[cmd/opampsupervisor] Fix restart delay when agent process exits unex…

…pectedly (#32150)

**Description:**

Reset should called only on stopped or expired timers with drained
channels. If the timer already expired (and the channel was not cleared)
it reads from the timer's channel to clear it.



**Link to tracking Issue:** Fixes
#27891

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

---------

Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>

.chloggen/opamp_fix_27891_restart_delay.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: opampsupervisor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix restart delay when agent process exits unexpectedly.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27891]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

cmd/opampsupervisor/supervisor/commander/commander.go

@@ -26,6 +26,7 @@ type Commander struct {
 	args    []string
 	cmd     *exec.Cmd
 	doneCh  chan struct{}
+	exitCh  chan struct{}
 	running *atomic.Int64
 }
 
@@ -39,6 +40,9 @@ func NewCommander(logger *zap.Logger, cfg *config.Agent, args ...string) (*Comma
 		cfg:     cfg,
 		args:    args,
 		running: &atomic.Int64{},
+		// Buffer channels so we can send messages without blocking on listeners.
+		doneCh: make(chan struct{}, 1),
+		exitCh: make(chan struct{}, 1),
 	}, nil
 }
 
@@ -52,6 +56,21 @@ func (c *Commander) Start(ctx context.Context) error {
 		return nil
 	}
 
+	// Drain channels in case there are no listeners that
+	// drained messages from previous runs.
+	if len(c.doneCh) > 0 {
+		select {
+		case <-c.doneCh:
+		default:
+		}
+	}
+	if len(c.exitCh) > 0 {
+		select {
+		case <-c.exitCh:
+		default:
+		}
+	}
+
 	c.logger.Debug("Starting agent", zap.String("agent", c.cfg.Executable))
 
 	logFilePath := "agent.log"
@@ -67,8 +86,6 @@ func (c *Commander) Start(ctx context.Context) error {
 	c.cmd.Stdout = logFile
 	c.cmd.Stderr = logFile
 
-	c.doneCh = make(chan struct{})
-
 	if err := c.cmd.Start(); err != nil {
 		return err
 	}
@@ -101,12 +118,13 @@ func (c *Commander) watch() {
 	}
 
 	c.running.Store(0)
-	close(c.doneCh)
+	c.doneCh <- struct{}{}
+	c.exitCh <- struct{}{}
 }
 
-// Done returns a channel that will send a signal when the Agent process is finished.
-func (c *Commander) Done() <-chan struct{} {
-	return c.doneCh
+// Exited returns a channel that will send a signal when the Agent process exits.
+func (c *Commander) Exited() <-chan struct{} {
+	return c.exitCh
 }
 
 // Pid returns Agent process PID if it is started or 0 if it is not.

cmd/opampsupervisor/supervisor/supervisor.go

@@ -797,7 +797,7 @@ func (s *Supervisor) runAgentProcess() {
 			s.stopAgentApplyConfig()
 			s.startAgent()
 
-		case <-s.commander.Done():
+		case <-s.commander.Exited():
 			if s.shuttingDown {
 				return
 			}
@@ -817,7 +817,12 @@ func (s *Supervisor) runAgentProcess() {
 			// https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/21079
 
 			// Wait 5 seconds before starting again.
-			restartTimer.Stop()
+			if !restartTimer.Stop() {
+				select {
+				case <-restartTimer.C: // Try to drain the channel
+				default:
+				}
+			}
 			restartTimer.Reset(5 * time.Second)
 
 		case <-restartTimer.C: