Prevent Valgrind Memcheck errors around px_acquire_system_randomness().

This function uses uninitialized stack and heap buffers as supplementary
entropy sources.  Mark them so Memcheck will not complain.  Back-patch
to 9.4, where Valgrind Memcheck cooperation first appeared.

Marko Tiikkaja

contrib/pgcrypto/random.c

@@ -32,6 +32,7 @@
 #include "postgres.h"
 
 #include "px.h"
+#include "utils/memdebug.h"
 
 /* how many bytes to ask from system random provider */
 #define RND_BYTES  32
@@ -195,7 +196,7 @@ try_unix_std(uint8 *dst)
 	memcpy(dst, (uint8 *) &x, sizeof(x));
 	dst += sizeof(x);
 
-	/* let's be desperate */
+	/* hash of uninitialized stack and heap allocations */
 	res = px_find_digest("sha1", &md);
 	if (res >= 0)
 	{
@@ -203,8 +204,10 @@ try_unix_std(uint8 *dst)
 		uint8		stack[8192];
 		int			alloc = 32 * 1024;
 
+		VALGRIND_MAKE_MEM_DEFINED(stack, sizeof(stack));
 		px_md_update(md, stack, sizeof(stack));
 		ptr = px_alloc(alloc);
+		VALGRIND_MAKE_MEM_DEFINED(ptr, alloc);
 		px_md_update(md, ptr, alloc);
 		px_free(ptr);