[GR-14882] Fix race condition with interpolate-once regular expressions.

PullRequest: truffleruby/765

Author: nirvdrum

CHANGELOG.md

@@ -7,6 +7,8 @@ Bug fixes:
   encoding and has non-ASCII characters.
 * Fixed `puts` for strings with non-ASCII-compatible encodings.
 * `rb_protect` now returns `Qnil` when an error occurs.
+* Fixed a race condition when using the interpolate-once (`/o`) modifier in
+  regular expressions.
 
 New features:
 

src/main/java/org/truffleruby/language/control/OnceNode.java

@@ -35,7 +35,11 @@ public Object execute(VirtualFrame frame) {
         if (value == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
             synchronized (this) {
-                if (cachedValue == null) {
+                // Read `cachedValue` again to check if the value was updated by another thread while this thread
+                // was waiting on the lock. If it's still null, this thread is the first one to get the lock and
+                // must update the cache.
+                value = cachedValue;
+                if (value == null) {
                     value = cachedValue = child.execute(frame);
                     assert value != null;
                 }