Add partitionAllocGetSize() for determining actual size of allocation

Source/wtf/PartitionAllocTest.cpp

Index: Source/wtf/PartitionAllocTest.cpp
diff --git a/Source/wtf/PartitionAllocTest.cpp b/Source/wtf/PartitionAllocTest.cpp
index aef6f5c86b35ca3d8c32e08be56288b9d716ef1f..232c85a208224a8f22f9283c66e898dd0be13c1a 100644
--- a/Source/wtf/PartitionAllocTest.cpp
+++ b/Source/wtf/PartitionAllocTest.cpp
@@ -574,6 +574,56 @@ TEST(WTF_PartitionAlloc, GenericAllocSizes)
     TestShutdown();
 }
 
+// Test that we can fetch the real allocated size after an allocation.
+TEST(WTF_PartitionAlloc, GenericAllocGetSize)
+{
+    TestSetup();
+
+    void* ptr;
+    size_t requestedSize, actualSize;
+
+    EXPECT_TRUE(partitionAllocSupportsGetSize());
+
+    // Allocate something small.
+    requestedSize = 511 - kExtraAllocSize;
+    ptr = partitionAllocGeneric(genericAllocator.root(), requestedSize);
+    EXPECT_TRUE(ptr);
+    actualSize = partitionAllocGetSize(ptr);
+    EXPECT_LT(requestedSize, actualSize);
+    partitionFreeGeneric(genericAllocator.root(), ptr);
+
+    // Allocate a size that should be a perfect match for a bucket, because it
+    // is an exact power of 2.
+    requestedSize = (256 * 1024) - kExtraAllocSize;
+    ptr = partitionAllocGeneric(genericAllocator.root(), requestedSize);
+    EXPECT_TRUE(ptr);
+    actualSize = partitionAllocGetSize(ptr);
+    EXPECT_EQ(requestedSize, actualSize);
+    partitionFreeGeneric(genericAllocator.root(), ptr);
+
+    // Allocate a size that is a system page smaller than a bucket. GetSize()
+    // should return a larger size than we asked for now.
+    requestedSize = (256 * 1024) - WTF::kSystemPageSize - kExtraAllocSize;
+    ptr = partitionAllocGeneric(genericAllocator.root(), requestedSize);
+    EXPECT_TRUE(ptr);
+    actualSize = partitionAllocGetSize(ptr);
+    EXPECT_EQ(requestedSize + WTF::kSystemPageSize, actualSize);
+    // Check that we can write at the end of the reported size too.
+    char* charPtr = reinterpret_cast<char*>(ptr);
+    *(charPtr + (actualSize - 1)) = 'A';
+    partitionFreeGeneric(genericAllocator.root(), ptr);
+
+    // Allocate something very large, and uneven.
+    requestedSize = 512 * 1024 * 1024 - 1;
+    ptr = partitionAllocGeneric(genericAllocator.root(), requestedSize);
+    EXPECT_TRUE(ptr);
+    actualSize = partitionAllocGetSize(ptr);
+    EXPECT_LT(requestedSize, actualSize);
+    partitionFreeGeneric(genericAllocator.root(), ptr);
+
+    TestShutdown();
+}
+
 // Test the realloc() contract.
 TEST(WTF_PartitionAlloc, Realloc)
 {