Refactoring of job dispatch in HostResolverImpl in preparation for DnsClient.

net/base/priority_dispatch.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_BASE_PRIORITY_DISPATCH_H_
+#define NET_BASE_PRIORITY_DISPATCH_H_
+#pragma once
+
+#include "net/base/net_export.h"
+#include "net/base/priority_queue.h"
+#include "net/base/request_priority.h"
+
+namespace net {
+
+// A priority-based dispatcher of jobs. The dispatcher enforces limits on
+// the number of running jobs. It never revokes a job once started. The job
+// must call OnJobFinished once it finishes in order to dispatch further jobs.
+// All operations are O(1) time. The class is fully reentrant: it is safe to
+// execute any method (incl. destructor) from within Job callbacks.
+class NET_EXPORT_PRIVATE PriorityDispatch {
+ public:
+  // Describes the number of slots reserved for each priority (and higher).
+  // For example, with |reserved_slots| = { 5, 10, 5, 0, 0 }, there can be at
+  // most 20 running jobs in total. If there are already 14 jobs running, then
+  // there can be 6 more jobs started of which at most 1 can be MEDIUM but the
+  // rest has to be HIGHEST. { 0, 0, 0, 0, 100 } allows up to 100 running jobs
+  // regardless of priority.
+  struct NET_EXPORT_PRIVATE Limits {
+    // Constructs Limits that permit |max_idle| jobs regardless of priority.
+    static Limits MakeAny(size_t max_idle);
+    // Sum of all |reserved_slots|.
+    size_t Total() const;
+    size_t reserved_slots[NUM_PRIORITIES];
+  };
+
+  // An interface to the job dispatched by PriorityDispatch. A job enqueued in
+  // the dispatcher is not owned by the dispatcher but must outlive it. Use

[mmenke, 2011/12/21 16:22:58]

I think you mean must not outlive it.  Or must not still be enqueued on the
dispatcher's destruction, at least.

[szym, 2011/12/21 22:19:34]

I wasn't clear: the dispatcher expects the Job to be available for call to
Start/OnEvicted as long as the Job is queued in the dispatcher. After the call
to Job::Start/OnEvicted, or after the dispatcher is destroyed, the Job can go
away.

+  // Cancel to remove job from queue.
+  class Job {
+   public:
+    // Note: PriorityDispatch will never delete a Job.
+    virtual ~Job() {}
+    // Called when the dispatcher starts the job. Must call OnJobFinished when
+    // done.
+    virtual void Start() = 0;
+    // Called when the job is evicted due to |max_queued|.
+    virtual void OnEvicted() = 0;
+  };

[mmenke, 2011/12/21 16:22:58]

nit:  Please add a linebreak here.

[szym, 2011/12/28 01:24:10]

Done.

+  // A handle to the enqueued job. The handle becomes invalid when the job is
+  // canceled, updated, evicted, or started.
+  typedef PriorityQueue<Job*>::Pointer Handle;
+
+  // Creates a dispatcher enforcing |limits| on number of running jobs and
+  // |max_queued| on queued jobs.
+  PriorityDispatch(const Limits& limits, size_t max_queued);
+
+  ~PriorityDispatch();
+
+  size_t num_running_jobs() const { return num_running_jobs_; }
+  size_t num_queued_jobs() const { return queue_.size(); }
+
+  // Configures |max_queued|. Allowed only when queue is empty.
+  void SetMaxQueued(size_t max_queued);
+
+  // Adds |job| with |priority| to the dispatcher. If limits permit, |job| is
+  // started immediately. Returns handle to the job or null-handle if the job is
+  // started or evicted.
+  Handle Add(Job* job, RequestPriority priority);
+
+  // Removes the job at |handle| from the queue.
+  void Cancel(const Handle& handle);
+
+  // Moves the queued job at |handle| to the end of all values with priority
+  // |priority| and returns the updated handle or null-handle if the job is
+  // started.
+  Handle Update(const Handle& handle, RequestPriority priority);
+
+  // Notifies the dispatcher that a running job has finished. Could start a job.
+  void OnJobFinished();
+
+ private:
+  // Queue for jobs that need to wait for a spare slot.
+  PriorityQueue<Job*> queue_;
+  // Maximum total number of running jobs permitting to start a new job at each
+  // priority.
+  size_t max_running_jobs_[NUM_PRIORITIES];
+  // Total number of running jobs.
+  size_t num_running_jobs_;
+  // Maximum number of jobs in |queue_|.
+  size_t max_queued_jobs_;
+
+  DISALLOW_COPY_AND_ASSIGN(PriorityDispatch);
+};
+
+}  // namespace net
+
+#endif  // NET_BASE_PRIORITY_DISPATCH_H_
+