gh-159111: Mark remaining CPython-specific changes in vendored mimalloc with a comment

Author: corona10

Include/internal/mimalloc/mimalloc/internal.h

@@ -102,6 +102,8 @@ terms of the MIT license. A copy of the license can be found in the file
 #define mi_decl_externc
 #endif
 
+// CPython: upstream defines __wasi__ on Emscripten here. It is removed so that
+// CPython code can distinguish the two platforms (gh-158628).
 
 // --------------------------------------------------------------------------
 // Internal functions

Objects/mimalloc/os.c

@@ -276,6 +276,7 @@ static void* mi_os_prim_alloc_aligned(size_t size, size_t alignment, bool commit
   else {
     // if not aligned, free it, overallocate, and unmap around it
     #if !MI_TRACK_ASAN
+    // CPython: downgraded from _mi_warning_message, the warning is noisy in the test suite (gh-112027)
     _mi_verbose_message("unable to allocate aligned OS memory directly, fall back to over-allocation (size: 0x%zx bytes, address: %p, alignment: 0x%zx, commit: %d)\n", size, p, alignment, commit);
     #endif
     if (p != NULL) { mi_os_prim_free(p, size, (commit ? size : 0)); }

Objects/mimalloc/prim/unix/prim.c

@@ -124,7 +124,7 @@ static bool unix_detect_overcommit(void) {
   #if defined(__linux__)
     int fd = mi_prim_open("/proc/sys/vm/overcommit_memory", O_RDONLY);
     if (fd >= 0) {
-      char buf[32] = {0};
+      char buf[32] = {0};  // CPython: zero-initialize to avoid an MSAN report
       ssize_t nread = mi_prim_read(fd, &buf, sizeof(buf));
       mi_prim_close(fd);
       // <https://www.kernel.org/doc/Documentation/vm/overcommit-accounting>

Objects/mimalloc/random.c

@@ -178,7 +178,7 @@ uintptr_t _mi_os_random_weak(uintptr_t extra_seed) {
 }
 
 static void mi_random_init_ex(mi_random_ctx_t* ctx, bool use_weak) {
-  uint8_t key[32] = {0};
+  uint8_t key[32] = {0};  // CPython: zero-initialize to avoid an MSAN report
   if (use_weak || !_mi_prim_random_buf(key, sizeof(key))) {
     // if we fail to get random data from the OS, we fall back to a
     // weak random source based on the current time