Disallow mutation of values being iterated by a for loop or comprehension

This entails adding a read-locking mechanism to Mutability contexts.

RELNOTES[INC]: Updating list/dicts while they are being looped over is not allowed. Use an explicit copy if needed ("for x in list(mylist):").

--
MOS_MIGRATED_REVID=134442701

1 files changed, 12 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index 386a9aefeb..5b1834d868 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -369,6 +369,18 @@ public final class EvalUtils {
     }
   }
 
+  public static void lock(Object object, Location loc) {
+    if (object instanceof SkylarkMutable) {
+      ((SkylarkMutable) object).lock(loc);
+    }
+  }
+
+  public static void unlock(Object object, Location loc) {
+    if (object instanceof SkylarkMutable) {
+      ((SkylarkMutable) object).unlock(loc);
+    }
+  }
+
   private static ImmutableList<String> split(String value) {
     ImmutableList.Builder<String> builder = new ImmutableList.Builder<>();
     for (char c : value.toCharArray()) {