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Fix race condition around supervisor's Commander #291

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Fix race condition around supervisor's Commander #291

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d191a3b
Fix race condition around supervisor's Commander
tpaschalis Jul 1, 2024
b45a84f
Reuse new condition
tpaschalis Jul 9, 2024
1337ec4
Synchronize stopping explicitly
tpaschalis Aug 14, 2024
f1840f7
Set IsStopping flag first things first
tpaschalis Aug 29, 2024
f135e3b
Add test for verifying proper shutdown
tpaschalis Aug 29, 2024
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14 changes: 7 additions & 7 deletions internal/examples/supervisor/supervisor/commander/commander.go
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ type Commander struct {
cmd *exec.Cmd
doneCh chan struct{}
waitCh chan struct{}
running int64
running atomic.Bool
}

func NewCommander(logger types.Logger, cfg *config.Agent, args ...string) (*Commander, error) {
Expand Down Expand Up @@ -63,7 +63,7 @@ func (c *Commander) Start(ctx context.Context) error {
}

c.logger.Debugf(ctx, "Agent process started, PID=%d", c.cmd.Process.Pid)
atomic.StoreInt64(&c.running, 1)
c.running.Store(true)

go c.watch()

Expand All @@ -83,7 +83,7 @@ func (c *Commander) Restart(ctx context.Context) error {
func (c *Commander) watch() {
c.cmd.Wait()
c.doneCh <- struct{}{}
atomic.StoreInt64(&c.running, 0)
c.running.Store(false)
close(c.waitCh)
}

Expand All @@ -102,21 +102,21 @@ func (c *Commander) Pid() int {

// ExitCode returns Agent process exit code if it exited or 0 if it is not.
func (c *Commander) ExitCode() int {
if c.cmd == nil || c.cmd.ProcessState == nil {
if c.IsRunning() {
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Here I've actually flipped the condition from what it was before, as per the method's docstring.

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@srikanthccv srikanthccv Jul 2, 2024 •
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Could this potentially leave the agent process running in some cases? The NewSupervisor method starts a goroutine runAgentProcess to launch the agent process. There's a possibility that Shutdown might be called before c.cmd.Start() finishes. If c.cmd.Start() completes successfully, the process would keep running, which isn't expected.

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Hmm, let me think about this (and if the previous code prevented that from happening). We could potentially store the running value a little earlier to avoid that IIUC.

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Hey there, sorry for taking so long to get back to you. As far as I can tell from the code, this was already the case and it isn't a regression from this PR.

To avoid this, we'd make it the caller's responsibility to call only call Stop() after IsRunning() returns true (for example having a backoff mechanism for properly cleaning up resources), or add more explicit synchronization like we do in clientcommon

opamp-go/client/internal/clientcommon.go

Lines 53 to 55 in ed38d5f

// True when stopping is in progress.
isStoppingFlag bool
isStoppingMutex sync.RWMutex

IMO this is a separate issue than the race condition the one's fixed here, so tackling the cleanup of resoueces might be a new discussion. WDYT?

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I do not think it was possible to leave the process running in the background because we are checking if c.cmd is non-nil (which is the source of the race condition) instead of running status. I think the Stop should be updated to handle this.

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Sorry for the delay.

Here is the case I think this still doesn't address

  1. Start() is called
  2. The process begins starting up but hasn't yet reached the point where c.running.Store(true) is executed
  3. Shutdown() is called during this window
  4. Since !c.IsRunning() evaluates to true at this point, Shutdown() returns immediately
  5. This could result in the process continuing to run

Does that make sense?

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FYI I've pushed another commit in the Stop method to set the isStoppingFlag immediately.

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So, let me try to get this right.

The commander's Start is called only in two cases
a) In the Supervisor's startAgent.
b) During a restart (not pertinent to our discussion)

The startAgent method is only called via the runAgentProcess; in turn, this is only called as a new goroutine in the NewSupervisor function.

In the Shutdown method, the Commander's Stop sets the isStopping flag.

So let's say that NewSupervisor starts, and the new goroutine is launched.

Immediately, the commander's Shutdown method is called, to call the commander's Stop and set the isStoppingFlag. When the call stack gets to the commander's Start it will immediately exit with the IsStopping method without having a chance to launch any process.

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cc @srikanthccv I've put out a test in f135e3b to try and verify the behavior we want.

I couldn't find any more elegant way than the ugly DelayLogger, but I'd like to see what you think. (This test fails in main, without my fix here).

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So let's say that NewSupervisor starts, and the new goroutine is launched.

Immediately, the commander's Shutdown method is called, to call the commander's Stop and set the isStoppingFlag. When the call stack gets to the commander's Start it will immediately exit with the IsStopping method without having a chance to launch any process.

Take this case

NewSupervisor starts, and the goroutine runAgentProcess is launched, -> startAgent -> s.commander.Start -> the execution reaches c.cmd.Start

opamp-go/internal/examples/supervisor/supervisor/commander/commander.go

Line 61 in 7cdd395

if err := c.cmd.Start(); err != nil {

And say cmd.Start didn't return yet

Now, assume the commander's Shutdown method is called (c.cmd.Start still didn't return) so c.running is false. In this flow isStoppingFlag is not helping because we would still return as IsRunning evaluates to true.

I think if we add a waitgroup before the c.cmd.Start and wait in shutdown should solve the issue.

return 0
}
return c.cmd.ProcessState.ExitCode()
}

func (c *Commander) IsRunning() bool {
return atomic.LoadInt64(&c.running) != 0
return c.running.Load()
}

// Stop the Agent process. Sends SIGTERM to the process and wait for up 10 seconds
// and if the process does not finish kills it forcedly by sending SIGKILL.
// Returns after the process is terminated.
func (c *Commander) Stop(ctx context.Context) error {
if c.cmd == nil || c.cmd.Process == nil {
if !c.IsRunning() {
// Not started, nothing to do.
return nil
}
Expand Down Expand Up @@ -159,7 +159,7 @@ func (c *Commander) Stop(ctx context.Context) error {
// Wait for process to terminate
<-c.waitCh

atomic.StoreInt64(&c.running, 0)
c.running.Store(false)

// Let goroutine know process is finished.
close(finished)
Expand Down
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