[tsan] Fix behavior of realloc(nullptr, 0) on Darwin

[kubamracek]

Summary

On Darwin, there are some apps that rely on realloc(nullptr, 0) returning a valid pointer. TSan currently returns nullptr in this case, let's fix it to avoid breaking binary compatibility.

[dvyukov]

LGTM with a nit

test/tsan/Darwin/realloc-zero.cc

16

add \n

[kubamracek]

I also submitted a follow-up change to update the tsan_mman_test.cc unit test in r277463.

lib/tsan/rtl/tsan_mman.cc

void OnUserFree(ThreadState *thr, uptr pc, uptr p, bool write) {
   CHECK_NE(p, (void*)0);
   uptr sz = ctx->metamap.FreeBlock(thr->proc(), p);
   DPrintf("#%d: free(%p, %zu)\n", thr->tid, p, sz);
   if (write && thr->ignore_reads_and_writes == 0)
     MemoryRangeFreed(thr, pc, (uptr)p, sz);
 }
 
 void *user_realloc(ThreadState *thr, uptr pc, void *p, uptr sz) {
-  void *p2 = 0;
   // FIXME: Handle "shrinking" more efficiently,
   // it seems that some software actually does this.
-  if (sz) {
-    p2 = user_alloc(thr, pc, sz);
-    if (p2 == 0)
-      return 0;
-    if (p) {
-      uptr oldsz = user_alloc_usable_size(p);
-      internal_memcpy(p2, p, min(oldsz, sz));
-    }
-  }
-  if (p)
+  void *p2 = user_alloc(thr, pc, sz);
     user_free(thr, pc, p);
+  }
   return p2;
 }
 
 uptr user_alloc_usable_size(const void *p) {
   if (p == 0)
     return 0;
   MBlock *b = ctx->metamap.GetBlock((uptr)p);
   if (!b)

test/tsan/Darwin/realloc-zero.cc

+// Test that realloc(nullptr, 0) return a non-NULL pointer.
+
+// RUN: %clang_tsan %s -o %t
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include <malloc/malloc.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+
+int main() {
+  void *p = realloc(nullptr, 0);
+  if (!p) {
+    abort();
+  }
+  fprintf(stderr, "Okay.");
+  return 0;
+}
+
+// CHECK: Okay.