Add yields to hash join iterator

Summary: Queries performing joins are lacking yields and monopolizing thread pool scheduler.

Test Plan:
This fix was confirmed with a manual validation.
For the regression tests, instead of adding tests for every possible query and command, we'll soon add a generic test during regular thread pool runs performed by the scheduler monitor, that every scheduler is progressing. If the current worker on the scheduler doesn't yield for a long time (e.g. 5 seconds) then the diagnostic callstack of the offending query will be printed to the errorlog and will help to identify and instrument code with missing yields.

Reviewers: sunxiayi, prerit, #mysql_mt

Reviewed By: prerit

Subscribers: pgl, webscalesql-eng@fb.com

Differential Revision: https://phabricator.intern.facebook.com/D39948376

Tasks: T133239442

sql/iterators/hash_join_iterator.cc

@@ -323,6 +323,12 @@ static bool WriteRowToChunk(
   }
 }
 
+// Helper to check if yielding is OK for this query.
+static bool YieldCondition() {
+  // Allow yielding in all cases.
+  return true;
+}
+
 // Write all the remaining rows from the given iterator out to chunk files
 // on disk. If the function returns true, an unrecoverable error occurred
 // (IO error etc.).
@@ -340,6 +346,8 @@ static bool WriteRowsToChunks(
       return true;
     }
 
+    thd->check_yield(YieldCondition);
+
     if (res == -1) {
       return false;  // EOF; success.
     }
@@ -459,6 +467,8 @@ bool HashJoinIterator::BuildHashTable() {
       return true;
     }
 
+    thd()->check_yield(YieldCondition);
+
     if (res == -1) {
       m_build_iterator_has_more_rows = false;
       // If the build input was empty, the result of inner joins and semijoins