Обсуждение: [MASSMAIL]elog/ereport VS misleading backtrace_function function address

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[MASSMAIL]elog/ereport VS misleading backtrace_function function address

От
Jakub Wartak
Дата:
Hi -hackers,

While chasing some other bug I've learned that backtrace_functions
might be misleading with top elog/ereport() address.

Reproducer:

# using Tom's reproducer on master:
wget https://www.postgresql.org/message-id/attachment/112394/ri-collation-bug-example.sql
echo '' >> ri-collation-bug-example.sql
echo '\errverbose' >> ri-collation-bug-example.sql
-- based on grepping the source code locations many others could go in here:
psql -p 5437 -c "alter system set backtrace_functions =
'RI_FKey_cascade_del,get_collation_isdeterministic';"
psql -p 5437 -c "select pg_reload_conf();"
dropdb -p 5437 test1 ; createdb -p 5437 test1 ; psql -p 5437 -d test1
-f ri-collation-bug-example.sql

gives (note "get_collation_isdeterministic"):
    psql:ri-collation-bug-example.sql:42: ERROR:  cache lookup failed
for collation 0
    psql:ri-collation-bug-example.sql:44: error: ERROR:  XX000: cache
lookup failed for collation 0
    LOCATION:  get_collation_isdeterministic, lsyscache.c:1088

and in log note the 0x14c6bb:
2024-03-27 14:39:13.065 CET [12899] postgres@test1 ERROR:  cache
lookup failed for collation 0
2024-03-27 14:39:13.065 CET [12899] postgres@test1 BACKTRACE:
        postgres: 16/main: postgres test1 [local] DELETE(+0x14c6bb)
[0x5565c5a9c6bb]
        postgres: 16/main: postgres test1 [local]
DELETE(RI_FKey_cascade_del+0x323) [0x5565c5ec0973]
        postgres: 16/main: postgres test1 [local] DELETE(+0x2d401f)
[0x5565c5c2401f]
        postgres: 16/main: postgres test1 [local] DELETE(+0x2d5a04)
[0x5565c5c25a04]
        postgres: 16/main: postgres test1 [local]
DELETE(AfterTriggerEndQuery+0x78) [0x5565c5c2a918]
[..]
2024-03-27 14:39:13.065 CET [12899] postgres@test1 STATEMENT:  delete
from revisions where id='5c617ce7-688d-4bea-9d66-c0f0ebc635da';

based on \errverbose it is OK (error matches the code, thanks to
Alvaro for this hint):

     9   bool
     8   get_collation_isdeterministic(Oid colloid)
     7   {
     6   │   HeapTuple>  tp;
     5   │   Form_pg_collation colltup;
     4   │   bool>   >   result;
     3   │
     2   │   tp = SearchSysCache1(COLLOID, ObjectIdGetDatum(colloid));
     1   │   if (!HeapTupleIsValid(tp))
  1088   │   │   elog(ERROR, "cache lookup failed for collation %u", colloid);
[..]


but based on backtrace address 0x14c6bb (!) and it resolves
differently which seems to be highly misleading (note the
"get_language_name.cold"):

$ addr2line -e /usr/lib/postgresql/16/bin/postgres -a -f 0x14c6bb
0x000000000014c6bb
get_language_name.cold
./build/src/backend/utils/cache/./build/../src/backend/utils/cache/lsyscache.c:1181

When i disassemble the get_collation_isdeterministic() (and I know the
name from \errverbose):

Dump of assembler code for function get_collation_isdeterministic:
Address range 0x5c7680 to 0x5c76c1:
   0x00000000005c7680 <+0>:     push   %rbp
[..]
   0x00000000005c7694 <+20>:    call   0x5d63b0 <SearchSysCache1>
   0x00000000005c7699 <+25>:    test   %rax,%rax
   0x00000000005c769c <+28>:    je     0x14c686
<get_collation_isdeterministic.cold>
   0x00000000005c76a2 <+34>:    mov    %rax,%rdi
[..]
   0x00000000005c76bf <+63>:    leave
   0x00000000005c76c0 <+64>:    ret
Address range 0x14c686 to 0x14c6bb:
   0x000000000014c686 <-4698106>:       xor    %esi,%esi
   0x000000000014c688 <-4698104>:       mov    $0x15,%edi
   0x000000000014c68d <-4698099>:       call   0x14ec86 <errstart_cold>
   0x000000000014c692 <-4698094>:       mov    %r12d,%esi
   0x000000000014c695 <-4698091>:       lea    0x5028dc(%rip),%rdi
   # 0x64ef78
   0x000000000014c69c <-4698084>:       xor    %eax,%eax
   0x000000000014c69e <-4698082>:       call   0x5df320 <errmsg_internal>
   0x000000000014c6a3 <-4698077>:       lea    0x6311a6(%rip),%rdx
   # 0x77d850 <__func__.34>
   0x000000000014c6aa <-4698070>:       mov    $0x440,%esi
   0x000000000014c6af <-4698065>:       lea    0x630c8a(%rip),%rdi
   # 0x77d340
   0x000000000014c6b6 <-4698058>:       call   0x5df100 <errfinish>
   << NOTE the exact 0x14c6bb is even missing here(!)

