cmd/go/internal/work: avoid panic for a repeated //go:debug setting

The creation of a bytes.Buffer in one code path is missing causing a nil
pointer dereference.

Changed (as rec. by Bryan Mills) to use fmt.Appendf() on []byte instead of
fmt.Fprintf on *bytes.Buffer - simpler and avoids duplicated code (but
requires Go 1.19 or later).

Added test to verify the change (as rec. by Michael Matloob) at
src\cmd\go\testdata\script\build_repeated_godebug_issue62346.txt

Fixes #62346

Change-Id: Ic3267d878a6f7ebedb1cde64e6206de404176b10
Reviewed-on: https://go-review.googlesource.com/c/go/+/523836
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>

Author: AndrewWPhillips

src/cmd/go/internal/work/exec.go

@@ -989,21 +989,18 @@ OverlayLoop:
 }
 
 func (b *Builder) checkDirectives(a *Action) error {
-	var msg *bytes.Buffer
+	var msg []byte
 	p := a.Package
 	var seen map[string]token.Position
 	for _, d := range p.Internal.Build.Directives {
 		if strings.HasPrefix(d.Text, "//go:debug") {
 			key, _, err := load.ParseGoDebug(d.Text)
 			if err != nil && err != load.ErrNotGoDebug {
-				if msg == nil {
-					msg = new(bytes.Buffer)
-				}
-				fmt.Fprintf(msg, "%s: invalid //go:debug: %v\n", d.Pos, err)
+				msg = fmt.Appendf(msg, "%s: invalid //go:debug: %v\n", d.Pos, err)
 				continue
 			}
 			if pos, ok := seen[key]; ok {
-				fmt.Fprintf(msg, "%s: repeated //go:debug for %v\n\t%s: previous //go:debug\n", d.Pos, key, pos)
+				msg = fmt.Appendf(msg, "%s: repeated //go:debug for %v\n\t%s: previous //go:debug\n", d.Pos, key, pos)
 				continue
 			}
 			if seen == nil {
@@ -1012,12 +1009,12 @@ func (b *Builder) checkDirectives(a *Action) error {
 			seen[key] = d.Pos
 		}
 	}
-	if msg != nil {
+	if len(msg) > 0 {
 		// We pass a non-nil error to reportCmd to trigger the failure reporting
 		// path, but the content of the error doesn't matter because msg is
 		// non-empty.
 		err := errors.New("invalid directive")
-		return b.Shell(a).reportCmd("", "", msg.Bytes(), err)
+		return b.Shell(a).reportCmd("", "", msg, err)
 	}
 	return nil
 }

src/cmd/go/testdata/script/build_repeated_godebug_issue62346.txt

@@ -0,0 +1,14 @@
+[short] skip # runs go build
+! go build file.go
+! stderr 'panic:'
+! stderr 'runtime error'
+stderr 'file.go:2:1: repeated //go:debug for panicnil'
+
+-- file.go --
+//go:debug panicnil=1
+//go:debug panicnil=1
+
+package main
+
+func main() {
+}