Avoid crashes when system libraries use the purgeable zone allocator

src/jemalloc.c

@@ -1175,7 +1175,7 @@ je_valloc(size_t size)
  * to inconsistently reference libc's malloc(3)-compatible functions
  * (https://bugzilla.mozilla.org/show_bug.cgi?id=493541).
  *
- * These definitions interpose hooks in glibc.  The functions are actually
+ * These definitions interpose hooks in glibc.  The functions are actually
  * passed an extra argument for the caller return address, which will be
  * ignored.
  */

src/zone.c

@@ -3,6 +3,13 @@
 #  error "This source file is for zones on Darwin (OS X)."
 #endif
 
+/*
+ * The malloc_default_purgeable_zone function is only available on >= 10.6.
+ * We need to check whether it is present at runtime, thus the weak_import.
+ */
+extern malloc_zone_t *malloc_default_purgeable_zone(void)
+JEMALLOC_ATTR(weak_import);
+
 /******************************************************************************/
 /* Data. */
 
@@ -207,6 +214,20 @@ register_zone(void)
 #endif
 #endif
 
+	/*
+	 * The default purgeable zone is created lazily by OSX's libc.  It uses
+	 * the default zone when it is created for "small" allocations
+	 * (< 15 KiB), but assumes the default zone is a scalable_zone.  This
+	 * obviously fails when the default zone is the jemalloc zone, so
+	 * malloc_default_purgeable_zone is called beforehand so that the
+	 * default purgeable zone is created when the default zone is still
+	 * a scalable_zone.  As purgeable zones only exist on >= 10.6, we need
+	 * to check for the existence of malloc_default_purgeable_zone() at
+	 * run time.
+	 */
+	if (malloc_default_purgeable_zone != NULL)
+		malloc_default_purgeable_zone();
+
 	/* Register the custom zone.  At this point it won't be the default. */
 	malloc_zone_register(&zone);