There is a problem in PL/TCL that can cause the postgres backend to
become multithreaded. Postgres is not designed to be multithreaded, so
this causes downstream errors in signal handling. We have seen this
cause a number of "unexpected state" errors associated with notification
handling; however, unpredictable signal handling would be likely to
cause other errors as well.

Some sample scripts are attached which will reproduce this problem when
running against a multithreaded version of TCL, but will work without
error with single-threaded TCL library. The scripts are a combination
of Unix shell, perl DBI, and SQL commands. The postgres process can be
seen to have multiple threads using the Linux command "ps -Lwfu
postgres". In this command the NLWP columns will be 2 for multithreaded
backend processes. The threaded/non-threaded state of the TCL library
can be ascertained on Linux using ldd to determine if libpthread.so is
linked to the TCL library (e.g. "ldd /usr/lib/libtcl8.4.so").

The multithreaded behavior occurs the first time PL/TCL is used in a
postgres backend, but only when postgres is linked against a
multithread-enabled version of libtcl. Thus, this problem can be
side-stepped by linking against the proper TCL library. However
multithreaded TCL libraries are becoming the norm in Linux distributions
and seems ubiquitous in the Windows world. Therefore a fix to the
PL/TCL code is warrented.

We determined that postgres became multithreaded during the creation of
the TCL interpreter in a function called tcl_InitNotifier. This
function is part of TCL's Notifier subsystem, which is used to monitor
for events asynchronously from the TCL event loop. Although initialized
when an interpreter is created, the Notifier subsystem is not used until
a process enters the TCL event loop. This never happens within a
postgres process, because postgres implements its own event loop.
Therefore the initialization of the Notifier subsystem is not necessary
within the context of PL/TCL.

Our solution was to disable the Notifier subsystem by overriding the
functions associated with it using the Tcl_SetNotifier function. This
allows 8 functions related to the Notifier to overriden. Even though we
found only two of the functions were ever called within postgres, we
overrode 8 functions with no-op versions, just for completeness. A
patch file containing the changes to pltcl.c from its 8.2.4 version is
also attached.

We tested this patch with PostgreSQL 8.2.4 on both RedHat Enterprise 4.0
usingTCL 8.4 (single threaded) and RHE 5.0 using TCL 8.4.13
(multithreaded). We expect this solution to work with Windows as well,
although we have not tested it. There may be some problems using this
solution with old versions of TCL that pre-date the Tcl_SetNotifier
function. However this function has been around for quite a while; it
was added in in the TCL 8.2 release, circa 2000.

We hope this patch will be considered for a future PostgreSQL release.

Steve Marshall
Paul Bayer
Doug Knight
WSI Corporation



*** pltcl.c.orig 2007-09-10 12:58:34.000000000 -0400
--- pltcl.c 2007-09-11 11:37:33.363222114 -0400
***************
*** 163,168 ****
--- 163,258 ----
static void pltcl_build_tuple_argument(HeapTuple tuple, TupleDesc tupdesc,
Tcl_DString *retval);

