Bug 42467: Make OS HTTP User-Agent OS spoofing configurable by pref

This commits makes it possible to disable OS spoofing in the HTTP
User-Agent header, to see if matching header and JS property improve
usability.

netwerk/protocol/http/nsHttpHandler.cpp

@@ -497,6 +497,9 @@ nsresult nsHttpHandler::Init() {
     // obsService->AddObserver(this, "net:failed-to-process-uri-content", true);
   }
 
+  Preferences::AddWeakObserver(
+      this, "privacy.resistFingerprinting.spoofOsInUserAgentHeader"_ns);
+
   MakeNewRequestTokenBucket();
   mWifiTickler = new Tickler();
   if (NS_FAILED(mWifiTickler->Init())) mWifiTickler = nullptr;
@@ -2064,6 +2067,9 @@ nsHttpHandler::Observe(nsISupports* subject, const char* topic,
     // Inform nsIOService that network is tearing down.
     gIOService->SetHttpHandlerAlreadyShutingDown();
 
+    Preferences::RemoveObserver(
+        this, "privacy.resistFingerprinting.spoofOsInUserAgentHeader"_ns);
+
     ShutdownConnectionManager();
 
     // need to reset the session start time since cache validation may
@@ -2189,6 +2195,11 @@ nsHttpHandler::Observe(nsISupports* subject, const char* topic,
     ShutdownConnectionManager();
     mConnMgr = nullptr;
     Unused << InitConnectionMgr();
+  } else if (!strcmp(topic, "nsPref:changed") &&
+             !NS_strcmp(
+                 data,
+                 u"privacy.resistFingerprinting.spoofOsInUserAgentHeader")) {
+    nsRFPService::GetSpoofedUserAgent(mSpoofedUserAgent, true);
   }
 
   return NS_OK;

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -939,11 +939,16 @@ void nsRFPService::GetSpoofedUserAgent(nsACString& userAgent,
   // https://developer.mozilla.org/en-US/docs/Web/API/NavigatorID/userAgent
   // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent
 
+  const bool spoofOs =
+      isForHTTPHeader &&
+      Preferences::GetBool(
+          "privacy.resistFingerprinting.spoofOsInUserAgentHeader", true);
+
   // These magic numbers are the lengths of the UA string literals below.
   // Assume three-digit Firefox version numbers so we have room to grow.
   size_t preallocatedLength =
       13 +
-      (isForHTTPHeader ? mozilla::ArrayLength(SPOOFED_HTTP_UA_OS)
+      (spoofOs ? mozilla::ArrayLength(SPOOFED_HTTP_UA_OS)
                : mozilla::ArrayLength(SPOOFED_UA_OS)) -
       1 + 5 + 3 + 10 + mozilla::ArrayLength(LEGACY_UA_GECKO_TRAIL) - 1 + 9 + 3 +
       2;
@@ -954,7 +959,7 @@ void nsRFPService::GetSpoofedUserAgent(nsACString& userAgent,
   // "Mozilla/5.0 (%s; rv:%d.0) Gecko/%d Firefox/%d.0"
   userAgent.AssignLiteral("Mozilla/5.0 (");
 
-  if (isForHTTPHeader) {
+  if (spoofOs) {
     userAgent.AppendLiteral(SPOOFED_HTTP_UA_OS);
   } else {
     userAgent.AppendLiteral(SPOOFED_UA_OS);