Implement the gamepad API in the IPC proxy

ppapi/proxy/gamepad_resource.cc

Index: ppapi/proxy/gamepad_resource.cc
diff --git a/ppapi/proxy/gamepad_resource.cc b/ppapi/proxy/gamepad_resource.cc
index 21ee02d9de9fa1cce6f3dc6b4ce99885691e4a7d..3f6952a62014f171960e1da4a0b2e61641b2750c 100644
--- a/ppapi/proxy/gamepad_resource.cc
+++ b/ppapi/proxy/gamepad_resource.cc
@@ -6,16 +6,47 @@
 
 #include <string.h>
 
-#include "ppapi/c/ppb_gamepad.h"
+#include "base/threading/platform_thread.h"
 #include "ppapi/proxy/dispatch_reply_message.h"
 #include "ppapi/proxy/ppapi_messages.h"
+#include "ppapi/shared_impl/ppb_gamepad_shared.h"
 
 namespace ppapi {
 namespace proxy {
 
+namespace {
+
+// This is the read logic from content/common/gamepad_seqlock.h
+base::subtle::Atomic32 ReadBegin(const base::subtle::Atomic32* sequence) {
+  base::subtle::Atomic32 version;
+  for (;;) {
+    version = base::subtle::NoBarrier_Load(sequence);
+
+    // If the counter is even, then the associated data might be in a
+    // consistent state, so we can try to read.
+    if ((version & 1) == 0)
+      break;
+
+    // Otherwise, the writer is in the middle of an update. Retry the read.
+    base::PlatformThread::YieldCurrentThread();
+  }
+  return version;
+}
+
+bool ReadRetry(const base::subtle::Atomic32* sequence,
+               base::subtle::Atomic32 version) {
+  // If the sequence number was updated then a read should be re-attempted.
+  // -- Load fence, read membarrier
+  return base::subtle::Release_Load(sequence) != version;
+}
+
+}  // namespace
+
 GamepadResource::GamepadResource(Connection connection, PP_Instance instance)
     : PluginResource(connection, instance),
       buffer_(NULL) {
+  memset(&last_read_, 0, sizeof(last_read_));
+
   SendCreateToBrowser(PpapiHostMsg_Gamepad_Create());
   CallBrowser(PpapiHostMsg_Gamepad_RequestMemory());
 }
@@ -28,9 +59,36 @@ void GamepadResource::Sample(PP_GamepadsSampleData* data) {
     // Browser hasn't sent back our shared memory, give the plugin gamepad
     // data corresponding to "not connected".
     memset(data, 0, sizeof(PP_GamepadsSampleData));
-  } else {
-    memcpy(data, buffer_, sizeof(PP_GamepadsSampleData));
+    return;
   }
+
+  // ==========
+  //   DANGER
+  // ==========
+  //
+  // This logic is duplicated in the renderer as well. If you change it, that
+  // also needs to be in sync. See gamepad_shared_memory_reader.cc.
+
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer.
+  const int kMaximumContentionCount = 10;
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  WebKitGamepads read_into;
+  do {
+    version = ReadBegin(&buffer_->sequence);
+    memcpy(&read_into, &buffer_->buffer, sizeof(read_into));
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (ReadRetry(&buffer_->sequence, version));
+
+  // In the event of a read failure, just leave the last read data as-is (the
+  // hardware thread is taking unusally long).
+  if (contention_count < kMaximumContentionCount)
+    ConvertWebKitGamepadData(read_into, &last_read_);
+
+  memcpy(data, &last_read_, sizeof(PP_GamepadsSampleData));
 }
 
 void GamepadResource::OnReplyReceived(const ResourceMessageReplyParams& params,
@@ -44,14 +102,11 @@ void GamepadResource::OnReplyReceived(const ResourceMessageReplyParams& params,
 void GamepadResource::OnPluginMsgSendMemory(
     const ResourceMessageReplyParams& params,
     base::SharedMemoryHandle shared_memory_handle) {
-  /* TODO(brettw) implement this when we have shared gamepad code. It would be
-     something like this:
   shared_memory_.reset(
       new base::SharedMemory(shared_memory_handle, true));
-  CHECK(shared_memory_->Map(sizeof(GamepadHardwareBuffer)));
-  void *memory = shared_memory_->memory();
-  // Use the memory...
-  */
+  CHECK(shared_memory_->Map(sizeof(ContentGamepadHardwareBuffer)));
+  buffer_ = static_cast<const ContentGamepadHardwareBuffer*>(
+      shared_memory_->memory());
 }
 
 }  // namespace proxy