c4eb682606
This diff forward ports 90bf0629b957c912a5a6f3bb6c98ad3abb5a2ff6 to `master`. If we close the channel to signal the end of a task we may panic when some background goroutine tries to post on the channel. This bug is rare but may happen. See for example https://github.com/ooni/probe/issues/1438. How can we improve? First, let us add a timeout when sending to the channel. Given that the channel is buffered and we have a generous timeout (1/4 of a second), it's unlikely we will really block. But, in the event in which a late message appears, we'll eventually _unblock_ when sending with a timeout. So, now we don't have to worry anymore about leaking forever a goroutine. Then, let us change the protocol with which we signal that a task is done. We used to close the channel. Now, instead we just synchronously post a nil on the channel when done. In turn, we interpret this nil to mean that the task is done when we receive messages. The _main_ different with respect to before is that now we are asking the consumer of our API to drain the channel. Because before we had a blocking channel, it seems to me we were already requiring the consumer of the API to do that. Which means, I think in practical terms it did not change much. Finally, acknowledge that we don't need a specific state variable to tell us we're done and simplify a little bit the API by just making isRunning private and using the "we're done" signal to determine whether we've stopped running the task. All these changes should be enough to close https://github.com/ooni/probe/issues/1438.
118 lines
3.8 KiB
Go
118 lines
3.8 KiB
Go
// Package oonimkall implements APIs used by OONI mobile apps. We
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// expose these APIs to mobile apps using gomobile.
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//
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// We expose two APIs: the task API, which is derived from the
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// API originally exposed by Measurement Kit, and the session API,
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// which is a Go API that mobile apps can use via `gomobile`.
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//
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// This package is named oonimkall because it contains a partial
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// reimplementation of the mkall API implemented by Measurement Kit
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// in, e.g., https://github.com/measurement-kit/mkall-ios.
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//
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// Task API
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//
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// The basic tenet of the task API is that you define an experiment
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// task you wanna run using a JSON, then you start a task for it, and
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// you receive events as serialized JSONs. In addition to this
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// functionality, we also include extra APIs used by OONI mobile.
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//
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// The task API was first defined in Measurement Kit v0.9.0. In this
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// context, it was called "the FFI API". The API we expose here is not
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// strictly an FFI API, but is close enough for the purpose of using
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// OONI from Android and iOS. See https://git.io/Jv4Rv
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// (measurement-kit/measurement-kit@v0.10.9) for a comprehensive
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// description of MK's FFI API.
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//
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// See also https://github.com/ooni/probe-cli/v3/internal/engine/pull/347 for the
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// design document describing the task API.
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//
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// See also https://github.com/ooni/probe-cli/v3/internal/engine/blob/master/DESIGN.md,
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// which explains why we implemented the oonimkall API.
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//
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// Session API
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//
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// The Session API is a Go API that can be exported to mobile apps
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// using the gomobile tool. The latest design document for this API is
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// at https://github.com/ooni/probe-cli/v3/internal/engine/pull/954.
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//
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// The basic tenet of the session API is that you create an instance
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// of `Session` and use it to perform the operations you need.
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package oonimkall
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import (
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"context"
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"encoding/json"
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"github.com/ooni/probe-cli/v3/internal/atomicx"
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"github.com/ooni/probe-cli/v3/internal/runtimex"
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)
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// Task is an asynchronous task running an experiment. It mimics the
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// namesake concept initially implemented in Measurement Kit.
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//
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// Future directions
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//
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// Currently Task and Session are two unrelated APIs. As part of
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// evolving the APIs with which apps interact with the engine, we
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// will modify Task to run in the context of a Session. We will
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// do that to save extra lookups and to allow several experiments
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// running as subsequent Tasks to reuse the Session connections
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// created with the OONI probe services backends.
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type Task struct {
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cancel context.CancelFunc
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isdone *atomicx.Int64
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out chan *event
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}
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// StartTask starts an asynchronous task. The input argument is a
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// serialized JSON conforming to MK v0.10.9's API.
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func StartTask(input string) (*Task, error) {
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var settings settings
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if err := json.Unmarshal([]byte(input), &settings); err != nil {
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return nil, err
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}
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const bufsiz = 128 // common case: we don't want runner to block
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ctx, cancel := context.WithCancel(context.Background())
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task := &Task{
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cancel: cancel,
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isdone: &atomicx.Int64{},
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out: make(chan *event, bufsiz),
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}
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go func() {
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run(ctx, &settings, task.out)
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task.out <- nil // signal that we're done w/o closing the channel
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}()
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return task, nil
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}
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// WaitForNextEvent blocks until the next event occurs. The returned
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// string is a serialized JSON following MK v0.10.9's API.
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func (t *Task) WaitForNextEvent() string {
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const terminated = `{"key":"task_terminated","value":{}}` // like MK
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if t.isdone.Load() != 0 {
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return terminated
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}
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evp := <-t.out
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if evp == nil {
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t.isdone.Add(1)
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return terminated
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}
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data, err := json.Marshal(evp)
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runtimex.PanicOnError(err, "json.Marshal failed")
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return string(data)
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}
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// IsDone returns true if the task is done.
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func (t *Task) IsDone() bool {
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return t.isdone.Load() != 0
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}
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func (t *Task) isRunning() bool {
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return !t.IsDone()
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}
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// Interrupt interrupts the task.
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func (t *Task) Interrupt() {
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t.cancel()
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}
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