Bug 817381 - Add sThreadCount and sTelemetryMaxThreadCount to count the number of maximum concurrent threads in a download session and implemented Telemetry for the same. r=biesi

netwerk/base/src/BackgroundFileSaver.cpp

@@ -16,6 +16,7 @@
 #include "nsXPCOMStrings.h"
 
 #include "BackgroundFileSaver.h"
+#include "mozilla/Telemetry.h"
 
 namespace mozilla {
 namespace net {
@@ -67,6 +68,9 @@ private:
 ////////////////////////////////////////////////////////////////////////////////
 //// BackgroundFileSaver
 
+uint32_t BackgroundFileSaver::sThreadCount = 0;
+uint32_t BackgroundFileSaver::sTelemetryMaxThreadCount = 0;
+
 BackgroundFileSaver::BackgroundFileSaver()
 : mControlThread(nullptr)
 , mWorkerThread(nullptr)
@@ -117,6 +121,8 @@ BackgroundFileSaver::virtualDestroyNSSReference()
 nsresult
 BackgroundFileSaver::Init()
 {
+  MOZ_ASSERT(NS_IsMainThread(), "This should be called on the main thread");
+
   nsresult rv;
 
   rv = NS_NewPipe2(getter_AddRefs(mPipeInputStream),
@@ -130,6 +136,11 @@ BackgroundFileSaver::Init()
   rv = NS_NewThread(getter_AddRefs(mWorkerThread));
   NS_ENSURE_SUCCESS(rv, rv);
 
+  sThreadCount++;
+  if (sThreadCount > sTelemetryMaxThreadCount) {
+    sTelemetryMaxThreadCount = sThreadCount;
+  }
+
   return NS_OK;
 }
 
@@ -600,6 +611,8 @@ BackgroundFileSaver::NotifyTargetChange(nsIFile *aTarget)
 nsresult
 BackgroundFileSaver::NotifySaveComplete()
 {
+  MOZ_ASSERT(NS_IsMainThread(), "This should be called on the main thread");
+
   nsresult status;
   {
     MutexAutoLock lock(mLock);
@@ -619,6 +632,18 @@ BackgroundFileSaver::NotifySaveComplete()
   // reference to it through the event object.
   mWorkerThread->Shutdown();
 
+  sThreadCount--;
+
+  // When there are no more active downloads, we consider the download session
+  // finished. We record the maximum number of concurrent downloads reached
+  // during the session in a telemetry histogram, and we reset the maximum
+  // thread counter for the next download session
+  if (sThreadCount == 0) {
+    Telemetry::Accumulate(Telemetry::BACKGROUNDFILESAVER_THREAD_COUNT,
+                          sTelemetryMaxThreadCount);
+    sTelemetryMaxThreadCount = 0;
+  }
+
   return NS_OK;
 }
 

netwerk/base/src/BackgroundFileSaver.h

@@ -53,6 +53,21 @@ public:
    */
   void virtualDestroyNSSReference();
 
+  /**
+   * Number of worker threads that are currently running.
+   */
+  static uint32_t sThreadCount;
+
+  /**
+   * Maximum number of worker threads reached during the current download session,
+   * used for telemetry.
+   *
+   * When there are no more worker threads running, we consider the download
+   * session finished, and this counter is reset.
+   */
+  static uint32_t sTelemetryMaxThreadCount;
+
+
 protected:
   virtual ~BackgroundFileSaver();
 

toolkit/components/telemetry/Histograms.json

@@ -23,6 +23,11 @@
     "extended_statistics_ok": true,
     "description": "time spent updating accessibility (ms)"
   },
+  "BACKGROUNDFILESAVER_THREAD_COUNT": {
+    "kind": "enumerated",
+    "n_values": 21,
+    "description": "Maximum number of concurrent threads reached during a given download session"
+  },
   "CYCLE_COLLECTOR": {
     "kind": "exponential",
     "high": "10000",