Cancel loading when body stream reader is detached.

content/child/shared_memory_data_consumer_handle.cc

Index: content/child/shared_memory_data_consumer_handle.cc
diff --git a/content/child/shared_memory_data_consumer_handle.cc b/content/child/shared_memory_data_consumer_handle.cc
index beb470d96d31e91d05214d1b13b4e1909ae242cc..01f9d5abd5fa072bcfb04bde17ea123054c37453 100644
--- a/content/child/shared_memory_data_consumer_handle.cc
+++ b/content/child/shared_memory_data_consumer_handle.cc
@@ -49,10 +49,13 @@ using Result = blink::WebDataConsumerHandle::Result;
 class SharedMemoryDataConsumerHandle::Context final
     : public base::RefCountedThreadSafe<Context> {
  public:
-  Context()
+  explicit Context(const base::Closure& on_clear)
       : result_(Ok),
         first_offset_(0),
         client_(nullptr),
+        writer_task_runner_(base::MessageLoop::current()->task_runner()),
+        on_clear_(on_clear),
+        is_on_clear_valid_(!on_clear_.is_null()),
         is_handle_active_(true),
         is_two_phase_read_in_progress_(false) {}
 
@@ -60,6 +63,10 @@ class SharedMemoryDataConsumerHandle::Context final
   void ClearIfNecessary() {
     if (!is_handle_locked() && !is_handle_active()) {
       // No one is interested in the contents.
+      if (is_on_clear_valid_) {
+        // We post a task even in the writer thread in order to keep reentrancy.

[hiroshige, 2015/06/22 18:23:16]

how about adding "because didGetReadable() can destruct the reader, which causes
ClearIfNecessary(), and calling |on_clear_| can cause didGetReadable() call" or
so (this explanation is somehow specific to CancelBodyStreaming() though).

nit optional: s/keep/avoid/? (not sure though)

[yhirano, 2015/06/23 05:28:37]

Done.

+        writer_task_runner_->PostTask(FROM_HERE, on_clear_);
+      }
       Clear();
     }
   }
@@ -105,6 +112,22 @@ class SharedMemoryDataConsumerHandle::Context final
                      base::Bind(&Context::NotifyInternal, this, false));
   }
   void Notify() { NotifyInternal(true); }
+  void ResetOnClear() {
+    if (on_clear_.is_null()) {
+      DCHECK(!is_on_clear_valid_);
+      return;
+    }
+    is_on_clear_valid_ = false;
+    if (writer_task_runner_->BelongsToCurrentThread()) {
+      // We can reset the closure immediately.
+      on_clear_.Reset();
+    } else {
+      // We need to reset |on_clear_| on the right thread because it might
+      // lead to the object destruction.
+      writer_task_runner_->PostTask(FROM_HERE,
+                                    base::Bind(&Context::ResetOnClear, this));
+    }
+  }
   bool is_handle_locked() const { return notification_task_runner_; }
   bool IsReaderBoundToCurrentThread() const {
     return notification_task_runner_ &&
@@ -159,10 +182,15 @@ class SharedMemoryDataConsumerHandle::Context final
     queue_.clear();
     first_offset_ = 0;
     client_ = nullptr;
+    // Note this doesn't work in the destructor if |on_clear_| is not null. We

[hiroshige, 2015/06/22 18:23:16]

This comment might be better to be placed just before ResetOnClear() definition.

[yhirano, 2015/06/23 05:28:37]

Done.

+    // have an assert in the destructor.
+    ResetOnClear();
   }
 
   friend class base::RefCountedThreadSafe<Context>;
   ~Context() {
+    DCHECK(on_clear_.is_null());
+
     // This is necessary because the queue stores raw pointers.
     Clear();
   }
@@ -177,6 +205,12 @@ class SharedMemoryDataConsumerHandle::Context final
   size_t first_offset_;
   Client* client_;
   scoped_refptr<base::SingleThreadTaskRunner> notification_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> writer_task_runner_;
+  base::Closure on_clear_;
+  // We need this boolean variable to remember if |on_clear_| is callable
+  // because we need to set |on_clear_| only on the writer thread and hence

[hiroshige, 2015/06/22 18:23:16]

optional: how about "... because |on_clear_| is reset on the writer thread
asynchronously and hence ..."?

[yhirano, 2015/06/23 05:28:38]

I think it misleading the callback will be reset synchronously when reset on the
writer thread.

+  // |on_clear_.is_null()| is untrustworthy.
+  bool is_on_clear_valid_;

[hiroshige, 2015/06/22 18:23:16]

Please add a comment that |on_clear_| and |is_on_clear_valid_| is protected by
|lock_|. (except for in the destructor?)

[yhirano, 2015/06/23 05:28:37]

Other variables are also protected by the lock, so I don't see a reason to say
that only for these variables.

   bool is_handle_active_;
   bool is_two_phase_read_in_progress_;
 
@@ -191,6 +225,8 @@ SharedMemoryDataConsumerHandle::Writer::Writer(
 
 SharedMemoryDataConsumerHandle::Writer::~Writer() {
   Close();
+  base::AutoLock lock(context_->lock());
+  context_->ResetOnClear();
 }
 
 void SharedMemoryDataConsumerHandle::Writer::AddData(
@@ -234,6 +270,7 @@ void SharedMemoryDataConsumerHandle::Writer::Close() {
     base::AutoLock lock(context_->lock());
     if (context_->result() == Ok) {
       context_->set_result(Done);
+      context_->ResetOnClear();
       needs_notification = context_->IsEmpty();
     }
   }
@@ -260,6 +297,7 @@ void SharedMemoryDataConsumerHandle::Writer::Fail() {
         context_->ClearQueue();
       }
 
+      context_->ResetOnClear();
       needs_notification = true;
     }
   }
@@ -362,7 +400,14 @@ Result SharedMemoryDataConsumerHandle::ReaderImpl::endRead(size_t read_size) {
 SharedMemoryDataConsumerHandle::SharedMemoryDataConsumerHandle(
     BackpressureMode mode,
     scoped_ptr<Writer>* writer)
-    : context_(new Context) {
+    : SharedMemoryDataConsumerHandle(mode, base::Closure(), writer) {
+}
+
+SharedMemoryDataConsumerHandle::SharedMemoryDataConsumerHandle(
+    BackpressureMode mode,
+    const base::Closure& on_clear,
+    scoped_ptr<Writer>* writer)
+    : context_(new Context(on_clear)) {
   writer->reset(new Writer(context_, mode));
 }