+ /************************************************** ********************
+ * Declarations for functions overriden using Tcl_SetNotifier.
+ ************************************************** ********************/
+ static int fakeThreadKey; /* To give valid address for ClientData */
+
+ static ClientData
+ pltcl_InitNotifier _ANSI_ARGS_((void));
+
+ static void
+ pltcl_FinalizeNotifier _ANSI_ARGS_((ClientData clientData));
+
+ static void
+ pltcl_SetTimer _ANSI_ARGS_((Tcl_Time *timePtr));
+
+ static void
+ pltcl_AlertNotifier _ANSI_ARGS_((ClientData clientData));
+
+ static void
+ pltcl_CreateFileHandler _ANSI_ARGS_((int fd, int mask, Tcl_FileProc *proc, ClientData clientData));
+
+ static void
+ pltcl_DeleteFileHandler _ANSI_ARGS_((int fd));
+
+ static void
+ pltcl_ServiceModeHook _ANSI_ARGS_((int mode));
+
+ static int
+ pltcl_WaitForEvent _ANSI_ARGS_((Tcl_Time *timePtr));
+
+ /************************************************** ********************
+ * Definitions for functions overriden using Tcl_SetNotifier.
+ * These implementations effectively disable the TCL Notifier subsystem.
+ * This is okay because we never enter the TCL event loop from postgres,
+ * so the notifier capabilities are initialized, but never used.
+ *
+ * NOTE: Only InitNotifier and DeleteFileHandler ever seem to get called
+ * by postgres, but we implement all the functions for completeness.
+ ************************************************** ********************/
+
+ ClientData
+ pltcl_InitNotifier()
+ {
+ return (ClientData) &(fakeThreadKey);
+ }
+
+ void
+ pltcl_FinalizeNotifier(clientData)
+ ClientData clientData; /* Not used. */
+ {
+ }
+
+ void
+ pltcl_SetTimer(timePtr)
+ Tcl_Time *timePtr;
+ {
+ }
+
+ void
+ pltcl_AlertNotifier(clientData)
+ ClientData clientData;
+ {
+ }
+
+ void
+ pltcl_CreateFileHandler(fd, mask, proc, clientData)
+ int fd;
+ int mask;
+ Tcl_FileProc *proc;
+ ClientData clientData;
+ {
+ }
+
+ void
+ pltcl_DeleteFileHandler(fd)
+ int fd;
+ {
+ }
+
+ void
+ pltcl_ServiceModeHook(mode)
+ int mode;
+ {
+ }
+
+ int
+ pltcl_WaitForEvent(timePtr)
+ Tcl_Time *timePtr; /* Maximum block time, or NULL. */
+ {
+ return 0;
+ }

/*
* This routine is a crock, and so is everyplace that calls it. The problem
***************
*** 189,194 ****
--- 279,287 ----
void
_PG_init(void)
{
+ /* Notifier structure used to override functions in Notifier subsystem*/
+ Tcl_NotifierProcs notifier;
+
/* Be sure we do initialization only once (should be redundant now) */
if (pltcl_pm_init_done)
return;
***************
*** 199,204 ****
--- 292,316 ----
#endif

/************************************************** **********
+ * Override the functions in the Notifier subsystem.
+ *
+ * We do this to prevent the postgres backend from becoming
+ * multithreaded, which happens in the default version of
+ * Tcl_InitNotifier if the TCL library has been compiled with
+ * multithreading support (i.e. when TCL_THREADS is defined
+ * under Unix, and in all cases under Windows).
+ ************************************************** **********/
+ notifier.setTimerProc = pltcl_SetTimer;
+ notifier.waitForEventProc = pltcl_WaitForEvent;
+ notifier.createFileHandlerProc = pltcl_CreateFileHandler;
+ notifier.deleteFileHandlerProc = pltcl_DeleteFileHandler;
+ notifier.initNotifierProc = pltcl_InitNotifier;
+ notifier.finalizeNotifierProc = pltcl_FinalizeNotifier;
+ notifier.alertNotifierProc = pltcl_AlertNotifier;
+ notifier.serviceModeHookProc = pltcl_ServiceModeHook;
+ Tcl_SetNotifier(&notifier);
+
+ /************************************************** **********
* Create the dummy hold interpreter to prevent close of
* stdout and stderr on DeleteInterp
************************************************** **********/


---------------------------(end of broadcast)---------------------------
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This has been saved for the 8.4 release:

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---------------------------------------------------------------------------

