Remove obsolete Net.SpdyHighestQueuedCappedFramesCount

Now that we have confirmed that 10000 is the correct limit here,
remove the histogram and add a comment with the relevant data.

Bug: 1053079
Change-Id: Idd767fb995295419f8b97c04960acadf445d56f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083993
Auto-Submit: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747041}

net/spdy/spdy_session.h

@@ -87,6 +87,13 @@ const spdy::SpdyStreamId kFirstStreamId = 1;
 const spdy::SpdyStreamId kLastStreamId = 0x7fffffff;
 
 // Maximum number of capped frames that can be queued at any time.
+// We measured how many queued capped frames were ever in the
+// SpdyWriteQueue at one given time between 2019-08 and 2020-02.
+// The numbers showed that in 99.94% of cases it would always
+// stay below 10, and that it would exceed 1000 only in
+// 10^-8 of cases. Therefore we picked 10000 as a number that will
+// virtually never be hit in practice, while still preventing an
+// attacker from growing this queue unboundedly.
 const int kSpdySessionMaxQueuedCappedFrames = 10000;
 
 class NetLog;

net/spdy/spdy_write_queue.cc

@@ -10,7 +10,6 @@
 
 #include "base/containers/circular_deque.h"
 #include "base/logging.h"
-#include "base/metrics/histogram_macros.h"
 #include "base/trace_event/memory_usage_estimator.h"
 #include "net/spdy/spdy_buffer.h"
 #include "net/spdy/spdy_buffer_producer.h"
@@ -53,9 +52,6 @@ SpdyWriteQueue::SpdyWriteQueue() : removing_writes_(false) {}
 
 SpdyWriteQueue::~SpdyWriteQueue() {
   DCHECK_GE(num_queued_capped_frames_, 0);
-  DCHECK_GT(highest_num_queued_capped_frames_, 0);
-  UMA_HISTOGRAM_COUNTS_100000("Net.SpdyHighestQueuedCappedFramesCount",
-                              highest_num_queued_capped_frames_);
   Clear();
 }
 
@@ -84,11 +80,6 @@ void SpdyWriteQueue::Enqueue(
   if (IsSpdyFrameTypeWriteCapped(frame_type)) {
     DCHECK_GE(num_queued_capped_frames_, 0);
     num_queued_capped_frames_++;
-    if (num_queued_capped_frames_ > highest_num_queued_capped_frames_) {
-      DCHECK_EQ(num_queued_capped_frames_,
-                highest_num_queued_capped_frames_ + 1);
-      highest_num_queued_capped_frames_ = num_queued_capped_frames_;
-    }
   }
 }
 

net/spdy/spdy_write_queue.h

@@ -110,8 +110,6 @@ class NET_EXPORT_PRIVATE SpdyWriteQueue {
 
   // Number of currently queued capped frames including all priorities.
   int num_queued_capped_frames_ = 0;
-  // Highest seen value of num_queued_capped_frames_.
-  int highest_num_queued_capped_frames_ = 1;
 
   // The actual write queue, binned by priority.
   base::circular_deque<PendingWrite> queue_[NUM_PRIORITIES];

tools/metrics/histograms/histograms.xml

@@ -92818,6 +92818,9 @@ uploading your change for review.
 
 <histogram name="Net.SpdyHighestQueuedCappedFramesCount" units="units"
     expires_after="M82">
+  <obsolete>
+    Removed in 2020-03 once we confirmed the numbers looked as expected.
+  </obsolete>
   <owner>dschinazi@chromium.org</owner>
   <summary>
     The highest number of capped frames queued in the SPDY write queue.