Waiting on result of a callback

[baile...@google.com]

Hi!

I was hoping someone could point me in the right direction. I'm trying to accomplish the following:

BarResult TwoCalls(arg) {

  // callback signature: void MyCallback(result_value, error_code)

  call Foo(arg, callback)

  error_code = WaitOnCallbackAndGetErrorCode(callback)

  if FAILED(error_code) return error_code

  return Bar(arg)

}

I'm wondering if there's some Chromium magic to wait for a callback. The reason we are making this synchronous is because if Foo fails, Bar must not be called, but the caller of TwoCalls should be returned the result of Bar().

Thanks!

[Ken Rockot]

You should almost always prefer to make TwoCalls itself asynchronous in this case, especially if this is production code.

Can you please provide more context? If this is for test-only code, it is fairly common to spin a RunLoop while waiting for a condition to be fulfilled.

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[baile...@google.com]

Thanks for the reply. Sure- let me add some details. We are working on adding native support for smb filesystems. We have refactored file_system_provider::service to support native providers in addition to extensions (currently supported). The entry point for mounting an SMB filesystem will eventually be a bit of UI with a mount button, that should call SmbService::Mount() [1]. Here is a hacky example from awhile ago calling Service::Mount() from a button press [2] (but now this would be a call to SmbService::Mount(). SmbService::Mount() in turn should first call SmbProvider::Mount() which communicates w/ a Daemon via DBUS. SmbService::Mount() would then call Service::Mount() on success, and return the result of that function.  Here is some more detailed psuedocode:

BaseSmbService::Mount(share_path)

- // void MountCallback(mount_id, error)

- SmbProviderClient.Mount(string share_path, MountCallback callback)

- Wait on the callback

- Check the error, and map and return if failed

- return Service::Mount(...)

Sorry if this is too much detail, but maybe it helps explain our issue better.

[1] https://cs.chromium.org/chromium/src/chrome/browser/chromeos/smb_client/smb_service.h?rcl=0fdb7a80614645df89b8e538a3d8f54688fc2347&l=36

[2] https://chromium-review.googlesource.com/c/chromium/src/+/726101/1/chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc#518

On Tuesday, December 5, 2017 at 12:11:06 PM UTC-8, Ken Rockot wrote:

You should almost always prefer to make TwoCalls itself asynchronous in this case, especially if this is production code.

Can you please provide more context? If this is for test-only code, it is fairly common to spin a RunLoop while waiting for a condition to be fulfilled.

On Tue, Dec 5, 2017 at 12:04 PM, baileyberro via Chromium-dev <chromi...@chromium.org> wrote:

Hi!

I was hoping someone could point me in the right direction. I'm trying to accomplish the following:

BarResult TwoCalls(arg) {

  // callback signature: void MyCallback(result_value, error_code)

  call Foo(arg, callback)

  error_code = WaitOnCallbackAndGetErrorCode(callback)

  if FAILED(error_code) return error_code

  return Bar(arg)

}

I'm wondering if there's some Chromium magic to wait for a callback. The reason we are making this synchronous is because if Foo fails, Bar must not be called, but the caller of TwoCalls should be returned the result of Bar().

Thanks!

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[Ken Rockot]

On Tue, Dec 5, 2017 at 12:23 PM, baileyberro via Chromium-dev <chromi...@chromium.org> wrote:

Thanks for the reply. Sure- let me add some details. We are working on adding native support for smb filesystems. We have refactored file_system_provider::service to support native providers in addition to extensions (currently supported). The entry point for mounting an SMB filesystem will eventually be a bit of UI with a mount button, that should call SmbService::Mount() [1]. Here is a hacky example from awhile ago calling Service::Mount() from a button press [2] (but now this would be a call to SmbService::Mount(). SmbService::Mount() in turn should first call SmbProvider::Mount() which communicates w/ a Daemon via DBUS. SmbService::Mount() would then call Service::Mount() on success, and return the result of that function.  Here is some more detailed psuedocode:

BaseSmbService::Mount(share_path)

- // void MountCallback(mount_id, error)

- SmbProviderClient.Mount(string share_path, MountCallback callback)

- Wait on the callback

- Check the error, and map and return if failed

- return Service::Mount(...)

Sorry if this is too much detail, but maybe it helps explain our issue better.

That's all fine, but I still don't understand why you need the "wait on the callback" step. That means blocking a Chrome thread that could otherwise be getting work done while the SMB service is off doing whatever it needs to do. I would strongly encourage you to consider making the Mount API itself asynchronous to eliminate any need for blocking operations.

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[Alex Clarke]

Would PostTaskAndReply help here?

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