BUG #5906: assertion failure in AtCleanup_Portals

The following bug has been logged online:

Bug reference: 5906
Logged by: YAMAMOTO Takashi
Email address: yamt(at)mwd(dot)biglobe(dot)ne(dot)jp
PostgreSQL version: 9.1devel
Operating system: NetBSD current
Description: assertion failure in AtCleanup_Portals
Details:

i got the following with my application, which uses
PQsendPrepare+PQsendQueryPrepared for nearly everything
including ROLLBACK.

PostgreSQL version is 4c966d920fb75a5d0366b887c2ef28e6d87c1eda.

(gdb) bt
#0 0xbbba4cc7 in _lwp_kill () from /usr/lib/libc.so.12
#1 0xbbba4c85 in raise (s=6) at /siro/nbsd/src/lib/libc/gen/raise.c:48
#2 0xbbba445a in abort () at /siro/nbsd/src/lib/libc/stdlib/abort.c:74
#3 0x0833eae2 in ExceptionalCondition (
conditionName=0x84bf3b8 "!(portal->cleanup == ((void *)0))",
errorType=0x8371631 "FailedAssertion", fileName=0x84bf200 "portalmem.c",

lineNumber=766) at assert.c:57
#4 0x0835d4e3 in AtCleanup_Portals () at portalmem.c:766
#5 0x080c49a8 in CleanupTransaction () at xact.c:2325
#6 0x080c809b in CommitTransactionCommand () at xact.c:2503
#7 0x082845a3 in finish_xact_command () at postgres.c:2450
#8 0x08287b34 in PostgresMain (argc=2, argv=0xbb9126a4,
username=0xbb9125f8 "takashi") at postgres.c:1980
#9 0x08242306 in ServerLoop () at postmaster.c:3590
#10 0x082430b8 in PostmasterMain (argc=3, argv=0xbfbfe5a8) at
postmaster.c:1110
#11 0x081e1ac9 in main (argc=3, argv=0xbfbfe5a8) at main.c:199
(gdb) fr 4
#4 0x0835d4e3 in AtCleanup_Portals () at portalmem.c:766
766 Assert(portal->cleanup == NULL);
(gdb) p *portal
$2 = {name = 0xbb9e3d48 "", prepStmtName = 0x99b35838 "41", heap =
0x99b323e8,
resowner = 0x99b44b8c, cleanup = 0x817dd70 <PortalCleanup>, createSubid =
1,
sourceText = 0x99b3581c "ROLLBACK", commandTag = 0x8498daf "ROLLBACK",
stmts = 0x99ba681c, cplan = 0x99ba6868, portalParams = 0x0,
strategy = PORTAL_MULTI_QUERY, cursorOptions = 4, status = PORTAL_DONE,
portalPinned = 0 '\0', queryDesc = 0x0, tupDesc = 0x0, formats = 0x0,
holdStore = 0x0, holdContext = 0x0, atStart = 1 '\001', atEnd = 1 '\001',

posOverflow = 0 '\0', portalPos = 0, creation_time = 352351237783155,
visible = 1 '\001'}
(gdb)