Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Wed, Mar 8, 2017 at 9:05 PM, Masahiko Sawada  wrote:
> On Wed, Feb 8, 2017 at 12:04 AM, Fujii Masao  wrote:
>> On Tue, Feb 7, 2017 at 2:13 AM, Petr Jelinek
>>  wrote:
>>> On 06/02/17 17:33, Fujii Masao wrote:
 On Sun, Feb 5, 2017 at 5:11 AM, Petr Jelinek
  wrote:
> On 03/02/17 19:38, Fujii Masao wrote:
>> On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  
>> wrote:
>>> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>>>  wrote:
 At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
 wrote in 
 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Masahiko Sawada]

On Wed, Feb 8, 2017 at 12:04 AM, Fujii Masao  wrote:
> On Tue, Feb 7, 2017 at 2:13 AM, Petr Jelinek
>  wrote:
>> On 06/02/17 17:33, Fujii Masao wrote:
>>> On Sun, Feb 5, 2017 at 5:11 AM, Petr Jelinek
>>>  wrote:
 On 03/02/17 19:38, Fujii Masao wrote:
> On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  
> wrote:
>> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>>  wrote:
>>> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
>>> wrote in 
>>> 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Robert Haas]

On Sat, Feb 4, 2017 at 3:11 PM, Petr Jelinek
 wrote:
> That was the reason why DropSubscription didn't release the lock in the
> first place. It was supposed to be released at the end of the
> transaction though.

Holding an LWLock until end-of-transaction is a phenomenally bad idea,
both because you lose interruptibility and because of the deadlock
risk.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Tue, Feb 7, 2017 at 2:13 AM, Petr Jelinek
 wrote:
> On 06/02/17 17:33, Fujii Masao wrote:
>> On Sun, Feb 5, 2017 at 5:11 AM, Petr Jelinek
>>  wrote:
>>> On 03/02/17 19:38, Fujii Masao wrote:
 On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  wrote:
> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>  wrote:
>> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
>> wrote in 
>> 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Petr Jelinek]

On 06/02/17 17:33, Fujii Masao wrote:
> On Sun, Feb 5, 2017 at 5:11 AM, Petr Jelinek
>  wrote:
>> On 03/02/17 19:38, Fujii Masao wrote:
>>> On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  wrote:
 On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
  wrote:
> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
> wrote in 
> 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Sun, Feb 5, 2017 at 5:11 AM, Petr Jelinek
 wrote:
> On 03/02/17 19:38, Fujii Masao wrote:
>> On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  wrote:
>>> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>>>  wrote:
 At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
 wrote in 
 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Petr Jelinek]

On 03/02/17 19:38, Fujii Masao wrote:
> On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  wrote:
>> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>>  wrote:
>>> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  
>>> wrote in 
>>> 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao  wrote:
> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
>  wrote:
>> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  wrote 
>> in 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
 wrote:
> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  wrote 
> in 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Kyotaro HORIGUCHI]

At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao  wrote in 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Thu, Feb 2, 2017 at 2:36 PM, Michael Paquier
 wrote:
> On Thu, Feb 2, 2017 at 2:13 PM, Tom Lane  wrote:
>> Kyotaro HORIGUCHI  writes:
>>> Then, the reason for the TRY-CATCH cluase is that I found that
>>> some functions called from there can throw exceptions.
>>
>> Yes, but all LWLocks should be released by normal error recovery.
>> It should not be necessary for this code to clean that up by hand.
>> If it were necessary, there would be TRY-CATCH around every single
>> LWLockAcquire in the backend, and we'd have an unreadable and
>> unmaintainable system.  Please don't add a TRY-CATCH unless it's
>> *necessary* -- and you haven't explained why this one is.

Yes.

> Putting hands into the code and at the problem, I can see that
> dropping a subscription on a node makes it unresponsive in case of a
> stop. And that's just because calls to LWLockRelease are missing as in
> the patch attached. A try/catch problem should not be necessary.

Thanks for the patch!

With the patch, LogicalRepLauncherLock is released at the end of
DropSubscription(). But ISTM that the lock should be released just after
logicalrep_worker_stop() and there is no need to protect the removal of
replication slot with the lock.

/*
* If we found worker but it does not have proc set it is starting up,
* wait for it to finish and then kill it.
*/
while (worker && !worker->proc)
{

ISTM that the above loop in logicalrep_worker_stop() is not necessary
because LogicalRepLauncherLock ensures that the above condition is
always false. Thought? Am I missing something?

