
@@ -353,8 +353,16 @@
@Override
public LocateOnPathResult lookup(
InclusionWithContext inclusion, Map<PathFragment, Artifact> pathToDeclaredHeader) {
- LocateOnPathResult result =
- cache.computeIfAbsent(inclusion, key -> locateOnPaths(key, pathToDeclaredHeader, false));
+ LocateOnPathResult result = cache.get(inclusion);
+ if (result == null) {
+ // Do not use computeIfAbsent() as the implementation of locateOnPaths might do multiple
+ // file stats and this creates substantial contention given CompactHashMap's locking.
+ // Do not use futures as the few duplicate executions are cheaper than the additional memory
+ // that would be required.
+ result = locateOnPaths(inclusion, pathToDeclaredHeader, false);
+ cache.put(inclusion, result);
+ return result;
+ }
// If the previous computation for this inclusion had a different pathToDeclaredHeader
// map, result may not be valid for this lookup. Because this is a hot spot, we tolerate a
// known correctness bug but try to catch most issues.
