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Fix incorrect sentinel byte logic in GenerationRealloc()

This only affects MEMORY_CONTEXT_CHECKING builds.This fixes an off-by-one issue in GenerationRealloc() where thefast-path code which tries to reuse the existing allocation if theexisting chunk is >= the new requested size. The code there thought itwas always ok to use the existing chunk, but when oldsize == size thereisn't enough space to store the sentinel byte. If both sizes matchedexactly set_sentinel() would overwrite the first byte beyond the chunkand then subsequent GenerationRealloc() calls could then fail theAssert(chunk->requested_size < oldsize) check which is trying to ensurethe chunk is large enough to store the sentinel.The same issue does not exist in aset.c as the sentinel checking codeonly adds a sentinel byte if there's enough space in the chunk.Reported-by: Alexander Lakhin <exclusion@gmail.com>Discussion:https://postgr.es/m/49275921-7b39-41af-5eb8-97b50ce3312e@gmail.comBackpatch-through: 16, where the problem was introduced by0e48038

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src/backend/utils/mmgr
- generation.c

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`‎src/backend/utils/mmgr/generation.c`

Lines changed: 7 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -780,16 +780,21 @@ GenerationRealloc(void *pointer, Size size)`
`780`	`780`	`#endif`
`781`	`781`
`782`	`782`	`/*`
`783`		`- * Maybe the allocated area alreadyis >= the new size. (In particular,`
`784`		`- *we alwaysfall out here if the requested size is a decrease.)`
	`783`	`+ * Maybe the allocated area alreadybig enough. (In particular, we always`
	`784`	`+ * fall out here if the requested size is a decrease.)`
`785`	`785`	`*`
`786`	`786`	`* This memory context does not use power-of-2 chunk sizing and instead`
`787`	`787`	`* carves the chunks to be as small as possible, so most repalloc() calls`
`788`	`788`	`* will end up in the palloc/memcpy/pfree branch.`
`789`	`789`	`*`
`790`	`790`	`* XXX Perhaps we should annotate this condition with unlikely()?`
`791`	`791`	`*/`
	`792`	`+#ifdefMEMORY_CONTEXT_CHECKING`
	`793`	`+/* With MEMORY_CONTEXT_CHECKING, we need an extra byte for the sentinel */`
	`794`	`+if (oldsize>size)`
	`795`	`+#else`
`792`	`796`	`if (oldsize >=size)`
	`797`	`+#endif`
`793`	`798`	`{`
`794`	`799`	`#ifdefMEMORY_CONTEXT_CHECKING`
`795`	`800`	`Sizeoldrequest=chunk->requested_size;`

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Commit49e29cb

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`‎src/backend/utils/mmgr/generation.c`

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