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Increase the number of possible random seeds per time period.

Commit197e4af refactored the initialization of the libc random()seed, but reduced the number of possible seeds values that could bechosen in a given time period. This negation of the effects ofcommit98c5065 was unintentional. Replace with code thatshifts the fast moving timestamp bits left, similar to the originalalgorithm (though not the previous float-tolerating coding, whichis no longer necessary).Author: Thomas MunroReported-by: Noah MischReviewed-by: Tom LaneDiscussion:https://postgr.es/m/20181112083358.GA1073796%40rfd.leadboat.com

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src/backend/postmaster
- postmaster.c

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`‎src/backend/postmaster/postmaster.c`

Lines changed: 10 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -2525,8 +2525,16 @@ InitProcessGlobals(void)`
`2525`	`2525`	`random_start_time.tv_usec=0;`
`2526`	`2526`	`#endif`
`2527`	`2527`
`2528`		`-/* Set a different seed for random() in every backend. */`
`2529`		`-srandom((unsignedint)MyProcPid ^ (unsignedint)MyStartTimestamp);`
	`2528`	`+/*`
	`2529`	`+ * Set a different seed for random() in every backend. Since PIDs and`
	`2530`	`+ * timestamps tend to change more frequently in their least significant`
	`2531`	`+ * bits, shift the timestamp left to allow a larger total number of seeds`
	`2532`	`+ * in a given time period. Since that would leave only 20 bits of the`
	`2533`	`+ * timestamp that cycle every ~1 second, also mix in some higher bits.`
	`2534`	`+ */`
	`2535`	`+srandom(((unsignedint)MyProcPid) ^`
	`2536`	`+((unsignedint)MyStartTimestamp <<12) ^`
	`2537`	`+((unsignedint)MyStartTimestamp >>20));`
`2530`	`2538`	`}`
`2531`	`2539`
`2532`	`2540`

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Commit5b0ce3e

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`‎src/backend/postmaster/postmaster.c`

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