btf: fix data race in (*mutableTypes).copy

Move the creation of the mutableTypes copy under the read lock, so that
the access to the len of the source types is protected when
pre-allocating the maps.

Signed-off-by: Paul Cacheux <[email protected]>

btf/btf.go

@@ -99,17 +99,17 @@ func (mt *mutableTypes) copy() *mutableTypes {
 		return nil
 	}
 
+	// Prevent concurrent modification of mt.copiedTypeIDs.
+	mt.mu.RLock()
+	defer mt.mu.RUnlock()
+
 	mtCopy := &mutableTypes{
 		mt.imm,
 		sync.RWMutex{},
 		make(map[Type]Type, len(mt.copies)),
 		make(map[Type]TypeID, len(mt.copiedTypeIDs)),
 	}
 
-	// Prevent concurrent modification of mt.copiedTypeIDs.
-	mt.mu.RLock()
-	defer mt.mu.RUnlock()
-
 	copiesOfCopies := make(map[Type]Type, len(mt.copies))
 	for orig, copy := range mt.copies {
 		// NB: We make a copy of copy, not orig, so that changes to mutable types