Add macro to disable address safety instrumentation

fasthash_accum_cstring_aligned() uses a technique, found in various
strlen() implementations, to detect a string's NUL terminator by
reading a word at at time. That triggers failures when testing with
"-fsanitize=address", at least with frontend code. To enable using
this function anywhere, add a function attribute macro to disable
such testing.

Reviewed by Jeff Davis

Discussion: https://postgr.es/m/CANWCAZbwvp7oUEkbw-xP4L0_S_WNKq-J-ucP4RCNDPJnrakUPw%40mail.gmail.com

src/include/c.h

@@ -135,6 +135,19 @@
 #define pg_nodiscard
 #endif
 
+/*
+ * This macro will disable address safety instrumentation for a function
+ * when running with "-fsanitize=address". Think twice before using this!
+ */
+#if defined(__clang__) || __GNUC__ >= 8
+#define pg_attribute_no_sanitize_address() __attribute__((no_sanitize("address")))
+#elif __has_attribute(no_sanitize_address)
+/* This would work for clang, but it's deprecated. */
+#define pg_attribute_no_sanitize_address() __attribute__((no_sanitize_address))
+#else
+#define pg_attribute_no_sanitize_address()
+#endif
+
 /*
  * Place this macro before functions that should be allowed to make misaligned
  * accesses.  Think twice before using it on non-x86-specific code!

src/include/common/hashfn_unstable.h

@@ -213,8 +213,11 @@ fasthash_accum_cstring_unaligned(fasthash_state *hs, const char *str)
  *
  * With an aligned pointer, we consume the string a word at a time.
  * Loading the word containing the NUL terminator cannot segfault since
- * allocation boundaries are suitably aligned.
+ * allocation boundaries are suitably aligned. To keep from setting
+ * off alarms with address sanitizers, exclude this function from
+ * such testing.
  */
+pg_attribute_no_sanitize_address()
 static inline size_t
 fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
 {