Bug 1975014: Add a pref for Compression Dictionaries r=necko-reviewers,kershaw

Differential Revision: https://phabricator.services.mozilla.com/D255666

Author: jesup

modules/libpref/init/StaticPrefList.yaml

@@ -15751,6 +15751,12 @@
   value: false
   mirror: always
 
+# Enable HTTP Compression Dictionary support
+- name: network.http.dictionaries.enable
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "nglayout."
 #---------------------------------------------------------------------------

netwerk/protocol/http/nsHttpChannel.cpp

@@ -5935,7 +5935,9 @@ nsresult DoAddCacheEntryHeaders(nsHttpChannel* self, nsICacheEntry* entry,
   if (NS_FAILED(rv)) return rv;
 
   // If this is being marked as a dictionary, add it to the list
-  self->ParseDictionary(entry, responseHead);
+  if (StaticPrefs::network_http_dictionaries_enable() && self->IsHTTPS()) {
+    self->ParseDictionary(entry, responseHead);
+  }
 
   // Indicate we have successfully finished setting metadata on the cache entry.
   rv = entry->MetaDataReady();

netwerk/protocol/http/nsHttpHandler.cpp

@@ -665,9 +665,8 @@ nsresult nsHttpHandler::AddAcceptAndDictionaryHeaders(
     // XXX This would require that AddAcceptAndDictionaryHeaders be effectively
     // async, perhaps by passing a lambda to call AddAcceptAndDictionaryHeaders
     // and then unblock the request
-    aDict =
-        mDictionaryCache ? mDictionaryCache->GetDictionaryFor(aURI) : nullptr;
-    if (aDict) {
+    if (StaticPrefs::network_http_dictionaries_enable() &&
+        (aDict = mDictionaryCache->GetDictionaryFor(aURI))) {
       rv = aRequest->SetHeader(nsHttp::Accept_Encoding,
                                mDictionaryAcceptEncodings, false,
                                nsHttpHeaderArray::eVarietyRequestOverride);