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Plug race in dsa_attach.

With sufficiently bad luck, it was possible for a parallel worker toattempt attach to a DSA area after all other backends have detachedfrom it, which is not legal. If the worker had waited a little longerto get started, the DSM itself would have been destroyed, which is whythis wasn't noticed before.Thomas Munro, per a report from Andreas SeltenreichDiscussion:http://postgr.es/m/87h92g83t3.fsf@credativ.de

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src/backend/utils/mmgr
- dsa.c

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`‎src/backend/utils/mmgr/dsa.c`

Lines changed: 7 additions & 0 deletions

Original file line number	Diff line number	Diff line change
`@@ -1314,6 +1314,13 @@ attach_internal(void place, dsm_segment segment, dsa_handle handle)`
`1314`	`1314`
`1315`	`1315`	`/* Bump the reference count. */`
`1316`	`1316`	`LWLockAcquire(DSA_AREA_LOCK(area),LW_EXCLUSIVE);`
	`1317`	`+if (control->refcnt==0)`
	`1318`	`+{`
	`1319`	`+/* We can't attach to a DSA area that has already been destroyed. */`
	`1320`	`+ereport(ERROR,`
	`1321`	`+(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),`
	`1322`	`+errmsg("could not attach to dsa_area")));`
	`1323`	`+}`
`1317`	`1324`	`++control->refcnt;`
`1318`	`1325`	`LWLockRelease(DSA_AREA_LOCK(area));`
`1319`	`1326`

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Commitfddf45b

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`‎src/backend/utils/mmgr/dsa.c`

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