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, the
new version depends on pg_erand48() which returns a value in [0,1).
The possibility of returning zero creates hazards of division by zero
or trying to compute log(0) at some call sites, and it might well
break third-party modules using anl_random_fract() too.  So let's
change it to never return zero.  Spotted by Coverity.

Michael Paquier, cosmetically adjusted by me

diff --git a/src/backend/utils/misc/sampling.c b/src/backend/utils/misc/sampling.c
index aaf1d6c4108b4d668fc702cc4306c7e3b7475556..6191f7973441b2ac7dbc473cfc2058e35d0da4d3 100644 (file)
--- a/src/backend/utils/misc/sampling.c
+++ b/src/backend/utils/misc/sampling.c
@@ -237,7 +237,14 @@ sampler_random_init_state(long seed, SamplerRandomState randstate)
 double
 sampler_random_fract(SamplerRandomState randstate)
 {
-       return pg_erand48(randstate);
+       double          res;
+
+       /* pg_erand48 returns a value in [0.0 - 1.0), so we must reject 0 */
+       do
+       {
+               res = pg_erand48(randstate);
+       } while (res == 0.0);
+       return res;
 }