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author | kevin.xu <cming.xu@gmail.com> | 2020-04-27 21:51:31 +0800 |
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committer | GitHub <noreply@github.com> | 2020-04-27 21:51:31 +0800 |
commit | e896ca54db67524afc20b644d43c72185e72dc0e (patch) | |
tree | 2a16f3a62a5cafd098f1f028c621f1b655589d69 /pkg/syncevent/syncevent_example_test.go | |
parent | 1f19624fa127d7d59cabe29593cc80b7fe6c81f8 (diff) | |
parent | 3c67754663f424f2ebbc0ff2a4c80e30618d5355 (diff) |
Merge pull request #1 from google/master
catch up
Diffstat (limited to 'pkg/syncevent/syncevent_example_test.go')
-rw-r--r-- | pkg/syncevent/syncevent_example_test.go | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/pkg/syncevent/syncevent_example_test.go b/pkg/syncevent/syncevent_example_test.go new file mode 100644 index 000000000..bfb18e2ea --- /dev/null +++ b/pkg/syncevent/syncevent_example_test.go @@ -0,0 +1,108 @@ +// Copyright 2020 The gVisor Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package syncevent + +import ( + "fmt" + "sync/atomic" + "time" +) + +func Example_ioReadinessInterrputible() { + const ( + evReady = Set(1 << iota) + evInterrupt + ) + errNotReady := fmt.Errorf("not ready for I/O") + + // State of some I/O object. + var ( + br Broadcaster + ready uint32 + ) + doIO := func() error { + if atomic.LoadUint32(&ready) == 0 { + return errNotReady + } + return nil + } + go func() { + // The I/O object eventually becomes ready for I/O. + time.Sleep(100 * time.Millisecond) + // When it does, it first ensures that future calls to isReady() return + // true, then broadcasts the readiness event to Receivers. + atomic.StoreUint32(&ready, 1) + br.Broadcast(evReady) + }() + + // Each user of the I/O object owns a Waiter. + var w Waiter + w.Init() + // The Waiter may be asynchronously interruptible, e.g. for signal + // handling in the sentry. + go func() { + time.Sleep(200 * time.Millisecond) + w.Receiver().Notify(evInterrupt) + }() + + // To use the I/O object: + // + // Optionally, if the I/O object is likely to be ready, attempt I/O first. + err := doIO() + if err == nil { + // Success, we're done. + return /* nil */ + } + if err != errNotReady { + // Failure, I/O failed for some reason other than readiness. + return /* err */ + } + // Subscribe for readiness events from the I/O object. + id := br.SubscribeEvents(w.Receiver(), evReady) + // When we are finished blocking, unsubscribe from readiness events and + // remove readiness events from the pending event set. + defer UnsubscribeAndAck(&br, w.Receiver(), evReady, id) + for { + // Attempt I/O again. This must be done after the call to SubscribeEvents, + // since the I/O object might have become ready between the previous call + // to doIO and the call to SubscribeEvents. + err = doIO() + if err == nil { + return /* nil */ + } + if err != errNotReady { + return /* err */ + } + // Block until either the I/O object indicates it is ready, or we are + // interrupted. + events := w.Wait() + if events&evInterrupt != 0 { + // In the specific case of sentry signal handling, signal delivery + // is handled by another system, so we aren't responsible for + // acknowledging evInterrupt. + return /* errInterrupted */ + } + // Note that, in a concurrent context, the I/O object might become + // ready and then not ready again. To handle this: + // + // - evReady must be acknowledged before calling doIO() again (rather + // than after), so that if the I/O object becomes ready *again* after + // the call to doIO(), the readiness event is not lost. + // + // - We must loop instead of just calling doIO() once after receiving + // evReady. + w.Ack(evReady) + } +} |