internal/pkg: return writer after sync

This fixes a use-after-free.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -441,14 +441,6 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 		}()
 
 		bw := f.cache.getWriter(status)
-		defer func() {
-			flushErr := bw.Flush()
-			if err == nil {
-				err = flushErr
-			}
-			f.cache.putWriter(bw)
-		}()
-
 		stdoutDone, stderrDone := make(chan struct{}), make(chan struct{})
 		go scanVerbose(
 			msg, cancel, stdoutDone,
@@ -460,7 +452,17 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 			"("+a.name+":2)",
 			io.TeeReader(stderr, bw),
 		)
-		defer func() { <-stdoutDone; <-stderrDone }()
+		defer func() {
+			<-stdoutDone
+			<-stderrDone
+
+			flushErr := bw.Flush()
+			if err == nil {
+				err = flushErr
+			}
+			f.cache.putWriter(bw)
+
+		}()
 	} else {
 		z.Stdout, z.Stderr = status, status
 	}