Add a new error code for a null callback on the main thread.

Convert the NaCl and ChromeIPC proxies to use the new value.

Review URL: http://codereview.chromium.org/7885014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@101556 0039d316-1c4b-4281-b951-d872f2087c98

16 files changed, 93 insertions, 34 deletions

diff --git a/ppapi/api/pp_errors.idl b/ppapi/api/pp_errors.idl
index 950b77b..bd49081 100644
--- a/ppapi/api/pp_errors.idl
+++ b/ppapi/api/pp_errors.idl
@@ -9,7 +9,10 @@
 
 /**
  * This enumeration contains enumerators of all PPAPI error codes.
- * Errors are negative valued.
+ *
+ * Errors are negative valued. Callers should treat all negative values as a
+ * failure, even if it's not in the list, since the possible errors are likely
+ * to expand and change over time.
  */
 [unnamed] enum PP_Error {
   /**
@@ -26,11 +29,20 @@
    */
   PP_OK_COMPLETIONPENDING = -1,
 
-  /** This value indicates failure for unspecified reasons. */
+  /**This value indicates failure for unspecified reasons. */
   PP_ERROR_FAILED         = -2,
+
   /**
    * This value indicates failure due to an asynchronous operation being
-   * interrupted, typically as a result of user action.
+   * interrupted. The most common cause of this error code is destroying a
+   * resource that still has a callback pending. All callbacks are guaranteed
+   * to execute, so any callbacks pending on a destroyed resource will be
+   * issued with PP_ERROR_ABORTED.
+   *
+   * If you get an aborted notification that you aren't expecting, check to
+   * make sure that the resource you're using is still in scope. A common
+   * mistake is to create a resource on the stack, which will destroy the
+   * resource as soon as the function returns.
    */
   PP_ERROR_ABORTED        = -3,
 
@@ -67,6 +79,20 @@
    */
   PP_ERROR_NOTSUPPORTED = -12,
 
+  /**
+   * Returned if you try to use a null completion callback to "block until
+   * complete" on the main thread. Blocking the main thread is not permitted
+   * to keep the browser responsive (otherwise, you may not be able to handle
+   * input events, and there are reentrancy and deadlock issues).
+   *
+   * The goal is to provide blocking calls from background threads, but PPAPI
+   * calls on background threads are not currently supported. Until this
+   * support is complete, you must either do asynchronous operations on the
+   * main thread, or provide an adaptor for a blocking background thread to
+   * execute the operaitions on the main thread.
+   */
+  PP_ERROR_BLOCKS_MAIN_THREAD = -13,
+
   PP_ERROR_FILENOTFOUND   = -20,
   /** This value indicates failure due to a file that already exists. */
   PP_ERROR_FILEEXISTS     = -21,
diff --git a/ppapi/c/pp_errors.h b/ppapi/c/pp_errors.h
index 1394aa7..724ff9b 100644
--- a/ppapi/c/pp_errors.h
+++ b/ppapi/c/pp_errors.h
@@ -3,7 +3,7 @@
  * found in the LICENSE file.
  */
 
-/* From pp_errors.idl modified Sat Jul 16 16:50:26 2011. */
+/* From pp_errors.idl modified Wed Sep 14 08:34:03 2011. */
 
