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Replace memcpy() calls in xlog.c critical sections with struct assignments.

This gets rid of a dangerous-looking use of the not-volatile XLogCtlpointer in a couple of spinlock-protected sections, where the normalcoding rule is that you should only access shared memory through apointer-to-volatile. I think the risk is only hypothetical not actual,since for there to be a bug the compiler would have to move the spinlockacquire or release across the memcpy() call, which one sincerely hopesit will not. Still, it looks cleaner this way.Per comment from Daniel Farina and subsequent discussion.

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src/backend/access/transam
- xlog.c

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`‎src/backend/access/transam/xlog.c`

Lines changed: 3 additions & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -8158,8 +8158,8 @@ RecoveryRestartPoint(const CheckPoint *checkPoint)`
`8158`	`8158`	`* work out the next time it wants to perform a restartpoint.`
`8159`	`8159`	`*/`
`8160`	`8160`	`SpinLockAcquire(&xlogctl->info_lck);`
`8161`		`-XLogCtl->lastCheckPointRecPtr=ReadRecPtr;`
`8162`		`-memcpy(&XLogCtl->lastCheckPoint,checkPoint,sizeof(CheckPoint));`
	`8161`	`+xlogctl->lastCheckPointRecPtr=ReadRecPtr;`
	`8162`	`+xlogctl->lastCheckPoint=*checkPoint;`
`8163`	`8163`	`SpinLockRelease(&xlogctl->info_lck);`
`8164`	`8164`	`}`
`8165`	`8165`
`@@ -8194,7 +8194,7 @@ CreateRestartPoint(int flags)`
`8194`	`8194`	`/* Get a local copy of the last safe checkpoint record. */`
`8195`	`8195`	`SpinLockAcquire(&xlogctl->info_lck);`
`8196`	`8196`	`lastCheckPointRecPtr=xlogctl->lastCheckPointRecPtr;`
`8197`		`-memcpy(&lastCheckPoint,&XLogCtl->lastCheckPoint,sizeof(CheckPoint));`
	`8197`	`+lastCheckPoint=xlogctl->lastCheckPoint;`
`8198`	`8198`	`SpinLockRelease(&xlogctl->info_lck);`
`8199`	`8199`
`8200`	`8200`	`/*`

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Commit2a2352e

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`‎src/backend/access/transam/xlog.c`

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