[#22010] YSQL: Failed txn restarts with UPDATE ... RETURNING queries

Summary:
### Issue

UPDATE ... REUTRNING queries fail txn restarts due to conflicts.

### Root Cause

yb_restart_portal does not reset the value of portal->holdContext to NULL.

This in turn fails the assertion in PortalCreateHoldStore.

### Impact

Only affects debug builds.

Backport-through: 2024.1
Jira: DB-10929

Test Plan:
Jenkins

yb_build.sh --java-test TestPgTransparentRestarts#testUpdateLong

Reviewers: pjain, smishra

Reviewed By: pjain

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D34242

java/yb-pgsql/src/test/java/org/yb/pgsql/TestPgTransparentRestarts.java

@@ -496,6 +496,23 @@ public PreparedStatement createStatement(Connection conn) throws Exception {
     }
   }
 
+  /*
+   * Regression test for #22010
+   *
+   * We do not expect read restarts even in the long string case
+   * since the RETUNRING output is cached in a temporary table
+   * and only sent back to the client after the update is done.
+   */
+  @Test
+  public void testUpdateLong() throws Exception {
+    new RegularDmlStatementTester(
+        getConnectionBuilder(),
+        "UPDATE test_rr set i=1 RETURNING *",
+        getLongString(),
+        false /* is_deadlock_possible */
+    ).runTest();
+  }
+
   //
   // Helpers methods
   //

src/postgres/src/backend/tcop/postgres.c

@@ -4477,7 +4477,10 @@ yb_restart_portal(const char* portal_name)
 
 	/* delete tuplestore storage, if any */
 	if (portal->holdContext)
+	{
 		MemoryContextDelete(portal->holdContext);
+		portal->holdContext = NULL;
+	}
 
 	/* the portal run context might not have been reset, so do it now */
 	MemoryContextReset(portal->ybRunContext);