 #ifndef PPAPI_C_PP_ERRORS_H_
 #define PPAPI_C_PP_ERRORS_H_
@@ -22,7 +22,10 @@
  */
 /**
  * This enumeration contains enumerators of all PPAPI error codes.
- * Errors are negative valued.
+ *
+ * Errors are negative valued. Callers should treat all negative values as a
+ * failure, even if it's not in the list, since the possible errors are likely
+ * to expand and change over time.
  */
 enum {
   /**
@@ -38,11 +41,19 @@ enum {
    * available.
    */
   PP_OK_COMPLETIONPENDING = -1,
-  /** This value indicates failure for unspecified reasons. */
+  /**This value indicates failure for unspecified reasons. */
   PP_ERROR_FAILED = -2,
   /**
    * This value indicates failure due to an asynchronous operation being
-   * interrupted, typically as a result of user action.
+   * interrupted. The most common cause of this error code is destroying a
+   * resource that still has a callback pending. All callbacks are guaranteed
+   * to execute, so any callbacks pending on a destroyed resource will be
+   * issued with PP_ERROR_ABORTED.
+   *
+   * If you get an aborted notification that you aren't expecting, check to
+   * make sure that the resource you're using is still in scope. A common
+   * mistake is to create a resource on the stack, which will destroy the
+   * resource as soon as the function returns.
    */
   PP_ERROR_ABORTED = -3,
   /** This value indicates failure due to an invalid argument. */
@@ -69,6 +80,19 @@ enum {
    * The requested command is not supported by the browser.
    */
   PP_ERROR_NOTSUPPORTED = -12,
+  /**
+   * Returned if you try to use a null completion callback to "block until
+   * complete" on the main thread. Blocking the main thread is not permitted
+   * to keep the browser responsive (otherwise, you may not be able to handle
+   * input events, and there are reentrancy and deadlock issues).
+   *
+   * The goal is to provide blocking calls from background threads, but PPAPI
+   * calls on background threads are not currently supported. Until this
+   * support is complete, you must either do asynchronous operations on the
+   * main thread, or provide an adaptor for a blocking background thread to
+   * execute the operaitions on the main thread.
+   */
+  PP_ERROR_BLOCKS_MAIN_THREAD = -13,
   PP_ERROR_FILENOTFOUND = -20,
   /** This value indicates failure due to a file that already exists. */
   PP_ERROR_FILEEXISTS = -21,
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_nacl_file.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_nacl_file.cc
index 1099ec1..4c8758c 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_nacl_file.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_nacl_file.cc
@@ -24,10 +24,10 @@ int32_t StreamAsFile(PP_Instance instance,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   NaClSrpcError srpc_result = NaClFileRpcClient::StreamAsFile(
-          GetMainSrpcChannel(), instance, const_cast<char*>(url), callback_id);
+      GetMainSrpcChannel(), instance, const_cast<char*>(url), callback_id);
   DebugPrintf("NaClFile::StreamAsFile: %s\n", NaClSrpcErrorString(srpc_result));
 
   if (srpc_result == NACL_SRPC_RESULT_OK)
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_io.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_io.cc
index 848b1ee..9f4851e 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_io.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_io.cc
@@ -66,7 +66,7 @@ int32_t Open(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
@@ -90,7 +90,7 @@ int32_t Query(PP_Resource file_io,
               struct PP_CompletionCallback callback) {
   DebugPrintf("PPB_FileIO::Query: file_io=%"NACL_PRIu32"\n", file_io);
   if (callback.func == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t callback_id = CompletionCallbackTable::Get()->AddCallback(
       callback, reinterpret_cast<char*>(info));
@@ -130,7 +130,7 @@ int32_t Touch(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
 
   int32_t pp_error = PP_ERROR_FAILED;
@@ -165,7 +165,7 @@ int32_t Read(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback, buffer);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error_or_bytes;
   NaClSrpcError srpc_result =
@@ -202,7 +202,7 @@ int32_t Write(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error_or_bytes = PP_ERROR_FAILED;
   NaClSrpcError srpc_result = PpbFileIORpcClient::PPB_FileIO_Write(
@@ -231,7 +231,7 @@ int32_t SetLength(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error = PP_ERROR_FAILED;
   NaClSrpcError srpc_result = PpbFileIORpcClient::PPB_FileIO_SetLength(
@@ -256,7 +256,7 @@ int32_t Flush(PP_Resource file_io,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error = PP_ERROR_FAILED;
   NaClSrpcError srpc_result = PpbFileIORpcClient::PPB_FileIO_Flush(
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_system.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_system.cc
index 64ebaf5..fb4fa40 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_system.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_file_system.cc
@@ -57,8 +57,8 @@ int32_t Open(PP_Resource file_system,
   DebugPrintf("PPB_FileSystem::Open: file_system=%"NACL_PRIu32"\n",
               file_system);
   int32_t callback_id = CompletionCallbackTable::Get()->AddCallback(callback);
-  if (callback_id == 0)
-    return PP_ERROR_BADARGUMENT;
+  if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_2d.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_2d.cc
index 3bab6d1..2bf2797 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_2d.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_2d.cc
@@ -153,7 +153,7 @@ int32_t Flush(PP_Resource graphics_2d,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_3d.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_3d.cc
index 974563a..736eee2 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_3d.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_graphics_3d.cc
@@ -238,7 +238,7 @@ int32_t PluginGraphics3D::SwapBuffers(PP_Resource graphics3d_id,
 
