Bug 1550108 - Avoid decompressing entries just to check if they exist r=kmag

This will not behave exactly the same if we had previously written bad
data for the entry that would fail to decompress. I imagine this is rare
enough, and the consequences are not severe enough, that this should be
fine.

Differential Revision: https://phabricator.services.mozilla.com/D30643

--HG--
extra : moz-landing-system : lando

dom/xul/nsXULPrototypeCache.cpp

@@ -353,10 +353,9 @@ nsresult nsXULPrototypeCache::HasData(nsIURI* uri, bool* exists) {
     return NS_OK;
   }
   UniquePtr<char[]> buf;
-  uint32_t len;
   StartupCache* sc = StartupCache::GetSingleton();
   if (sc) {
-    rv = sc->GetBuffer(spec.get(), &buf, &len);
+    *exists = sc->HasEntry(spec.get());
   } else {
     *exists = false;
     return NS_OK;

startupcache/StartupCache.cpp

@@ -230,6 +230,21 @@ nsresult GetBufferFromZipArchive(nsZipArchive* zip, bool doCRC, const char* id,
 
 } /* anonymous namespace */
 
+bool StartupCache::HasEntry(const char* id) {
+  AUTO_PROFILER_LABEL("StartupCache::HasEntry", OTHER);
+
+  MOZ_ASSERT(NS_IsMainThread(), "Startup cache only available on main thread");
+  WaitOnWriteThread();
+
+  if (!mStartupWriteInitiated) {
+    CacheEntry* entry;
+    mTable.Get(nsDependentCString(id), &entry);
+    return !!entry;
+  }
+
+  return mArchive && mArchive->GetItem(id);
+}
+
 // NOTE: this will not find a new entry until it has been written to disk!
 // Consumer should take ownership of the resulting buffer.
 nsresult StartupCache::GetBuffer(const char* id, UniquePtr<char[]>* outbuf,

startupcache/StartupCache.h

@@ -109,6 +109,9 @@ class StartupCache : public nsIMemoryReporter {
 
   // StartupCache methods. See above comments for a more detailed description.
 
+  // true if the archive has an entry for the buffer or not.
+  bool HasEntry(const char* id);
+
   // Returns a buffer that was previously stored, caller takes ownership.
   nsresult GetBuffer(const char* id, UniquePtr<char[]>* outbuf,
                      uint32_t* length);