Bug 1437152 - NS_ReadInputStreamToString doesn't reallocate the string if the size is passed, r=froydnj

netwerk/base/nsNetUtil.cpp

@@ -1874,11 +1874,30 @@ NS_ReadInputStreamToString(nsIInputStream* aInputStream,
         aWritten = &dummyWritten;
     }
 
+    // Nothing to do if aCount is 0.
+    if (aCount == 0) {
+        aDest.Truncate();
+        *aWritten = 0;
+        return NS_OK;
+    }
+
+    // If we have the size, we can pre-allocate the buffer.
+    if (aCount > 0) {
+        if (NS_WARN_IF(aCount  >= INT32_MAX) ||
+            NS_WARN_IF(!aDest.SetLength(aCount, mozilla::fallible))) {
+            return NS_ERROR_OUT_OF_MEMORY;
+        }
+
+        void* dest = aDest.BeginWriting();
+        return NS_ReadInputStreamToBuffer(aInputStream, &dest, aCount,
+                                          aWritten);
+    }
+
+    // If the size is unknown, BufferWriter will allocate the buffer.
     void* dest = nullptr;
     nsresult rv = NS_ReadInputStreamToBuffer(aInputStream, &dest, aCount,
                                              aWritten);
     MOZ_ASSERT_IF(NS_FAILED(rv), dest == nullptr);
-
     NS_ENSURE_SUCCESS(rv, rv);
 
     if (!dest) {

netwerk/test/gtest/TestReadStreamToString.cpp

@@ -3,6 +3,28 @@
 #include "nsCOMPtr.h"
 #include "nsNetUtil.h"
 
+// Here we test the reading a pre-allocated size
+TEST(TestReadStreamToString, SyncStreamPreAllocatedSize) {
+  nsCString buffer;
+  buffer.AssignLiteral("Hello world!");
+
+  nsCOMPtr<nsIInputStream> stream;
+  ASSERT_EQ(NS_OK, NS_NewCStringInputStream(getter_AddRefs(stream), buffer));
+
+  uint64_t written;
+  nsAutoCString result;
+  result.SetLength(5);
+
+  void* ptr = result.BeginWriting();
+
+  ASSERT_EQ(NS_OK, NS_ReadInputStreamToString(stream, result, 5, &written));
+  ASSERT_EQ((uint64_t)5, written);
+  ASSERT_TRUE(nsCString(buffer.get(), 5).Equals(result));
+
+  // The pointer should be equal: no relocation.
+  ASSERT_EQ(ptr, result.BeginWriting());
+}
+
 // Here we test the reading the full size of a sync stream
 TEST(TestReadStreamToString, SyncStreamFullSize) {
   nsCString buffer;