   int32_t callback_id = CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError retval =
diff --git a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_url_loader.cc b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_url_loader.cc
index 2118e90..be7a639 100644
--- a/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_url_loader.cc
+++ b/ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_url_loader.cc
@@ -60,7 +60,7 @@ int32_t Open(PP_Resource loader,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
@@ -80,7 +80,7 @@ int32_t FollowRedirect(PP_Resource loader,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
@@ -167,7 +167,7 @@ int32_t ReadResponseBody(PP_Resource loader,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback, buffer);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error_or_bytes;
   NaClSrpcError srpc_result =
@@ -195,7 +195,7 @@ int32_t FinishStreamingToFile(PP_Resource loader,
   int32_t callback_id =
       CompletionCallbackTable::Get()->AddCallback(callback);
   if (callback_id == 0)  // Just like Chrome, for now disallow blocking calls.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   int32_t pp_error;
   NaClSrpcError srpc_result =
diff --git a/ppapi/native_client/tests/ppapi_browser/ppb_file_system/ppapi_ppb_file_system.cc b/ppapi/native_client/tests/ppapi_browser/ppb_file_system/ppapi_ppb_file_system.cc
index 5d17cde..ebf8128 100644
--- a/ppapi/native_client/tests/ppapi_browser/ppb_file_system/ppapi_ppb_file_system.cc
+++ b/ppapi/native_client/tests/ppapi_browser/ppb_file_system/ppapi_ppb_file_system.cc
@@ -134,7 +134,7 @@ void TestOpen() {
     pp_error = ppb_file_system->Open(file_system, kSize,
                                      PP_BlockUntilComplete());
     ppb_core->ReleaseResource(file_system);
-    EXPECT(pp_error == PP_ERROR_BADARGUMENT);
+    EXPECT(pp_error == PP_ERROR_BLOCKS_MAIN_THREAD);
 #endif
 
     // Test success for asynchronous open.
diff --git a/ppapi/native_client/tests/ppapi_browser/ppb_graphics2d/ppapi_ppb_graphics2d.cc b/ppapi/native_client/tests/ppapi_browser/ppb_graphics2d/ppapi_ppb_graphics2d.cc
index cedeead..c8b4007 100644
--- a/ppapi/native_client/tests/ppapi_browser/ppb_graphics2d/ppapi_ppb_graphics2d.cc
+++ b/ppapi/native_client/tests/ppapi_browser/ppb_graphics2d/ppapi_ppb_graphics2d.cc
@@ -486,7 +486,7 @@ void TestFlush() {
   // Invalid args -> PP_ERROR_BAD..., no callback.
   EXPECT(PP_ERROR_BADRESOURCE == PPBGraphics2D()->Flush(kInvalidResource, cc));
   EXPECT(PP_ERROR_BADRESOURCE == PPBGraphics2D()->Flush(kNotAResource, cc));
-  EXPECT(PP_ERROR_BADARGUMENT ==
+  EXPECT(PP_ERROR_BLOCKS_MAIN_THREAD ==
          PPBGraphics2D()->Flush(graphics2d, PP_BlockUntilComplete()));
 
   // Valid args -> PP_OK_COMPLETIONPENDING, expect callback.
diff --git a/ppapi/native_client/tests/ppapi_browser/ppb_url_loader/ppapi_ppb_url_loader.cc b/ppapi/native_client/tests/ppapi_browser/ppb_url_loader/ppapi_ppb_url_loader.cc
index 1c5d30f..70d4d93 100644
--- a/ppapi/native_client/tests/ppapi_browser/ppb_url_loader/ppapi_ppb_url_loader.cc
+++ b/ppapi/native_client/tests/ppapi_browser/ppb_url_loader/ppapi_ppb_url_loader.cc
@@ -407,7 +407,7 @@ void TestOpenSimple() {
   loader = PPBURLLoader()->Create(pp_instance());
   // We are on the main thread, performing a sync call should fail
   rv = PPBURLLoader()->Open(loader, request, PP_BlockUntilComplete());
-  EXPECT(PP_ERROR_BADARGUMENT == rv);
+  EXPECT(PP_ERROR_BLOCKS_MAIN_THREAD == rv);
   PPBCore()->ReleaseResource(loader);
 
   LOG_TO_BROWSER("open (asynchronous) normal");
diff --git a/ppapi/proxy/ppb_broker_proxy.cc b/ppapi/proxy/ppb_broker_proxy.cc
index ff06759..f584cf0 100644
--- a/ppapi/proxy/ppb_broker_proxy.cc
+++ b/ppapi/proxy/ppb_broker_proxy.cc
@@ -98,7 +98,7 @@ PPB_Broker_API* Broker::AsPPB_Broker_API() {
 int32_t Broker::Connect(PP_CompletionCallback connect_callback) {
   if (!connect_callback.func) {
     // Synchronous calls are not supported.
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
   }
 
   if (current_connect_callback_.func)
diff --git a/ppapi/proxy/ppb_file_chooser_proxy.cc b/ppapi/proxy/ppb_file_chooser_proxy.cc
index 73f7a6a..c74ba4f 100644
--- a/ppapi/proxy/ppb_file_chooser_proxy.cc
+++ b/ppapi/proxy/ppb_file_chooser_proxy.cc
@@ -83,6 +83,9 @@ PPB_FileChooser_API* FileChooser::AsPPB_FileChooser_API() {
 }
 
 int32_t FileChooser::Show(const PP_CompletionCallback& callback) {
+  if (!callback.func)
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
+
   if (current_show_callback_.func)
     return PP_ERROR_INPROGRESS;  // Can't show more than once.
 
