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Commit1541632

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Add code to InternalIpcMemoryCreate() to handle the case where shmget()
returns EINVAL for an existing shared memory segment. Although it's notterribly sensible, that behavior does meet the POSIX spec because EINVALis the appropriate error code when the existing segment is smaller than therequested size, and the spec explicitly disclaims any particular ordering oferror checks. Moreover, it does in fact happen on OS X and probably otherBSD-derived kernels. (We were able to talk NetBSD into changing their code,but purging that behavior from the wild completely seems unlikely to happen.)We need to distinguish collision with a pre-existing segment from invalid sizerequest in order to behave sensibly, so it's worth some extra code here to getit right. Per report from Gavin Kistner and subsequent investigation.Back-patch to all supported versions, since any of them could get usedwith a kernel having the debatable behavior.
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‎src/backend/port/sysv_shmem.c

Lines changed: 43 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,7 @@
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.55 2010/01/02 16:57:50 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.56 2010/05/01 22:46:30 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -92,6 +92,48 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
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)
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returnNULL;
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/*
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* Some BSD-derived kernels are known to return EINVAL, not EEXIST,
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* if there is an existing segment but it's smaller than "size"
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* (this is a result of poorly-thought-out ordering of error tests).
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* To distinguish between collision and invalid size in such cases,
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* we make a second try with size = 0. These kernels do not test
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* size against SHMMIN in the preexisting-segment case, so we will
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* not get EINVAL a second time if there is such a segment.
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*/
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if (errno==EINVAL)
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{
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intsave_errno=errno;
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shmid=shmget(memKey,0,IPC_CREAT |IPC_EXCL |IPCProtection);
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if (shmid<0)
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{
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/* As above, fail quietly if we verify a collision */
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if (errno==EEXIST||errno==EACCES
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#ifdefEIDRM
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||errno==EIDRM
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#endif
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)
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returnNULL;
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/* Otherwise, fall through to report the original error */
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}
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else
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{
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/*
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* On most platforms we cannot get here because SHMMIN is
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* greater than zero. However, if we do succeed in creating
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* a zero-size segment, free it and then fall through to
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* report the original error.
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*/
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if (shmctl(shmid,IPC_RMID,NULL)<0)
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elog(LOG,"shmctl(%d, %d, 0) failed: %m",
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(int)shmid,IPC_RMID);
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}
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errno=save_errno;
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}
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/*
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* Else complain and abort
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*/

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