From 33e1e77cf1726548f4303a611779adb43aef1759 Mon Sep 17 00:00:00 2001
From: Shawn Debnath <sdn@amazon.com>
Date: Tue, 12 Mar 2019 23:21:41 +0000
Subject: [PATCH 1/2] Introduce timed waits for condition variables.

Provide ConditionVariableTimedSleep(), like ConditionVariableSleep()
but with a timeout argument.

Author: Shawn Debnath
Reviewed-by: Kyotaro Horiguchi, Thomas Munro
Discussion: https://postgr.es/m/eeb06007ccfe46e399df6af18bfcd15a@EX13D05UWC002.ant.amazon.com
---
 src/backend/storage/lmgr/condition_variable.c | 68 ++++++++++++++++---
 src/include/storage/condition_variable.h      |  2 +
 2 files changed, 62 insertions(+), 8 deletions(-)

diff --git a/src/backend/storage/lmgr/condition_variable.c b/src/backend/storage/lmgr/condition_variable.c
index 58b7b51472..528d865267 100644
--- a/src/backend/storage/lmgr/condition_variable.c
+++ b/src/backend/storage/lmgr/condition_variable.c
@@ -19,6 +19,7 @@
 #include "postgres.h"
 
 #include "miscadmin.h"
+#include "portability/instr_time.h"
 #include "storage/condition_variable.h"
 #include "storage/ipc.h"
 #include "storage/proc.h"
@@ -122,8 +123,24 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
 void
 ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
 {
-	WaitEvent	event;
-	bool		done = false;
+	(void) ConditionVariableTimedSleep(cv, -1 /* no timeout */ ,
+									   wait_event_info);
+}
+
+/*
+ * Wait for a condition variable to be signaled or a timeout to be reached.
+ *
+ * Returns true when timeout expires, otherwise returns false.
+ *
+ * See ConditionVariableSleep() for general usage.
+ */
+bool
+ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
+							uint32 wait_event_info)
+{
+	long		cur_timeout = -1;
+	instr_time	start_time;
+	instr_time	cur_time;
 
 	/*
 	 * If the caller didn't prepare to sleep explicitly, then do so now and
@@ -143,23 +160,42 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
 	if (cv_sleep_target != cv)
 	{
 		ConditionVariablePrepareToSleep(cv);
-		return;
+		return false;
 	}
 
-	do
+	/*
+	 * Record the current time so that we can calculate the remaining timeout
+	 * if we are woken up spuriously.
+	 */
+	if (timeout >= 0)
 	{
-		CHECK_FOR_INTERRUPTS();
+		INSTR_TIME_SET_CURRENT(start_time);
+		Assert(timeout >= 0 && timeout <= INT_MAX);
+		cur_timeout = timeout;
+	}
+
+	while (true)
+	{
+		WaitEvent	event;
+		bool		done = false;
+		int			rc;
 
 		/*
 		 * Wait for latch to be set.  (If we're awakened for some other
 		 * reason, the code below will cope anyway.)
 		 */
-		(void) WaitEventSetWait(cv_wait_event_set, -1, &event, 1,
-								wait_event_info);
+		rc = WaitEventSetWait(cv_wait_event_set, cur_timeout, &event, 1,
+							  wait_event_info);
 
 		/* Reset latch before examining the state of the wait list. */
 		ResetLatch(MyLatch);
 
+		CHECK_FOR_INTERRUPTS();
+
+		/* Timed out */
+		if (rc == 0)
+			return true;
+
 		/*
 		 * If this process has been taken out of the wait list, then we know
 		 * that it has been signaled by ConditionVariableSignal (or
@@ -182,7 +218,23 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
 			proclist_push_tail(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
 		}
 		SpinLockRelease(&cv->mutex);
-	} while (!done);
+
+		/* We were signaled, so return */
+		if (done)
+			return false;
+
+		/* If we're not done, update cur_timeout for next iteration */
+		if (timeout >= 0)
+		{
+			INSTR_TIME_SET_CURRENT(cur_time);
+			INSTR_TIME_SUBTRACT(cur_time, start_time);
+			cur_timeout = timeout - (long) INSTR_TIME_GET_MILLISEC(cur_time);
+
+			/* Have we crossed the timeout threshold? */
+			if (cur_timeout <= 0)
+				return true;
+		}
+	}
 }
 
 /*
diff --git a/src/include/storage/condition_variable.h b/src/include/storage/condition_variable.h
index 2a0249392c..ee06e051ce 100644
--- a/src/include/storage/condition_variable.h
+++ b/src/include/storage/condition_variable.h
@@ -43,6 +43,8 @@ extern void ConditionVariableInit(ConditionVariable *cv);
  * the condition variable.
  */
 extern void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info);
+extern bool ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
+										uint32 wait_event_info);
 extern void ConditionVariableCancelSleep(void);
 
 /*
-- 
2.21.0