internal/ceb: use CAS rather than set channel nil

This lets us continue to check using a channel read.

internal/ceb/ceb.go

@@ -72,7 +72,8 @@ type CEB struct {
 	// FIRST child command is ready to be started. This can be closed before
 	// any command is sent to childCmdCh. It indicates that the child process
 	// watcher can begin executing.
-	childReadyCh chan struct{}
+	childReadySent uint32
+	childReadyCh   chan struct{}
 
 	// childCmdBase is the base command to use for making any changes to the
 	// child; use the copyCmd() function to copy this safetly to make changes.

internal/ceb/child_exec.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -41,9 +42,8 @@ func (ceb *CEB) initChildCmd(ctx context.Context, cfg *config) error {
 // markChildCmdReady will allow watchChildCmd to begin executing commands.
 // This should be called once and should not be called concurrently.
 func (ceb *CEB) markChildCmdReady() {
-	if ceb.childReadyCh != nil {
+	if atomic.CompareAndSwapUint32(&ceb.childReadySent, 0, 1) {
 		close(ceb.childReadyCh)
-		ceb.childReadyCh = nil
 	}
 }