diff --git a/ppapi/proxy/ppb_flash_tcp_socket_proxy.cc b/ppapi/proxy/ppb_flash_tcp_socket_proxy.cc
index 7ff2ce3..020cf28 100644
--- a/ppapi/proxy/ppb_flash_tcp_socket_proxy.cc
+++ b/ppapi/proxy/ppb_flash_tcp_socket_proxy.cc
@@ -209,8 +209,10 @@ PP_Bool FlashTCPSocket::GetRemoteAddress(PP_Flash_NetAddress* remote_addr) {
 int32_t FlashTCPSocket::SSLHandshake(const char* server_name,
                                      uint16_t server_port,
                                      PP_CompletionCallback callback) {
-  if (!server_name || !callback.func)
+  if (!server_name)
     return PP_ERROR_BADARGUMENT;
+  if (!callback.func)
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   if (connection_state_ != CONNECTED)
     return PP_ERROR_FAILED;
@@ -230,8 +232,10 @@ int32_t FlashTCPSocket::SSLHandshake(const char* server_name,
 int32_t FlashTCPSocket::Read(char* buffer,
                              int32_t bytes_to_read,
                              PP_CompletionCallback callback) {
-  if (!buffer || bytes_to_read <= 0 || !callback.func)
+  if (!buffer || bytes_to_read <= 0)
     return PP_ERROR_BADARGUMENT;
+  if (!callback.func)
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   if (!IsConnected())
     return PP_ERROR_FAILED;
@@ -251,8 +255,10 @@ int32_t FlashTCPSocket::Read(char* buffer,
 int32_t FlashTCPSocket::Write(const char* buffer,
                               int32_t bytes_to_write,
                               PP_CompletionCallback callback) {
-  if (!buffer || bytes_to_write <= 0 || !callback.func)
+  if (!buffer || bytes_to_write <= 0)
     return PP_ERROR_BADARGUMENT;
+  if (!callback.func)
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   if (!IsConnected())
     return PP_ERROR_FAILED;
@@ -365,7 +371,7 @@ int32_t FlashTCPSocket::ConnectWithMessage(IPC::Message* msg,
                                            PP_CompletionCallback callback) {
   scoped_ptr<IPC::Message> msg_deletor(msg);
   if (!callback.func)
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
   if (connection_state_ != BEFORE_CONNECT)
     return PP_ERROR_FAILED;
   if (connect_callback_.func)
diff --git a/ppapi/proxy/ppb_graphics_2d_proxy.cc b/ppapi/proxy/ppb_graphics_2d_proxy.cc
index 232e1fe..dcf8d26 100644
--- a/ppapi/proxy/ppb_graphics_2d_proxy.cc
+++ b/ppapi/proxy/ppb_graphics_2d_proxy.cc
@@ -124,7 +124,7 @@ int32_t Graphics2D::Flush(PP_CompletionCallback callback) {
   // For now, disallow blocking calls. We'll need to add support for other
   // threads to this later.
   if (!callback.func)
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   if (current_flush_callback_.func)
     return PP_ERROR_INPROGRESS;  // Can't have >1 flush pending.
diff --git a/ppapi/proxy/ppb_surface_3d_proxy.cc b/ppapi/proxy/ppb_surface_3d_proxy.cc
index 70809da..c46f01b 100644
--- a/ppapi/proxy/ppb_surface_3d_proxy.cc
+++ b/ppapi/proxy/ppb_surface_3d_proxy.cc
@@ -62,7 +62,7 @@ int32_t Surface3D::SwapBuffers(PP_CompletionCallback callback) {
   // For now, disallow blocking calls. We'll need to add support for other
   // threads to this later.
   if (!callback.func)
-    return PP_ERROR_BADARGUMENT;
+    return PP_ERROR_BLOCKS_MAIN_THREAD;
 
   if (is_flush_pending())
     return PP_ERROR_INPROGRESS;  // Can't have >1 flush pending.