notice the interesting thing here: according to GDB
get_collation_isdeterministic() is @ 0x5c7680 + jump to 0x14c686
<get_collation_isdeterministic.cold> till 0x14c6bb (but without it)
without any stack setup for that .cold. But backtrace() just captured
the elog/ereport (cold) at the end of 0x14c6bb instead, so if I take
that exact address from backtrace_functions output as it is
("DELETE(+0x14c6bb)"), it also shows WRONG function (just like
addr2line):

(gdb) disassemble 0x14c6bb
Dump of assembler code for function get_language_name:
Address range 0x5c7780 to 0x5c77ee:
[..]
   0x00000000005c77ed <+109>:   ret
Address range 0x14c6bb to 0x14c6f0:
   0x000000000014c6bb <-4698309>:       xor    %esi,%esi
[..]
   0x000000000014c6eb <-4698261>:       call   0x5df100 <errfinish>
End of assembler dump.

so this is wrong (as are failing in "get_collation_isdeterministic"
not in "get_language_name").

I was very confused at the beginning with the main question being: why
are we compiling elog/ereport() so that it is incompatible with
backtrace? When looking for it I've found two threads [1] by David
that were actually helpful in understanding that this was done for
performance (unlikley(), cold attribute and similiar type of
discussions). Now my thought is that for >= ERROR in ereport_domain we
could add something more (?) before pg_unreachable() that would help
the backtrace to resolve the symbol correctly

If I try non-portable PoC with x86 nop instruction:

--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -145,8 +145,10 @@ struct Node;
                        errstart_cold(elevel, domain) : \
                        errstart(elevel, domain)) \
                        __VA_ARGS__, errfinish(__FILE__, __LINE__, __func__); \
-               if (__builtin_constant_p(elevel) && (elevel) >= ERROR) \
+               if (__builtin_constant_p(elevel) && (elevel) >= ERROR) { \
+                       __asm__ __volatile__("nop"); \
                        pg_unreachable(); \
+               } \

it partially works and the address can be now properly resolved!

Somewhat backtrace() still is unable to lookup the to do so in log itself:
        postgres: postgres test1 [local] DELETE(+0x15f84c) [0x55703bf5284c]
        postgres: postgres test1 [local]
DELETE(RI_FKey_cascade_del+0x2b0) [0x55703c34c850]

but at least:

postgres@hive:~$ addr2line -e /usr/pgsql17/bin/postgres -a -f 0x15f84c
0x000000000015f84c
get_collation_isdeterministic
/git/postgres/src/backend/utils/cache/lsyscache.c:1062 (discriminator 4)
postgres@hive:~$

in gdb it will point right to our new nop:
   0x000000000015f840 <-4490944>:       lea    0x608306(%rip),%rdi
   # 0x767b4d
   0x000000000015f847 <-4490937>:       call   0x5be1d0 <errfinish>
   0x000000000015f84c <-4490932>:       nop

Thoughts? Does it make sense to post a patch (for pg18?)? How to do it
in $arch-independent way? I've tried also to try to find a way with
e.g. -rdynamic to show that real function name, but it looks like
without some more serious unwind library it seems unrealistic (?) to
resolve that get_collation_isdeterministic.cold

-J.

