print chosen seed when building with -seed=random

The seedFlag.random field had never worked,
as my refactor in December 2021 never set it to true.

Even if the boolean was working, we only printed the random seed
when we failed. It's still useful to see it when a build succeeds,
for example when wanting to reproduce the same binary
or when wanting to reverse a panic from the produced binary.

Add a test this time.

Fixes #696.

main.go

@@ -78,6 +78,8 @@ func (f seedFlag) String() string {
 
 func (f *seedFlag) Set(s string) error {
 	if s == "random" {
+		f.random = true // to show the random seed we chose
+
 		f.bytes = make([]byte, 16) // random 128 bit seed
 		if _, err := cryptorand.Read(f.bytes); err != nil {
 			return fmt.Errorf("error generating random seed: %v", err)
@@ -234,18 +236,20 @@ func main1() int {
 		usage()
 		return 2
 	}
+
+	// If a random seed was used, the user won't be able to reproduce the
+	// same output or failure unless we print the random seed we chose.
+	// If the build failed and a random seed was used,
+	// the failure might not reproduce with a different seed.
+	// Print it before we exit.
+	if flagSeed.random {
+		fmt.Fprintf(os.Stderr, "-seed chosen at random: %s\n", base64.RawStdEncoding.EncodeToString(flagSeed.bytes))
+	}
 	if err := mainErr(args); err != nil {
 		if code, ok := err.(errJustExit); ok {
 			return int(code)
 		}
 		fmt.Fprintln(os.Stderr, err)
-
-		// If the build failed and a random seed was used,
-		// the failure might not reproduce with a different seed.
-		// Print it before we exit.
-		if flagSeed.random {
-			fmt.Fprintf(os.Stderr, "random seed: %s\n", base64.RawStdEncoding.EncodeToString(flagSeed.bytes))
-		}
 		return 1
 	}
 	return 0

testdata/script/seed.txtar

@@ -63,6 +63,7 @@ cp stderr importedpkg-seed-static-2
 
 # Use a random seed, which should always trigger a full build.
 garble -seed=random build -v
+stderr -count=1 '^-seed chosen at random: .+'
 stderr -count=1 '^runtime$'
 stderr -count=1 '^test/main$'
 exec ./main$exe