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Make sampler_random_fract() actually obey its API contract.

This function is documented to return a value in the range (0,1),which is what its predecessor anl_random_fract() did. However, thenew version depends on pg_erand48() which returns a value in [0,1).The possibility of returning zero creates hazards of division by zeroor trying to compute log(0) at some call sites, and it might wellbreak third-party modules using anl_random_fract() too. So let'schange it to never return zero. Spotted by Coverity.Michael Paquier, cosmetically adjusted by me

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src/backend/utils/misc
- sampling.c

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`‎src/backend/utils/misc/sampling.c`

Lines changed: 8 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -237,7 +237,14 @@ sampler_random_init_state(long seed, SamplerRandomState randstate)`
`237`	`237`	`double`
`238`	`238`	`sampler_random_fract(SamplerRandomStaterandstate)`
`239`	`239`	`{`
`240`		`-returnpg_erand48(randstate);`
	`240`	`+doubleres;`
	`241`	`+`
	`242`	`+/* pg_erand48 returns a value in [0.0 - 1.0), so we must reject 0 */`
	`243`	`+do`
	`244`	`+{`
	`245`	`+res=pg_erand48(randstate);`
	`246`	`+}while (res==0.0);`
	`247`	`+returnres;`
`241`	`248`	`}`
`242`	`249`
`243`	`250`

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Commitd7c19d6

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`‎src/backend/utils/misc/sampling.c`

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