[1] -
https://www.postgresql.org/message-id/flat/CAApHDvoWoxtbWiqZxrhO%2Bi9NoG56AWHDzuDDD%2B1OEf4PxzFhig%40mail.gmail.com#611566bd6fd06f27e51dbc3148a673d3
[2] - https://www.postgresql.org/message-id/flat/CAKJS1f8yqRW3qx2CO9r4bqqvA2Vx68%3D3awbh8CJWTP9zXeoHMw%40mail.gmail.com



Re: elog/ereport VS misleading backtrace_function function address

От
Tom Lane
Дата:
Jakub Wartak <jakub.wartak@enterprisedb.com> writes:
> While chasing some other bug I've learned that backtrace_functions
> might be misleading with top elog/ereport() address.

That was understood from the beginning: this type of backtrace is
inherently pretty imprecise, and I doubt there is much that can
be done to make it better.  IIRC the fundamental problem is it only
looks at global symbols, so static functions inherently defeat it.
It was argued that this is better than nothing, which is true, but
you have to take the info with a mountain of salt.

I recall speculating about whether we could somehow invoke gdb
to get a more comprehensive and accurate backtrace, but I don't
really have a concrete idea how that could be made to work.

            regards, tom lane



Re: elog/ereport VS misleading backtrace_function function address

От
Jakub Wartak
Дата:
Hi Tom and -hackers!

On Thu, Mar 28, 2024 at 7:36 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Jakub Wartak <jakub.wartak@enterprisedb.com> writes:
> > While chasing some other bug I've learned that backtrace_functions
> > might be misleading with top elog/ereport() address.
>
> That was understood from the beginning: this type of backtrace is
> inherently pretty imprecise, and I doubt there is much that can
> be done to make it better.  IIRC the fundamental problem is it only
> looks at global symbols, so static functions inherently defeat it.
> It was argued that this is better than nothing, which is true, but
> you have to take the info with a mountain of salt.

OK, point taken, thanks for describing the limitations, still I find
backtrace_functions often the best thing we have primarily due its
simplicity and ease of use (for customers). I found out simplest
portable way to generate NOP (or any other instruction that makes the
problem go away):

with the reproducer, psql returns:

psql:ri-collation-bug-example.sql:48: error: ERROR:  XX000: cache
lookup failed for collation 0
LOCATION:  get_collation_isdeterministic, lsyscache.c:1062

logfile without patch:

2024-05-07 09:05:43.043 CEST [44720] ERROR:  cache lookup failed for collation 0
2024-05-07 09:05:43.043 CEST [44720] BACKTRACE:
        postgres: postgres postgres [local] DELETE(+0x155883) [0x55ce5a86a883]
        postgres: postgres postgres [local]
DELETE(RI_FKey_cascade_del+0x2b0) [0x55ce5ac6dfd0]
        postgres: postgres postgres [local] DELETE(+0x2d488b) [0x55ce5a9e988b]
[..]

$ addr2line -e /usr/pgsql18/bin/postgres  -a -f 0x155883
0x0000000000155883
get_constraint_type.cold <<<<<< so it's wrong as the real function
should be get_collation_isdeterministic

logfile with attached patch:

2024-05-07 09:11:06.596 CEST [51185] ERROR:  cache lookup failed for collation 0
2024-05-07 09:11:06.596 CEST [51185] BACKTRACE:
        postgres: postgres postgres [local] DELETE(+0x168bf0) [0x560e1cdfabf0]
        postgres: postgres postgres [local]
DELETE(RI_FKey_cascade_del+0x2b0) [0x560e1d200c00]
        postgres: postgres postgres [local] DELETE(+0x2e90fb) [0x560e1cf7b0fb]
[..]

$ addr2line -e /usr/pgsql18/bin/postgres  -a -f 0x168bf0
0x0000000000168bf0
get_collation_isdeterministic.cold <<< It's ok with the patch

NOTE: in case one will be testing this: one cannot ./configure with
--enable-debug as it prevents the compiler optimizations that actually
end up with the ".cold" branch optimizations that cause backtrace() to
return the wrong address.

> I recall speculating about whether we could somehow invoke gdb
> to get a more comprehensive and accurate backtrace, but I don't
> really have a concrete idea how that could be made to work.

Oh no, I'm more about micro-fix rather than doing some bigger
revolution. The patch simply adds this one instruction in macro, so
that now backtrace_functions behaves better:

   0x0000000000773d28 <+105>:   call   0x79243f <errfinish>
   0x0000000000773d2d <+110>:   movzbl -0x12(%rbp),%eax  << this ends
up being added by the patch
   0x0000000000773d31 <+114>:   call   0xdc1a0 <abort@plt>

I'll put that as for PG18 in CF, but maybe it could be backpatched too
- no hard opinion on that (I don't see how that ERROR/FATAL path could
cause any performance regressions)

-J.

