[blink] Harden inspector ScriptResource content access

Check for an edge case where a ScriptResource may have a null resource
buffer without having been loaded. As part of this, extract the
inspector's logic for accessing a ScriptResource's text into the
ScriptResource, as it relies on ScriptResource internal state.

Bug: 865098
Bug: 899217
Change-Id: I5ceb6f105e21e4eeb7ce5f9bc82c7d7c7f7f6aad
Reviewed-on: https://chromium-review.googlesource.com/c/1352183
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611255}

third_party/blink/renderer/core/inspector/inspector_page_agent.cc

@@ -348,9 +348,7 @@ bool InspectorPageAgent::CachedResourceContent(Resource* cached_resource,
       return true;
     case blink::ResourceType::kScript:
       MaybeEncodeTextContent(
-          cached_resource->ResourceBuffer()
-              ? ToScriptResource(cached_resource)->DecodedText()
-              : ToScriptResource(cached_resource)->SourceText().ToString(),
+          ToScriptResource(cached_resource)->TextForInspector(),
           cached_resource->ResourceBuffer(), result, base64_encoded);
       return true;
     default:

third_party/blink/renderer/core/loader/resource/script_resource.cc

@@ -145,6 +145,35 @@ const ParkableString& ScriptResource::SourceText() {
   return source_text_;
 }
 
+String ScriptResource::TextForInspector() const {
+  // If the resource buffer exists, we can safely return the decoded text.
+  if (ResourceBuffer())
+    return DecodedText();
+
+  // If there is no resource buffer, then we have three cases.
+  // TODO(crbug.com/865098): Simplify the below code and remove the CHECKs once
+  // the assumptions are confirmed.
+
+  if (IsLoaded()) {
+    CHECK(IsFinishedInternal());
+    if (!source_text_.IsNull()) {
+      // 1. We have finished loading, and have already decoded the buffer into
+      //    the source text and cleared the resource buffer to save space.
+      return source_text_.ToString();
+    }
+
+    // 2. We have finished loading with no data received, so no streaming ever
+    //    happened or streaming was suppressed.
+    CHECK(!streamer_ || streamer_->StreamingSuppressedReason() ==
+                            ScriptStreamer::kScriptTooSmall);
+    return "";
+  }
+
+  // 3. We haven't started loading, and actually haven't received any data yet
+  //    at all to initialise the resource buffer, so the resource is empty.
+  return "";
+}
+
 SingleCachedMetadataHandler* ScriptResource::CacheHandler() {
   return static_cast<SingleCachedMetadataHandler*>(Resource::CacheHandler());
 }

third_party/blink/renderer/core/loader/resource/script_resource.h

@@ -132,6 +132,10 @@ class CORE_EXPORT ScriptResource final : public TextResource {
 
   const ParkableString& SourceText();
 
+  // Get the resource's current text. This can return partial data, so should
+  // not be used outside of the inspector.
+  String TextForInspector() const;
+
   SingleCachedMetadataHandler* CacheHandler();
 
   // Gets the script streamer from the ScriptResource, clearing the resource's