Fix jemalloc forward declarations (#6613)

Summary:
Add `nothrow` attribute to match declarations in jemalloc.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6613

Reviewed By: igorsugak

Differential Revision: D20749490

fbshipit-source-id: 9ac8df27f7b4268f27b32b130c23ce8a1f772b3a

port/jemalloc_helper.h

@@ -40,21 +40,21 @@ static inline bool HasJemalloc() { return true; }
 
 // Declare non-standard jemalloc APIs as weak symbols. We can null-check these
 // symbols to detect whether jemalloc is linked with the binary.
-extern "C" void* mallocx(size_t, int) __attribute__((__weak__));
-extern "C" void* rallocx(void*, size_t, int) __attribute__((__weak__));
-extern "C" size_t xallocx(void*, size_t, size_t, int) __attribute__((__weak__));
-extern "C" size_t sallocx(const void*, int) __attribute__((__weak__));
-extern "C" void dallocx(void*, int) __attribute__((__weak__));
-extern "C" void sdallocx(void*, size_t, int) __attribute__((__weak__));
-extern "C" size_t nallocx(size_t, int) __attribute__((__weak__));
+extern "C" void* mallocx(size_t, int) __attribute__((__nothrow__, __weak__));
+extern "C" void* rallocx(void*, size_t, int) __attribute__((__nothrow__, __weak__));
+extern "C" size_t xallocx(void*, size_t, size_t, int) __attribute__((__nothrow__, __weak__));
+extern "C" size_t sallocx(const void*, int) __attribute__((__nothrow__, __weak__));
+extern "C" void dallocx(void*, int) __attribute__((__nothrow__, __weak__));
+extern "C" void sdallocx(void*, size_t, int) __attribute__((__nothrow__, __weak__));
+extern "C" size_t nallocx(size_t, int) __attribute__((__nothrow__, __weak__));
 extern "C" int mallctl(const char*, void*, size_t*, void*, size_t)
-    __attribute__((__weak__));
+    __attribute__((__nothrow__, __weak__));
 extern "C" int mallctlnametomib(const char*, size_t*, size_t*)
-    __attribute__((__weak__));
+    __attribute__((__nothrow__, __weak__));
 extern "C" int mallctlbymib(const size_t*, size_t, void*, size_t*, void*,
-                            size_t) __attribute__((__weak__));
+                            size_t) __attribute__((__nothrow__, __weak__));
 extern "C" void malloc_stats_print(void (*)(void*, const char*), void*,
-                                   const char*) __attribute__((__weak__));
+                                   const char*) __attribute__((__nothrow__, __weak__));
 extern "C" size_t malloc_usable_size(JEMALLOC_USABLE_SIZE_CONST void*)
     JEMALLOC_CXX_THROW __attribute__((__weak__));