Вложения

Re: elog/ereport VS misleading backtrace_function function address

От
Peter Eisentraut
Дата:
On 07.05.24 09:43, Jakub Wartak wrote:
> NOTE: in case one will be testing this: one cannot ./configure with
> --enable-debug as it prevents the compiler optimizations that actually
> end up with the ".cold" branch optimizations that cause backtrace() to
> return the wrong address.

Is that configuration useful?  If you're interested in backtraces, 
wouldn't you also want debug symbols?  Don't production builds use debug 
symbols nowadays as well?

>> I recall speculating about whether we could somehow invoke gdb
>> to get a more comprehensive and accurate backtrace, but I don't
>> really have a concrete idea how that could be made to work.
> Oh no, I'm more about micro-fix rather than doing some bigger
> revolution. The patch simply adds this one instruction in macro, so
> that now backtrace_functions behaves better:
> 
>     0x0000000000773d28 <+105>:   call   0x79243f <errfinish>
>     0x0000000000773d2d <+110>:   movzbl -0x12(%rbp),%eax  << this ends
> up being added by the patch
>     0x0000000000773d31 <+114>:   call   0xdc1a0 <abort@plt>
> 
> I'll put that as for PG18 in CF, but maybe it could be backpatched too
> - no hard opinion on that (I don't see how that ERROR/FATAL path could
> cause any performance regressions)

I'm missing a principled explanation of what this does.  I just see that 
it sprinkles some no-op code to make this particular thing work a bit 
differently, but this might just behave differently with the next 
compiler next year.  I'd like to see a bit more of an abstract 
explanation of what is happening here.




Re: elog/ereport VS misleading backtrace_function function address

От
Jakub Wartak
Дата:
Hi Peter!

On Sun, May 12, 2024 at 10:33 PM Peter Eisentraut <peter@eisentraut.org> wrote:
>
> On 07.05.24 09:43, Jakub Wartak wrote:
> > NOTE: in case one will be testing this: one cannot ./configure with
> > --enable-debug as it prevents the compiler optimizations that actually
> > end up with the ".cold" branch optimizations that cause backtrace() to
> > return the wrong address.
>
> Is that configuration useful?  If you're interested in backtraces,
> wouldn't you also want debug symbols?

The use case here is that backtrace_functions is unreliable when we
ask customers on production builds to use it (so it's useful not just
for local tests)

> Don't production builds use debug
> symbols nowadays as well?

Reality is apparently mixed,at least from what I have checked :
- all RHEL 7.x/8.x (PGDG and our forks) do NOT come with
--enable-debug according to pg_config.
- on Debian 11/12 PGDG does come with --enable-debug

> >> I recall speculating about whether we could somehow invoke gdb
> >> to get a more comprehensive and accurate backtrace, but I don't
> >> really have a concrete idea how that could be made to work.
> > Oh no, I'm more about micro-fix rather than doing some bigger
> > revolution. The patch simply adds this one instruction in macro, so
> > that now backtrace_functions behaves better:
> >
> >     0x0000000000773d28 <+105>:   call   0x79243f <errfinish>
> >     0x0000000000773d2d <+110>:   movzbl -0x12(%rbp),%eax  << this ends
> > up being added by the patch
> >     0x0000000000773d31 <+114>:   call   0xdc1a0 <abort@plt>
> >
> > I'll put that as for PG18 in CF, but maybe it could be backpatched too
> > - no hard opinion on that (I don't see how that ERROR/FATAL path could
> > cause any performance regressions)
>
> I'm missing a principled explanation of what this does.  I just see that
> it sprinkles some no-op code to make this particular thing work a bit
> differently, but this might just behave differently with the next
> compiler next year.  I'd like to see a bit more of an abstract
> explanation of what is happening here.