If the above condition is true, which means that there is the worker slot
having the "subid" of the worker to kill, but its "proc" has not been set yet.
Without LogicalRepLauncherLock, this situation can happen after "subid"
is set by the launcher and before "proc" is set by the worker. But
LogicalRepLauncherLock protects those operations, so logicalrep_worker_stop()
called while holding the lock should always think the above condition is false.

Regards,

-- 
Fujii Masao


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Michael Paquier]

On Thu, Feb 2, 2017 at 2:13 PM, Tom Lane  wrote:
> Kyotaro HORIGUCHI  writes:
>> Then, the reason for the TRY-CATCH cluase is that I found that
>> some functions called from there can throw exceptions.
>
> Yes, but all LWLocks should be released by normal error recovery.
> It should not be necessary for this code to clean that up by hand.
> If it were necessary, there would be TRY-CATCH around every single
> LWLockAcquire in the backend, and we'd have an unreadable and
> unmaintainable system.  Please don't add a TRY-CATCH unless it's
> *necessary* -- and you haven't explained why this one is.

Putting hands into the code and at the problem, I can see that
dropping a subscription on a node makes it unresponsive in case of a
stop. And that's just because calls to LWLockRelease are missing as in
the patch attached. A try/catch problem should not be necessary.
-- 
Michael


drop-subs-locks.patch
Description: Binary data

-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Tom Lane]

Kyotaro HORIGUCHI  writes:
> Then, the reason for the TRY-CATCH cluase is that I found that
> some functions called from there can throw exceptions.

Yes, but all LWLocks should be released by normal error recovery.
It should not be necessary for this code to clean that up by hand.
If it were necessary, there would be TRY-CATCH around every single
LWLockAcquire in the backend, and we'd have an unreadable and
unmaintainable system.  Please don't add a TRY-CATCH unless it's
*necessary* -- and you haven't explained why this one is.

regards, tom lane


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Kyotaro HORIGUCHI]

At Thu, 2 Feb 2017 08:46:11 +0900, Michael Paquier  
wrote in 
> On Thu, Feb 2, 2017 at 2:14 AM, Fujii Masao  wrote:
> > The lwlock would be released when an exception occurs, so I don't think
> > that TRY-CATCH is necessary here. Or it's necessary for another reason?
> 
> +PG_CATCH();
> +{
> +LWLockRelease(LogicalRepLauncherLock);
> +PG_RE_THROW();
> +}
> +PG_END_TRY();
> Just to do that, a TRY/CATCH block looks like an overkill to me. Why
> not just call LWLockRelease in the ERROR and return code paths?

I though the same first. The modification at the "if (wrconn =="
is the remains of that. It is reverted inthe attached patch.

Then, the reason for the TRY-CATCH cluase is that I found that
some functions called from there can throw exceptions.

logicalrep_worker_stop and replorigin_drop have ereport in its path.
load_library apparently can throw exception.
(walrcv_(libpq_) functions don't seeem to.)

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
>From d0ca653bb2aa776742a2e7a697b02794b1ad66d9 Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi 
Date: Thu, 2 Feb 2017 11:33:40 +0900
Subject: [PATCH] Fix DROP SUBSCRIPTION's lock leak.

DROP SUBSCRIPTION acquires a lock on LogicalRepLauncherLock but never
releases. This fixes it.
---
 src/backend/commands/subscriptioncmds.c | 86 ++---
 1 file changed, 48 insertions(+), 38 deletions(-)

diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index 5de..223eea4 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -511,52 +511,62 @@ DropSubscription(DropSubscriptionStmt *stmt)
 	/* Protect against launcher restarting the worker. */
 	LWLockAcquire(LogicalRepLauncherLock, LW_EXCLUSIVE);
 
-	/* Kill the apply worker so that the slot becomes accessible. */
-	logicalrep_worker_stop(subid);
-
-	/* Remove the origin tracking if exists. */
-	snprintf(originname, sizeof(originname), "pg_%u", subid);
-	originid = replorigin_by_name(originname, true);
-	if (originid != InvalidRepOriginId)
-		replorigin_drop(originid);
-
-	/* If the user asked to not drop the slot, we are done mow.*/
-	if (!stmt->drop_slot)
-	{
-		heap_close(rel, NoLock);
-		return;
-	}
-
 	/*
-	 * Otherwise drop the replication slot at the publisher node using
-	 * the replication connection.
+	 * Some functions called here can throw exceptions. we must release
+	 * LogicalRepLauncherLock for the case.
 	 */
-	load_file("libpqwalreceiver", false);
+	PG_TRY();
+	{
+		/* Kill the apply worker so that the slot becomes accessible. */
+		logicalrep_worker_stop(subid);
 
