Randomize upb's map ordering

We use the address of a variable as the seed for upb's hash function, and this
allows us to get some randomness from ASLR.

The randomness is not perfect, especially since it won't necessarily help when
ASLR is not enabled. However, it seems to be enough to prevent unit tests from
relying on a deterministic map ordering.

PiperOrigin-RevId: 721950394

upb/hash/common.c

@@ -402,9 +402,14 @@ uint32_t _upb_Hash(const void* p, size_t n, uint64_t seed) {
   return Wyhash(p, n, seed, kWyhashSalt);
 }
 
-// Returns a seed for upb's hash function. For now this is just a hard-coded
+static const void* const _upb_seed;
-// constant, but we are going to randomize it soon.
+
-static uint64_t _upb_Seed(void) { return 0x69835f69597ec1cc; }
+// Returns a random seed for upb's hash function. This does not provide
+// high-quality randomness, but it should be enough to prevent unit tests from
+// relying on a deterministic map ordering. By returning the address of a
+// variable, we are able to get some randomness for free provided that ASLR is
+// enabled.
+static uint64_t _upb_Seed(void) { return (uint64_t)&_upb_seed; }
 
 static uint32_t _upb_Hash_NoSeed(const char* p, size_t n) {
   return _upb_Hash(p, n, _upb_Seed());