OK I'll try to explain using assembly, but I'm not an expert on this.
Let's go to the 1st post, assume we run with backtrace_function set:

get_collation_isdeterministic()  0x5c7680 to 0x5c76c1:
       might jump(!) to 0x14c686 <get_collation_isdeterministic.cold>
       note the two completely different address ranges (hot and separate cold)
       it's because of 913ec71d682 that added the cold branches
optimization via pg_attribute_cold to errstart_cold

Dump of assembler code for function get_collation_isdeterministic:
Address range 0x5c7680 to 0x5c76c1:
   0x00000000005c7680 <+0>:     push   %rbp
[..]
   0x00000000005c7694 <+20>:    call   0x5d63b0 <SearchSysCache1>
   0x00000000005c7699 <+25>:    test   %rax,%rax
   0x00000000005c769c <+28>:    je     0x14c686
<get_collation_isdeterministic.cold>
[..]
   0x00000000005c76b3 <+51>:    call   0x5d6430 <ReleaseSysCache>
   0x00000000005c76b8 <+56>:    mov    %r12d,%eax
   0x00000000005c76bb <+59>:    mov    -0x8(%rbp),%r12
   0x00000000005c76bf <+63>:    leave
   0x00000000005c76c0 <+64>:    ret
Address range 0x14c686 to 0x14c6bb:
                                    // note here that the last
instruction is 0x14c6b6 not 0x14c6bb(!)
                                    // note this cold path also has no
frame pointer setup
   0x000000000014c686 <-4698106>:       xor    %esi,%esi
   0x000000000014c688 <-4698104>:       mov    $0x15,%edi
   0x000000000014c68d <-4698099>:       call   0x14ec86 <errstart_cold>
   0x000000000014c692 <-4698094>:       mov    %r12d,%esi
   0x000000000014c695 <-4698091>:       lea    0x5028dc(%rip),%rdi
   # 0x64ef78
   0x000000000014c69c <-4698084>:       xor    %eax,%eax
   0x000000000014c69e <-4698082>:       call   0x5df320 <errmsg_internal>
   0x000000000014c6a3 <-4698077>:       lea    0x6311a6(%rip),%rdx
   # 0x77d850 <__func__.34>
   0x000000000014c6aa <-4698070>:       mov    $0x440,%esi
   0x000000000014c6af <-4698065>:       lea    0x630c8a(%rip),%rdi
   # 0x77d340
   0x000000000014c6b6 <-4698058>:       call   0x5df100 <errfinish>
End of assembler dump.

so it's
    get_collation_isdeterministic() ->
        get_collation_isdeterministic.cold() [but not real function ->
no proper fp/stack?] ->
            .. ->
                errfinish() ->
                    set_backtrace() // just builds and appends string
using backtrace()/backtrace_functions()/backtrace_symbol_list().
                    prints/logs, finishes

It seems that the thing is that the limited GNU libc
backtrace_symbol_list() won't resolve the 0x14c6b6..0x14c6bb to the
"get_collation_isdeterministic[.cold]" symbol name and it will just
simply put 0x14c6bb in that the text asm. It's wrong and it is
confusing:

2024-03-27 14:39:13.065 CET [12899] postgres(at)test1 ERROR:  cache
lookup failed for collation 0
2024-03-27 14:39:13.065 CET [12899] postgres(at)test1 BACKTRACE:
        postgres: 16/main: postgres test1 [local] DELETE(+0x14c6bb)
[0x5565c5a9c6bb]
        postgres: 16/main: postgres test1 [local]
DELETE(RI_FKey_cascade_del+0x323) [0x5565c5ec0973]
[..]

if you now take addr2line for 0x14c6bb it will be resolved to the next
assembly AFTER where it had happened: we did blow up in
<0x14c686..0x14c6bb), but it put 0x14c6bb as the IP:

$ addr2line -e /usr/lib/postgresql/16/bin/postgres -a -f 0x14c6bb
0x000000000014c6bb
get_language_name.cold

^ it's wrong,! but GDB also gets the same (GIGO):

(gdb) disassemble 0x14c6bb
Dump of assembler code for function get_language_name:
Address range 0x5c7780 to 0x5c77ee:
[..]
   0x00000000005c77da <+90>:    je     0x14c6bb <get_language_name.cold>
[..]
   0x00000000005c77ed <+109>:   ret
Address range 0x14c6bb to 0x14c6f0: // <<< HERE
   0x000000000014c6bb <-4698309>:       xor    %esi,%esi
[..]