Marshall, Steve wrote:
> There is a problem in PL/TCL that can cause the postgres backend to
> become multithreaded. Postgres is not designed to be multithreaded, so
> this causes downstream errors in signal handling. We have seen this
> cause a number of "unexpected state" errors associated with notification
> handling; however, unpredictable signal handling would be likely to
> cause other errors as well.
>
> Some sample scripts are attached which will reproduce this problem when
> running against a multithreaded version of TCL, but will work without
> error with single-threaded TCL library. The scripts are a combination
> of Unix shell, perl DBI, and SQL commands. The postgres process can be
> seen to have multiple threads using the Linux command "ps -Lwfu
> postgres". In this command the NLWP columns will be 2 for multithreaded
> backend processes. The threaded/non-threaded state of the TCL library
> can be ascertained on Linux using ldd to determine if libpthread.so is
> linked to the TCL library (e.g. "ldd /usr/lib/libtcl8.4.so").
>
> The multithreaded behavior occurs the first time PL/TCL is used in a
> postgres backend, but only when postgres is linked against a
> multithread-enabled version of libtcl. Thus, this problem can be
> side-stepped by linking against the proper TCL library. However
> multithreaded TCL libraries are becoming the norm in Linux distributions
> and seems ubiquitous in the Windows world. Therefore a fix to the
> PL/TCL code is warrented.
>
> We determined that postgres became multithreaded during the creation of
> the TCL interpreter in a function called tcl_InitNotifier. This
> function is part of TCL's Notifier subsystem, which is used to monitor
> for events asynchronously from the TCL event loop. Although initialized
> when an interpreter is created, the Notifier subsystem is not used until
> a process enters the TCL event loop. This never happens within a
> postgres process, because postgres implements its own event loop.
> Therefore the initialization of the Notifier subsystem is not necessary
> within the context of PL/TCL.
>
> Our solution was to disable the Notifier subsystem by overriding the
> functions associated with it using the Tcl_SetNotifier function. This
> allows 8 functions related to the Notifier to overriden. Even though we
> found only two of the functions were ever called within postgres, we
> overrode 8 functions with no-op versions, just for completeness. A
> patch file containing the changes to pltcl.c from its 8.2.4 version is
> also attached.
>
> We tested this patch with PostgreSQL 8.2.4 on both RedHat Enterprise 4.0
> usingTCL 8.4 (single threaded) and RHE 5.0 using TCL 8.4.13
> (multithreaded). We expect this solution to work with Windows as well,
> although we have not tested it. There may be some problems using this
> solution with old versions of TCL that pre-date the Tcl_SetNotifier
> function. However this function has been around for quite a while; it
> was added in in the TCL 8.2 release, circa 2000.
>
> We hope this patch will be considered for a future PostgreSQL release.
>
> Steve Marshall
> Paul Bayer
> Doug Knight
> WSI Corporation
>
>

[ application/x-gzip is not supported, skipping... ]

