Fix unavailable MAP_ANONYMOUS

Signed-off-by: Alexandre Terrasa <alexandre.terrasa@shopify.com>

Author: Morriar

src/util/rbs_allocator.c

@@ -22,16 +22,30 @@
     #include <sys/mman.h>
 #endif
 
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__sun)
-#define MAP_ANONYMOUS MAP_ANON
-#endif
-
-
 struct rbs_allocator {
     uintptr_t heap_ptr;
     uintptr_t size;
 };
 
+static void* portable_mmap_anon(size_t size) {
+    void* ptr;
+
+    #if defined(MAP_ANONYMOUS)
+        ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+    #elif defined(MAP_ANON)
+        ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
+    #else
+        /* Fallback to /dev/zero for systems without anonymous mapping */
+        int fd = open("/dev/zero", O_RDWR);
+        if (fd == -1) return MAP_FAILED;
+
+        ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
+        close(fd);  /* Can close fd after mapping */
+    #endif
+
+    return ptr;
+}
+
 static size_t get_system_page_size(void) {
 #ifdef _WIN32
     SYSTEM_INFO si;
@@ -49,8 +63,7 @@ static void *map_memory(size_t size) {
     LPVOID result = VirtualAlloc(NULL, size, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
     rbs_assert(result != NULL, "VirtualAlloc failed");
 #else
-    void *result = mmap(NULL, size, PROT_READ | PROT_WRITE,
-                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    void *result = portable_mmap_anon(size);
     rbs_assert(result != MAP_FAILED, "mmap failed");
 #endif
     return result;