[test] Make Multi-Mapped Mock Allocator threadsafe

TSan complains in "isolates" tests otherwise. Also further reduce
virtual memory requirements of the sample test to address flaky
allocation failures on 32-bit platforms.

Change-Id: I26c9a59965009d7083876b4ff4836ee879d33350
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000138
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65750}

src/d8/d8.cc

@@ -257,6 +257,7 @@ class MultiMappedAllocator : public ArrayBufferAllocatorBase {
         FATAL("mremap failed with error %d: %s", errno, strerror(errno));
       }
     }
+    base::MutexGuard lock_guard(&regions_mutex_);
     regions_[virtual_alloc] = real_alloc;
     return virtual_alloc;
   }
@@ -265,6 +266,7 @@ class MultiMappedAllocator : public ArrayBufferAllocatorBase {
     if (length < kChunkSize) {
       return ArrayBufferAllocatorBase::Free(data, length);
     }
+    base::MutexGuard lock_guard(&regions_mutex_);
     void* real_alloc = regions_[data];
     munmap(real_alloc, kChunkSize);
     size_t rounded_length = RoundUp(length, kChunkSize);
@@ -277,6 +279,7 @@ class MultiMappedAllocator : public ArrayBufferAllocatorBase {
   static constexpr size_t kChunkSize = 2 * 1024 * 1024;
 
   std::unordered_map<void*, void*> regions_;
+  base::Mutex regions_mutex_;
 };
 
 #endif  // V8_OS_LINUX

test/mjsunit/regress/regress-crbug-1041232.js

@@ -6,7 +6,7 @@
 
 // Chosen for stress runs on 32-bit systems. Physical memory is not an issue
 // thanks to the mock allocator, but virtual address space is still limited.
-let kSize = 512 * 1024 * 1024;
+let kSize = 128 * 1024 * 1024;
 // Must be >= MultiMappedMockAllocator::kChunkSize in d8.cc.
 let kChunkSize = 2 * 1024 * 1024;
 let a = new Uint8Array(kSize);