So something was saved in the wrong way (fp?), however adding that 1
trivial NOP instruction between errfinish() and pg_unreachable() where
afterwards somehow it returns, does make the returned address (using
addr2line) resolve to the proper function. My limited understanding is
that we simply pushed the compiler into generating that cold path
(that has no %ebp/%rbp frame pointer assembly) followed by no-exit in
asm code that prevents backtrace() from working. So it looks like we
are playing hard at micro-optimizations (cold branch optimizations,
again 913ec71d682), we provide backtrace_functions GUC and we provide
various packaging ways, and that somehow bite me. The backtrace(3)
says `Omission of the frame pointers (as implied by any of gcc(1)'s
nonzero optimization levels) may cause these assumptions to be
violated.`

-J.



Re: elog/ereport VS misleading backtrace_function function address

От
Alvaro Herrera
Дата:
On 2024-May-14, Jakub Wartak wrote:

> On Sun, May 12, 2024 at 10:33 PM Peter Eisentraut <peter@eisentraut.org> wrote:

> > Don't production builds use debug
> > symbols nowadays as well?
> 
> Reality is apparently mixed,at least from what I have checked :
> - all RHEL 7.x/8.x (PGDG and our forks) do NOT come with
> --enable-debug according to pg_config.

Ooh, yeah, that's true according to 

https://git.postgresql.org/gitweb/?p=pgrpms.git;a=blob;f=rpm/redhat/main/non-common/postgresql-16/main/postgresql-16.spec;h=ab2f6edc903f083e04b8f7a1d3bad8e1b7018791;hb=1a8b9fa7019d3f73322ca873b62dc0b33e73ed1d

 507 %if %beta
 508         --enable-debug \
 509         --enable-cassert \
 510 %endif

Maybe a better approach for this whole thing is to change the specs so
that --enable-debug is always given, not just for %beta.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/



Re: elog/ereport VS misleading backtrace_function function address

От
Tom Lane
Дата:
Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> On 2024-May-14, Jakub Wartak wrote:
>> Reality is apparently mixed,at least from what I have checked :
>> - all RHEL 7.x/8.x (PGDG and our forks) do NOT come with
>> --enable-debug according to pg_config.

> Ooh, yeah, that's true according to
>
https://git.postgresql.org/gitweb/?p=pgrpms.git;a=blob;f=rpm/redhat/main/non-common/postgresql-16/main/postgresql-16.spec;h=ab2f6edc903f083e04b8f7a1d3bad8e1b7018791;hb=1a8b9fa7019d3f73322ca873b62dc0b33e73ed1d

>  507 %if %beta
>  508         --enable-debug \
>  509         --enable-cassert \
>  510 %endif

> Maybe a better approach for this whole thing is to change the specs so
> that --enable-debug is always given, not just for %beta.

My recollection from my time at Red Hat is that their standard policy
is to build everything with debug symbols all the time; so this is
violating that policy, and we should change it just on those grounds.
However, I'm not sure how much the change will help Joe Average User
with respect to the thread topic.  RH actually has infrastructure that
splits the debug symbol tables out into separate "debuginfo" RPMs,
which you have to install manually if you want to debug a particular
package.  This is good for disk space consumption, but it means that
most users are still only going to see the same backtrace they see
currently.

I don't know how much of that applies to, eg, Debian.

            regards, tom lane



Re: elog/ereport VS misleading backtrace_function function address

От
Robert Haas
Дата:
On Tue, May 14, 2024 at 6:13 AM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
> OK I'll try to explain using assembly, but I'm not an expert on this.
> Let's go to the 1st post, assume we run with backtrace_function set:

I feel like this explanation doesn't really explain very much. I mean,
the question is not "how is it that adding a nop instruction fixes
anything?" but "is adding a nop instruction a principled way of fixing
things, and if so, for what reason?".  And as far as I can see, you
haven't answered that question anywhere. Unless we really understand
why the results are better with that nop instruction added, we can't
possibly have any confidence that this is anything other than random
good fortune, which isn't a sufficiently good reason to make a change.
And, while I'm no expert on this, I suspect that it is mostly just
random good fortune -- the fact that inserting a volatile variable
declaration here solved the problem seems like something that could
easily fail to work out on another platform or compiler or compiler
version. I also think it's the wrong idea on principle to insert junk
declarations into our code to try to get good backtraces.

I think the right answer here is probably what Alvaro said, namely,
that people have to have the debug symbols installed if they want to
get backtraces. Tom is probably correct when he says that there's
nothing we can do to ensure that users end up with debug symbols in
all cases, but that doesn't mean it's the wrong solution. It's at
least understandable why it works.

--
Robert Haas
EDB: http://www.enterprisedb.com