frontend/dockerfile: BFlags.Parse: include flag with "--" prefix in e…

…rrors

Before this change, only the name of the flag would be printed as part
of errors. Change the errors to include the flag-name _including_ the
"--" prefix to make it more clear the error is about a flag that was
provided but invalid.

Before this change:

    Dockerfile:2
    --------------------
    1 |     FROM alpine
    2 | >>> COPY --nosuchflag . .
    3 |
    --------------------
    ERROR: failed to solve: dockerfile parse error on line 2: unknown flag: nosuchflag

    Dockerfile:4
    --------------------
    2 |     FROM alpine
    3 |     WORKDIR /test
    4 | >>> COPY --exclude /.git . .
    5 |
    --------------------
    ERROR: failed to solve: dockerfile parse error on line 4: missing a value on flag: exclude

With this change;

    Dockerfile:2
    --------------------
    1 |     FROM alpine
    2 | >>> COPY --nosuchflag . .
    3 |
    --------------------
    ERROR: failed to solve: dockerfile parse error on line 2: unknown flag: --nosuchflag

    Dockerfile:4
    --------------------
    2 |     FROM alpine
    3 |     WORKDIR /test
    4 | >>> COPY --exclude /.git . .
    5 |
    --------------------
    ERROR: failed to solve: dockerfile parse error on line 4: missing a value on flag: --exclude

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

frontend/dockerfile/dockerfile_mount_test.go

@@ -115,7 +115,7 @@ RUN --mont=target=/mytmp,type=tmpfs /bin/true
 		},
 	}, nil)
 	require.Error(t, err)
-	require.Contains(t, err.Error(), "unknown flag: mont")
+	require.Contains(t, err.Error(), "unknown flag: --mont")
 	require.Contains(t, err.Error(), "did you mean mount?")
 
 	dockerfile = []byte(`

frontend/dockerfile/instructions/bflag.go

@@ -147,8 +147,8 @@ func (bf *BFlags) Parse() error {
 		return errors.Wrap(bf.Err, "error setting up flags")
 	}
 
-	for _, arg := range bf.Args {
-		if arg == "--" {
+	for _, a := range bf.Args {
+		if a == "--" {
 			// Stop processing further arguments as flags. We're matching
 			// the POSIX Utility Syntax Guidelines here;
 			// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02
@@ -158,21 +158,21 @@ func (bf *BFlags) Parse() error {
 			// > should be treated as operands, even if they begin with the '-' character.
 			return nil
 		}
-
-		if !strings.HasPrefix(arg, "--") {
-			return errors.Errorf("arg should start with -- : %s", arg)
+		if !strings.HasPrefix(a, "--") {
+			return errors.Errorf("arg should start with -- : %s", a)
 		}
 
-		arg, value, hasValue := strings.Cut(arg[2:], "=")
+		flagName, value, hasValue := strings.Cut(a, "=")
+		arg := flagName[2:]
 
 		flag, ok := bf.flags[arg]
 		if !ok {
-			err := errors.Errorf("unknown flag: %s", arg)
+			err := errors.Errorf("unknown flag: %s", flagName)
 			return suggest.WrapError(err, arg, allFlags(bf.flags), true)
 		}
 
 		if _, ok = bf.used[arg]; ok && flag.flagType != stringsType {
-			return errors.Errorf("duplicate flag specified: %s", arg)
+			return errors.Errorf("duplicate flag specified: %s", flagName)
 		}
 
 		bf.used[arg] = flag
@@ -181,7 +181,7 @@ func (bf *BFlags) Parse() error {
 		case boolType:
 			// value == "" is only ok if no "=" was specified
 			if hasValue && value == "" {
-				return errors.Errorf("missing a value on flag: %s", arg)
+				return errors.Errorf("missing a value on flag: %s", flagName)
 			}
 
 			switch strings.ToLower(value) {
@@ -190,18 +190,18 @@ func (bf *BFlags) Parse() error {
 			case "false":
 				flag.Value = "false"
 			default:
-				return errors.Errorf("expecting boolean value for flag %s, not: %s", arg, value)
+				return errors.Errorf("expecting boolean value for flag %s, not: %s", flagName, value)
 			}
 
 		case stringType:
 			if !hasValue {
-				return errors.Errorf("missing a value on flag: %s", arg)
+				return errors.Errorf("missing a value on flag: %s", flagName)
 			}
 			flag.Value = value
 
 		case stringsType:
 			if !hasValue {
-				return errors.Errorf("missing a value on flag: %s", arg)
+				return errors.Errorf("missing a value on flag: %s", flagName)
 			}
 			flag.StringValues = append(flag.StringValues, value)
 

frontend/dockerfile/instructions/parse_test.go

@@ -229,7 +229,7 @@ func TestErrorCases(t *testing.T) {
 		{
 			name:          "MAINTAINER unknown flag",
 			dockerfile:    "MAINTAINER --boo joe@example.com",
-			expectedError: "unknown flag: boo",
+			expectedError: "unknown flag: --boo",
 		},
 		{
 			name:          "Chaining ONBUILD",