> *** pltcl.c.orig 2007-09-10 12:58:34.000000000 -0400
> --- pltcl.c 2007-09-11 11:37:33.363222114 -0400
> ***************
> *** 163,168 ****
> --- 163,258 ----
> static void pltcl_build_tuple_argument(HeapTuple tuple, TupleDesc tupdesc,
> Tcl_DString *retval);
>
> + /************************************************** ********************
> + * Declarations for functions overriden using Tcl_SetNotifier.
> + ************************************************** ********************/
> + static int fakeThreadKey; /* To give valid address for ClientData */
> +
> + static ClientData
> + pltcl_InitNotifier _ANSI_ARGS_((void));
> +
> + static void
> + pltcl_FinalizeNotifier _ANSI_ARGS_((ClientData clientData));
> +
> + static void
> + pltcl_SetTimer _ANSI_ARGS_((Tcl_Time *timePtr));
> +
> + static void
> + pltcl_AlertNotifier _ANSI_ARGS_((ClientData clientData));
> +
> + static void
> + pltcl_CreateFileHandler _ANSI_ARGS_((int fd, int mask, Tcl_FileProc *proc, ClientData clientData));
> +
> + static void
> + pltcl_DeleteFileHandler _ANSI_ARGS_((int fd));
> +
> + static void
> + pltcl_ServiceModeHook _ANSI_ARGS_((int mode));
> +
> + static int
> + pltcl_WaitForEvent _ANSI_ARGS_((Tcl_Time *timePtr));
> +
> + /************************************************** ********************
> + * Definitions for functions overriden using Tcl_SetNotifier.
> + * These implementations effectively disable the TCL Notifier subsystem.
> + * This is okay because we never enter the TCL event loop from postgres,
> + * so the notifier capabilities are initialized, but never used.
> + *
> + * NOTE: Only InitNotifier and DeleteFileHandler ever seem to get called
> + * by postgres, but we implement all the functions for completeness.
> + ************************************************** ********************/
> +
> + ClientData
> + pltcl_InitNotifier()
> + {
> + return (ClientData) &(fakeThreadKey);
> + }
> +
> + void
> + pltcl_FinalizeNotifier(clientData)
> + ClientData clientData; /* Not used. */
> + {
> + }
> +
> + void
> + pltcl_SetTimer(timePtr)
> + Tcl_Time *timePtr;
> + {
> + }
> +
> + void
> + pltcl_AlertNotifier(clientData)
> + ClientData clientData;
> + {
> + }
> +
> + void
> + pltcl_CreateFileHandler(fd, mask, proc, clientData)
> + int fd;
> + int mask;
> + Tcl_FileProc *proc;
> + ClientData clientData;
> + {
> + }
> +
> + void
> + pltcl_DeleteFileHandler(fd)
> + int fd;
> + {
> + }
> +
> + void
> + pltcl_ServiceModeHook(mode)
> + int mode;
> + {
> + }
> +
> + int
> + pltcl_WaitForEvent(timePtr)
> + Tcl_Time *timePtr; /* Maximum block time, or NULL. */
> + {
> + return 0;
> + }
>
> /*
> * This routine is a crock, and so is everyplace that calls it. The problem
> ***************
> *** 189,194 ****
> --- 279,287 ----
> void
> _PG_init(void)
> {
> + /* Notifier structure used to override functions in Notifier subsystem*/
> + Tcl_NotifierProcs notifier;
> +
> /* Be sure we do initialization only once (should be redundant now) */
> if (pltcl_pm_init_done)
> return;
> ***************
> *** 199,204 ****
> --- 292,316 ----
> #endif
>
> /************************************************** **********
> + * Override the functions in the Notifier subsystem.
> + *
> + * We do this to prevent the postgres backend from becoming
> + * multithreaded, which happens in the default version of
> + * Tcl_InitNotifier if the TCL library has been compiled with
> + * multithreading support (i.e. when TCL_THREADS is defined
> + * under Unix, and in all cases under Windows).
> + ************************************************** **********/
> + notifier.setTimerProc = pltcl_SetTimer;
> + notifier.waitForEventProc = pltcl_WaitForEvent;
> + notifier.createFileHandlerProc = pltcl_CreateFileHandler;
> + notifier.deleteFileHandlerProc = pltcl_DeleteFileHandler;
> + notifier.initNotifierProc = pltcl_InitNotifier;
> + notifier.finalizeNotifierProc = pltcl_FinalizeNotifier;
> + notifier.alertNotifierProc = pltcl_AlertNotifier;
> + notifier.serviceModeHookProc = pltcl_ServiceModeHook;
> + Tcl_SetNotifier(&notifier);
> +
> + /************************************************** **********
> * Create the dummy hold interpreter to prevent close of
> * stdout and stderr on DeleteInterp
> ************************************************** **********/

>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
> http://www.postgresql.org/docs/faq

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?

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"Bruce Momjian" <bruce@momjian.us> writes:

> This has been saved for the 8.4 release:
>
> http://momjian.postgresql.org/cgi-bin/pgpatches_hold
>
> ---------------------------------------------------------------------------
>
> Marshall, Steve wrote:
>> There is a problem in PL/TCL that can cause the postgres backend to
>> become multithreaded. Postgres is not designed to be multithreaded, so
>> this causes downstream errors in signal handling.

Um, this is a bug fix. Unless you had some problem with it?

Do we have anyone actively maintaining pltcl these days? I'm intentionally
quite unfamiliar with Tcl or I would be happy to verify it's reasonable. But
the explanation seems pretty convincing. If we don't have anyone maintaining
it then we're pretty much at the mercy of applying whatever patches come in
from people who are more familiar with it than us. In my experience that's how
new maintainers for modules of free software are often found anyways.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

Gregory Stark <stark@enterprisedb.com> writes:
> "Bruce Momjian" <bruce@momjian.us> writes:
>>> There is a problem in PL/TCL that can cause the postgres backend to
>>> become multithreaded. Postgres is not designed to be multithreaded, so
>>> this causes downstream errors in signal handling.

