Use long instead of int for thread indices

This protects against an integer overflow which could occur for large key list size and large thread counts.

Regrettably, it's difficult to write a regression test for this scenario, as exercising this overflow requires lots of time and heap, so it would be a performance regression to our test suites.

Fixes #19445

PiperOrigin-RevId: 595420516
Change-Id: Ic0a475a6a273c50fe9895dd0852fa5b062859cb2

src/main/java/com/google/devtools/build/skyframe/InvalidatingNodeVisitor.java

@@ -274,18 +274,18 @@ protected void runInternal(ImmutableList<Pair<SkyKey, InvalidationType>> pending
               MIN_TIME_FOR_LOGGING)) {
         // To avoid contention and scheduling too many jobs for our #cpus, we start
         // DEFAULT_THREAD_COUNT jobs, each processing a chunk of the pending visitations.
-        int listSize = pendingList.size();
-        int numThreads = min(DEFAULT_THREAD_COUNT, listSize);
-        for (int i = 0; i < numThreads; i++) {
-          int index = i;
+        long listSize = pendingList.size();
+        long numThreads = min(DEFAULT_THREAD_COUNT, listSize);
+        for (long i = 0; i < numThreads; i++) {
+          // Use long multiplication to avoid possible overflow, as numThreads * listSize might be
+          // larger than max int.
+          int startIndex = (int) ((i * listSize) / numThreads);
+          int endIndex = (int) (((i + 1) * listSize) / numThreads);
           executor.execute(
               () ->
                   visit(
                       Collections2.transform(
-                          pendingList.subList(
-                              (index * listSize) / numThreads,
-                              ((index + 1) * listSize) / numThreads),
-                          Pair::getFirst),
+                          pendingList.subList(startIndex, endIndex), Pair::getFirst),
                       InvalidationType.DELETED));
         }
       }