[Tom Lane <tgl@sss.pgh.pa.us>]

I initially ran into this while trying to reproduce the recent
reports of trouble with LLVM 14 on ARM.  However, it also reproduces
with LLVM 17 on x86_64, and I see no reason to think it's at all
arch-specific.  I also reproduced it in back branches (only tried
v14, but it's definitely not new in HEAD).

To reproduce:

1. Build with --with-llvm

2. Create a config file containing

$ cat $HOME/tmp/temp_config
# enable jit at max
jit_above_cost = 1
jit_inline_above_cost = 1
jit_optimize_above_cost = 1

and do
export TEMP_CONFIG=$HOME/tmp/temp_config

3. cd to .../src/pl/plpgsql/src/, and do "make check".

It gets a SIGSEGV in plpgsql_transaction.sql's
cursor_fail_during_commit test.  The stack trace looks like

(gdb) bt
#0  __strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:77
#1  0x0000000000735c58 in pq_sendstring (buf=0x7ffd80f8eeb0, 
    str=0x7f77cffdf000 <error: Cannot access memory at address 0x7f77cffdf000>)
    at pqformat.c:197
#2  0x00000000009ca09c in err_sendstring (buf=0x7ffd80f8eeb0, 
    str=0x7f77cffdf000 <error: Cannot access memory at address 0x7f77cffdf000>)
    at elog.c:3449
#3  0x00000000009ca4ba in send_message_to_frontend (edata=0xf786a0 <errordata>)
    at elog.c:3568
#4  0x00000000009c73a3 in EmitErrorReport () at elog.c:1715
#5  0x00000000008987e7 in PostgresMain (dbname=<optimized out>, 
    username=0x29fdb00 "postgres") at postgres.c:4378
#6  0x0000000000893c5d in BackendMain (startup_data=<optimized out>, 
    startup_data_len=<optimized out>) at backend_startup.c:105

The errordata structure it's trying to print out contains

(gdb) p *edata
$1 = {elevel = 21, output_to_server = true, output_to_client = true, 
  hide_stmt = false, hide_ctx = false, 
  filename = 0x7f77cffdf000 <error: Cannot access memory at address 0x7f77cffdf000>, lineno = 843, 
  funcname = 0x7f77cffdf033 <error: Cannot access memory at address 0x7f77cffdf033>, domain = 0xbd3baa "postgres-17", 
  context_domain = 0x7f77c3343320 "plpgsql-17", sqlerrcode = 33816706, 
  message = 0x29fdc20 "division by zero", detail = 0x0, detail_log = 0x0, 
  hint = 0x0, 
  context = 0x29fdc50 "PL/pgSQL function cursor_fail_during_commit() line 6 at COMMIT", backtrace = 0x0, 
  message_id = 0x7f77cffdf022 <error: Cannot access memory at address 0x7f77cffdf022>, schema_name = 0x0, table_name = 0x0, column_name = 0x0, 
  datatype_name = 0x0, constraint_name = 0x0, cursorpos = 0, internalpos = 0, 
  internalquery = 0x0, saved_errno = 2, assoc_context = 0x29fdb20}

lineno = 843 matches the expected error location in int4_div().
The three string fields containing obviously-garbage pointers
are ones that elog.c expects to point at compile-time constants,
so it just records the caller's pointers without strdup'ing them.

Perhaps somebody else will know better, but what I think is happening
here is

A. Thanks to the low jit cost settings, we choose to jit-compile
the "1/(x-1000)" expression inside cursor_fail_during_commit().

B. When x reaches 1000, the division-by-zero error that the test
intends to provoke is thrown from jit-compiled code.

C. Somewhere between there and EmitErrorReport(), something decided
it could unmap the jit-compiled code.

D. Now filename/funcname are pointing into the void, and 
send_message_to_frontend() dumps core while trying to send them.

One way to fix this could be to pstrdup those strings even
though they should be constants.  I don't especially like
the amount of overhead that'd add though.

What I think is the right solution is to fix things so that
seemingly-no-longer-used jit compilations are not thrown away
until transaction cleanup.  I don't know the JIT code nearly
well enough to take point on fixing it like that, though.

			regards, tom lane