> Um, this is a bug fix. Unless you had some problem with it?

I haven't reviewed that patch yet, but I concur we should consider it
for 8.3.

regards, tom lane

---------------------------(end of broadcast)---------------------------
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Gregory Stark wrote:
> Do we have anyone actively maintaining pltcl these days? I'm intentionally
> quite unfamiliar with Tcl or I would be happy to verify it's reasonable. But
> the explanation seems pretty convincing. If we don't have anyone maintaining
> it then we're pretty much at the mercy of applying whatever patches come in
> from people who are more familiar with it than us. In my experience that's how
> new maintainers for modules of free software are often found anyways.
>
>

I was hoping that Jan or somebody else with some Tcl-fu would comment. I
agree it probably shouldn't be deferred.

It does bother me a bit that the Tcl engine startup just starts firing
up threads without advertisement - I wondered if we shouldn't kick back
and say it's their problem to fix.

cheers

andrew

---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend

OK, moved to patches list.

---------------------------------------------------------------------------

Andrew Dunstan wrote:
>
>
> Gregory Stark wrote:
> > Do we have anyone actively maintaining pltcl these days? I'm intentionally
> > quite unfamiliar with Tcl or I would be happy to verify it's reasonable. But
> > the explanation seems pretty convincing. If we don't have anyone maintaining
> > it then we're pretty much at the mercy of applying whatever patches come in
> > from people who are more familiar with it than us. In my experience that's how
> > new maintainers for modules of free software are often found anyways.
> >
> >
>
> I was hoping that Jan or somebody else with some Tcl-fu would comment. I
> agree it probably shouldn't be deferred.
>
> It does bother me a bit that the Tcl engine startup just starts firing
> up threads without advertisement - I wondered if we shouldn't kick back
> and say it's their problem to fix.
>
> cheers
>
> andrew

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

---------------------------(end of broadcast)---------------------------
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Tom Lane wrote:
> Gregory Stark <stark@enterprisedb.com> writes:
>> "Bruce Momjian" <bruce@momjian.us> writes:
>>>> There is a problem in PL/TCL that can cause the postgres backend to
>>>> become multithreaded. Postgres is not designed to be multithreaded, so
>>>> this causes downstream errors in signal handling.
>
>> Um, this is a bug fix. Unless you had some problem with it?
>
> I haven't reviewed that patch yet, but I concur we should consider it
> for 8.3.

hmm i wonder if that could be related to:

http://archives.postgresql.org/pgsql...1/msg00377.php


Stefan

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

Stefan Kaltenbrunner <stefan@kaltenbrunner.cc> writes:
> hmm i wonder if that could be related to:
> http://archives.postgresql.org/pgsql...1/msg00377.php

I had forgotten that thread, but it sure does look related doesn't it?
Do you want to try Steve's proposed patch and see if it fixes it?

regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend

Tom Lane wrote:
> Stefan Kaltenbrunner <stefan@kaltenbrunner.cc> writes:
>> hmm i wonder if that could be related to:
>> http://archives.postgresql.org/pgsql...1/msg00377.php
>
> I had forgotten that thread, but it sure does look related doesn't it?
> Do you want to try Steve's proposed patch and see if it fixes it?

yeah testing that patch now (seems to apply just fine on -HEAD) but it
seems that there is something strange going on because I just got:

http://www.kaltenbrunner.cc/files/regression.diffs

on the first manual buildfarm run on quagga...


Stefan

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Stefan Kaltenbrunner <stefan@kaltenbrunner.cc> writes:
> yeah testing that patch now (seems to apply just fine on -HEAD) but it
> seems that there is something strange going on because I just got:

! ERROR: could not read block 2 of relation 1663/16384/2606: read only 0 of 8192 bytes

Is that repeatable? What sort of filesystem are you testing on?
(soft-mounted NFS by any chance?)

regards, tom lane

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