Bug 1525762: Part 1l - Don't delay serving image resources until extensions are ready. r=aswan

Since we want themes to be loaded before the main browser window is loaded, we
really need their resources to be ready as soon as possible. We typically
delay serving moz-extension: requests until the extension is ready, since
extension page and CSS loads rely on the extension being fully initialized.

Image loads, though, are perfectly safe to load as early as we need, so this
patch whitelists them to bypass the delayed load logic.

--HG--
extra : rebase_source : 0f17a884e2981cc226e0004b40f1cfbb79d66c84
extra : histedit_source : 55d9f5ca67f135b1a62797b673bdb3a4ed8e3de5

netwerk/protocol/res/ExtensionProtocolHandler.cpp

@@ -6,6 +6,7 @@
 
 #include "ExtensionProtocolHandler.h"
 
+#include "mozilla/BinarySearch.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/Promise.h"
@@ -41,6 +42,7 @@
 #include "nsIOutputStream.h"
 #include "nsIStreamConverterService.h"
 #include "nsNetUtil.h"
+#include "nsReadableUtils.h"
 #include "nsURLHelper.h"
 #include "prio.h"
 #include "SimpleChannel.h"
@@ -54,6 +56,30 @@
 using mozilla::dom::Promise;
 using mozilla::ipc::FileDescriptor;
 
+// A list of file extensions containing purely static data, which can be loaded
+// from an extension before the extension is fully ready. The main purpose of
+// this is to allow image resources from theme XPIs to load quickly during
+// browser startup.
+//
+// The layout of this array is chosen in order to prevent the need for runtime
+// relocation, which an array of char* pointers would require. It also has the
+// benefit of being more compact when the difference in length between the
+// longest and average string is less than 8 bytes. The length of the
+// char[] array must match the size of the longest entry in the list.
+//
+// This list must be kept sorted.
+static const char sStaticFileExtensions[][5] = {
+  // clang-format off
+  "bmp",
+  "gif",
+  "ico",
+  "jpeg",
+  "jpg",
+  "png",
+  "svg",
+  // clang-format on
+};
+
 namespace mozilla {
 
 namespace net {
@@ -460,9 +486,10 @@ nsresult ExtensionProtocolHandler::SubstituteChannel(nsIURI* aURI,
 
   nsAutoCString ext;
   MOZ_TRY(url->GetFileExtension(ext));
+  ToLowerCase(ext);
 
   nsCOMPtr<nsIChannel> channel;
-  if (ext.LowerCaseEqualsLiteral("css")) {
+  if (ext.EqualsLiteral("css")) {
     // Filter CSS files to replace locale message tokens with localized strings.
     static const auto convert = [](nsIStreamListener* listener,
                                    nsIChannel* channel,
@@ -502,6 +529,16 @@ nsresult ExtensionProtocolHandler::SubstituteChannel(nsIURI* aURI,
           return RequestOrReason(origChannel);
         });
   } else if (readyPromise) {
+    size_t matchIdx;
+    if (BinarySearchIf(
+            sStaticFileExtensions, 0, ArrayLength(sStaticFileExtensions),
+            [&ext](const char* aOther) { return ext.Compare(aOther); },
+            &matchIdx)) {
+      // This is a static resource that shouldn't depend on the extension being
+      // ready. Don't bother waiting for it.
+      return NS_OK;
+    }
+
     channel = NS_NewSimpleChannel(
         aURI, aLoadInfo, *result,
         [readyPromise](nsIStreamListener* listener, nsIChannel* channel,