Avoid memcpy() with same source and destination during relmapper init.

tglsfdc · tglsfdc · commit 722eb325b951 · 2020-12-18T15:46:44.000-05:00
A narrow reading of the C standard says that memcpy(x,x,n) is undefined, although it's hard to envision an implementation that would really misbehave. However, analysis tools such as valgrind might whine about this; accordingly, let's band-aid relmapper.c to not do it. See also 5b63050, d3f4e8a, ad7b48e, and other similar fixes. Apparently, none of those folk tried valgrinding initdb? This has been like this for long enough that I'm surprised it hasn't been reported before. Back-patch, just in case anybody wants to use a back branch on a platform that complains about this; we back-patched those earlier fixes too. Discussion: https://postgr.es/m/161790.1608310142@sss.pgh.pa.us
diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
@@ -928,8 +928,15 @@ write_relmap_file(bool shared, RelMapFile *newmap,
 		}
 	}
 
-	/* Success, update permanent copy */
-	memcpy(realmap, newmap, sizeof(RelMapFile));
+	/*
+	 * Success, update permanent copy.  During bootstrap, we might be working
+	 * on the permanent copy itself, in which case skip the memcpy() to avoid
+	 * invoking nominally-undefined behavior.
+	 */
+	if (realmap != newmap)
+		memcpy(realmap, newmap, sizeof(RelMapFile));
+	else
+		Assert(!send_sinval);	/* must be bootstrapping */
 
 	/* Critical section done */
 	if (write_wal)
