Bug 1864123 - Optimize flushing to base stream; r=dom-storage-reviewers,asuth,hsingh

Differential Revision: https://phabricator.services.mozilla.com/D193286

dom/quota/EncryptingOutputStream.h

@@ -24,6 +24,7 @@
 #include "nscore.h"
 
 class nsIInputStream;
+class nsIRandomGenerator;
 
 namespace mozilla::dom::quota {
 class EncryptingOutputStreamBase : public nsIOutputStream {
@@ -83,6 +84,8 @@ class EncryptingOutputStream final : public EncryptingOutputStreamBase {
   // effective block size at a block boundary.
   nsTArray<uint8_t> mBuffer;
 
+  nsCOMPtr<nsIRandomGenerator> mRandomGenerator;
+
   // The next byte in the plain data to copy incoming data to.
   size_t mNextByte = 0;
 

dom/quota/EncryptingOutputStream_impl.h

@@ -18,6 +18,8 @@
 #include "nsDebug.h"
 #include "nsError.h"
 #include "nsIAsyncOutputStream.h"
+#include "nsIRandomGenerator.h"
+#include "nsServiceManagerUtils.h"
 
 namespace mozilla::dom::quota {
 template <typename CipherStrategy>
@@ -207,6 +209,26 @@ nsresult EncryptingOutputStream<CipherStrategy>::FlushToBaseStream() {
     return NS_OK;
   }
 
+  if (mNextByte < mEncryptedBlock->MaxPayloadLength()) {
+    if (!mRandomGenerator) {
+      mRandomGenerator =
+          do_GetService("@mozilla.org/security/random-generator;1");
+      if (NS_WARN_IF(!mRandomGenerator)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    const auto payload = mEncryptedBlock->MutablePayload();
+
+    const auto unusedPayload = payload.From(mNextByte);
+
+    nsresult rv = mRandomGenerator->GenerateRandomBytesInto(
+        unusedPayload.Elements(), unusedPayload.Length());
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
   // XXX The compressing stream implementation this was based on wrote a stream
   // identifier, containing e.g. the block size. Should we do something like
   // that as well? At the moment, we don't need it, but maybe this were