-	initStringInfo();
-	appendStringInfo(, "DROP_REPLICATION_SLOT \"%s\"", slotname);
+		/* Remove the origin tracking if exists. */
+		snprintf(originname, sizeof(originname), "pg_%u", subid);
+		originid = replorigin_by_name(originname, true);
+		if (originid != InvalidRepOriginId)
+			replorigin_drop(originid);
 
-	wrconn = walrcv_connect(conninfo, true, subname, );
-	if (wrconn == NULL)
-		ereport(ERROR,
-(errmsg("could not connect to publisher when attempting to "
-		"drop the replication slot \"%s\"", slotname),
- errdetail("The error was: %s", err)));
+		/* Do the follwoing only if the user asked to actually drop the slot */
+		if (stmt->drop_slot)
+		{
+			/*
+			 * Drop the replication slot at the publisher node using the
+			 * replication connection.
+			 */
+			load_file("libpqwalreceiver", false);
 
-	if (!walrcv_command(wrconn, cmd.data, ))
-		ereport(ERROR,
-(errmsg("could not drop the replication slot \"%s\" on publisher",
-		slotname),
- errdetail("The error was: %s", err)));
-	else
-		ereport(NOTICE,
-(errmsg("dropped replication slot \"%s\" on publisher",
-		slotname)));
+			initStringInfo();
+			appendStringInfo(, "DROP_REPLICATION_SLOT \"%s\"", slotname);
+
+			wrconn = walrcv_connect(conninfo, true, subname, );
+			if (wrconn == NULL)
+ereport(ERROR,
+		(errmsg("could not connect to publisher when attempting to "
+"drop the replication slot \"%s\"", slotname),
+		 errdetail("The error was: %s", err)));
 
-	walrcv_disconnect(wrconn);
+			else if (!walrcv_command(wrconn, cmd.data, ))
+ereport(ERROR,
+		(errmsg("could not drop the replication slot \"%s\" on publisher",
+slotname),
+		 errdetail("The error was: %s", err)));
+
+			ereport(NOTICE,
+	(errmsg("dropped replication slot \"%s\" on publisher",
+			slotname)));
 
-	pfree(cmd.data);
+			walrcv_disconnect(wrconn);
+			pfree(cmd.data);
+		}
+	}
+	PG_CATCH();
+	{
+		LWLockRelease(LogicalRepLauncherLock);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
 
+	LWLockRelease(LogicalRepLauncherLock);
 	heap_close(rel, NoLock);
 }
 
-- 
2.9.2


-- 
Sent via pgsql-hackers mailing list 

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Michael Paquier]

On Thu, Feb 2, 2017 at 2:14 AM, Fujii Masao  wrote:
> The lwlock would be released when an exception occurs, so I don't think
> that TRY-CATCH is necessary here. Or it's necessary for another reason?

+PG_CATCH();
+{
+LWLockRelease(LogicalRepLauncherLock);
+PG_RE_THROW();
+}
+PG_END_TRY();
Just to do that, a TRY/CATCH block looks like an overkill to me. Why
not just call LWLockRelease in the ERROR and return code paths?
-- 
Michael


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Fujii Masao]

On Wed, Feb 1, 2017 at 5:36 PM, Kyotaro HORIGUCHI
 wrote:
> Hello, while looking another bug, I found that standby cannot
> shutdown after DROP SUBSCRIPTION.
>
> standby=# CREATE SUBSCRPTION sub1 ...
> standby=# 
> standby=# DROP SUBSCRIPTION sub1;
>
> Ctrl-C to the standby fails to work. ApplyLauncherMain is waiting
> LogicalRepLauncherLock forever.
>
> The culprit is DropSbuscription. It acquires
> LogicalRepLauncherLock but never releases.
>
> The attached patch fixes it. Most part of the fucntion is now
> enclosed by PG_TRY-CATCH since some functions can throw
> exceptions.

The lwlock would be released when an exception occurs, so I don't think
that TRY-CATCH is necessary here. Or it's necessary for another reason?

