Bug 825519 - Add telemetry for the max number of times any one image was decoded. r=joe

image/src/RasterImage.cpp

@@ -67,6 +67,10 @@ static bool gHQDownscaling = false;
 // This is interpreted as a floating-point value / 1000
 static uint32_t gHQDownscalingMinFactor = 1000;
 
+// The maximum number of times any one RasterImage was decoded.  This is only
+// used for statistics.
+static int32_t sMaxDecodeCount = 0;
+
 static void
 InitPrefCaches()
 {
@@ -2573,6 +2577,16 @@ RasterImage::InitDecoder(bool aDoSizeDecode)
     Telemetry::GetHistogramById(Telemetry::IMAGE_DECODE_COUNT)->Subtract(mDecodeCount);
     mDecodeCount++;
     Telemetry::GetHistogramById(Telemetry::IMAGE_DECODE_COUNT)->Add(mDecodeCount);
+
+    if (mDecodeCount > sMaxDecodeCount) {
+      // Don't subtract out 0 from the histogram, because that causes its count
+      // to go negative, which is not kosher.
+      if (sMaxDecodeCount > 0) {
+        Telemetry::GetHistogramById(Telemetry::IMAGE_MAX_DECODE_COUNT)->Subtract(sMaxDecodeCount);
+      }
+      sMaxDecodeCount = mDecodeCount;
+      Telemetry::GetHistogramById(Telemetry::IMAGE_MAX_DECODE_COUNT)->Add(sMaxDecodeCount);
+    }
   }
 
   return NS_OK;

toolkit/components/telemetry/Histograms.json

@@ -507,6 +507,12 @@
     "n_buckets": 50,
     "description": "Decode count"
   },
+  "IMAGE_MAX_DECODE_COUNT": {
+    "kind": "exponential",
+    "high": "10000",
+    "n_buckets": 100,
+    "description": "Max decode count over all images"
+  },
   "IMAGE_DECODE_SPEED_JPEG": {
     "kind": "exponential",
     "low": 500,