improve the code comments around -seed

Explain why we print the random seed on failure, and why we accept
base64 padding when we don't use it.

While at it, the flagOptions.Random field is unused, so remove it. It
also seems wrong for it to exist; the random value is only for the seed
flag, which already has a field of its own.

main.go

@@ -155,6 +155,10 @@ func main1() int {
 		case errJustExit:
 		default:
 			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(opts.Seed))
 			}

shared.go

@@ -90,7 +90,6 @@ type flagOptions struct {
 	GarbleDir      string
 	DebugDir       string
 	Seed           []byte
-	Random         bool
 }
 
 // setFlagOptions sets flagOptions from the user supplied flags.
@@ -115,17 +114,17 @@ func setFlagOptions() error {
 			return fmt.Errorf("error generating random seed: %v", err)
 		}
 
-		opts.Random = true
-
-	} else {
+	} else if len(flagSeed) > 0 {
+		// We expect unpadded base64, but to be nice, accept padded
+		// strings too.
 		flagSeed = strings.TrimRight(flagSeed, "=")
 		seed, err := base64.RawStdEncoding.DecodeString(flagSeed)
 		if err != nil {
 			return fmt.Errorf("error decoding seed: %v", err)
 		}
 
-		if len(seed) != 0 && len(seed) < 8 {
-			return fmt.Errorf("the seed needs to be at least 8 bytes, but is only %v bytes", len(seed))
+		if len(seed) < 8 {
+			return fmt.Errorf("-seed needs at least 8 bytes, have %d", len(seed))
 		}
 
 		opts.Seed = seed