Regards,

-- 
Fujii Masao


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

[HACKERS] Cannot shutdown subscriber after DROP SUBSCRIPTION

[Kyotaro HORIGUCHI]

Hello, while looking another bug, I found that standby cannot
shutdown after DROP SUBSCRIPTION.

standby=# CREATE SUBSCRPTION sub1 ...
standby=# 
standby=# DROP SUBSCRIPTION sub1;

Ctrl-C to the standby fails to work. ApplyLauncherMain is waiting
LogicalRepLauncherLock forever.

The culprit is DropSbuscription. It acquires
LogicalRepLauncherLock but never releases.

The attached patch fixes it. Most part of the fucntion is now
enclosed by PG_TRY-CATCH since some functions can throw
exceptions.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index 5de..f07143e 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -511,51 +511,67 @@ DropSubscription(DropSubscriptionStmt *stmt)
 	/* Protect against launcher restarting the worker. */
 	LWLockAcquire(LogicalRepLauncherLock, LW_EXCLUSIVE);
 
-	/* Kill the apply worker so that the slot becomes accessible. */
-	logicalrep_worker_stop(subid);
-
-	/* Remove the origin tracking if exists. */
-	snprintf(originname, sizeof(originname), "pg_%u", subid);
-	originid = replorigin_by_name(originname, true);
-	if (originid != InvalidRepOriginId)
-		replorigin_drop(originid);
-
-	/* If the user asked to not drop the slot, we are done mow.*/
-	if (!stmt->drop_slot)
-	{
-		heap_close(rel, NoLock);
-		return;
-	}
-
 	/*
-	 * Otherwise drop the replication slot at the publisher node using
-	 * the replication connection.
+	 * replorigin_drop can throw an exception. we must release
+	 * LogicalRepLauncherLock for the case.
 	 */
-	load_file("libpqwalreceiver", false);
+	PG_TRY();
+	{
+		/* Kill the apply worker so that the slot becomes accessible. */
+		logicalrep_worker_stop(subid);
 
-	initStringInfo();
-	appendStringInfo(, "DROP_REPLICATION_SLOT \"%s\"", slotname);
+		/* Remove the origin tracking if exists. */
+		snprintf(originname, sizeof(originname), "pg_%u", subid);
+		originid = replorigin_by_name(originname, true);
+		if (originid != InvalidRepOriginId)
+			replorigin_drop(originid);
 
-	wrconn = walrcv_connect(conninfo, true, subname, );
-	if (wrconn == NULL)
-		ereport(ERROR,
-(errmsg("could not connect to publisher when attempting to "
-		"drop the replication slot \"%s\"", slotname),
- errdetail("The error was: %s", err)));
+		/* If the user asked to not drop the slot, we are done now.*/
+		if (stmt->drop_slot)
+		{
+			/*
+			 * Otherwise drop the replication slot at the publisher node using
+			 * the replication connection.
+			 */
+			load_file("libpqwalreceiver", false);
+
+			initStringInfo();
+			appendStringInfo(, "DROP_REPLICATION_SLOT \"%s\"", slotname);
+
+			wrconn = walrcv_connect(conninfo, true, subname, );
+			if (wrconn == NULL || !walrcv_command(wrconn, cmd.data, ))
+			{
+if (wrconn == NULL)
+	ereport(ERROR,
+			(errmsg("could not connect to publisher when "
+	"attempting to drop the "
+	"replication slot \"%s\"", slotname),
+			 errdetail("The error was: %s", err)));
 
-	if (!walrcv_command(wrconn, cmd.data, ))
-		ereport(ERROR,
-(errmsg("could not drop the replication slot \"%s\" on publisher",
-		slotname),
- errdetail("The error was: %s", err)));
-	else
-		ereport(NOTICE,
-(errmsg("dropped replication slot \"%s\" on publisher",
-		slotname)));
+ereport(ERROR,
+		(errmsg("could not drop the replication slot \"%s\" on publisher",
+slotname),
+		 errdetail("The error was: %s", err)));
+			}
 
-	walrcv_disconnect(wrconn);
+			ereport(NOTICE,
+	(errmsg("dropped replication slot \"%s\" on publisher",
+			slotname)));
 
-	pfree(cmd.data);
+			pfree(cmd.data);
+		}
+	}
+	PG_CATCH();
+	{
+		LWLockRelease(LogicalRepLauncherLock);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	LWLockRelease(LogicalRepLauncherLock);
+
+	if (wrconn)
+		walrcv_disconnect(wrconn);
 
 	heap_close(rel, NoLock);
 }

-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers