interp: Return invalid globs when nullglob is set

Prior to this commit invalid globs were returned as null when nullglob
was set, which resulted in test expressions failing:

  $ shopt -s nullglob; [ -n butter ] && echo bubbles
  "-n": executable file not found in $PATH

After this commit invalid globs are returned verbatim when nullglob is
set.

expand/expand.go

@@ -409,14 +409,17 @@ func Fields(cfg *Config, words ...*syntax.Word) ([]string, error) {
 			for _, field := range wfields {
 				path, doGlob := cfg.escapedGlobField(field)
 				var matches []string
+				var syntaxError *pattern.SyntaxError
 				if doGlob && cfg.ReadDir != nil {
 					matches, err = cfg.glob(dir, path)
-					if err != nil {
-						return nil, err
-					}
-					if len(matches) > 0 || cfg.NullGlob {
-						fields = append(fields, matches...)
-						continue
+					if !errors.As(err, &syntaxError) {
+						if err != nil {
+							return nil, err
+						}
+						if len(matches) > 0 || cfg.NullGlob {
+							fields = append(fields, matches...)
+							continue
+						}
 					}
 				}
 				fields = append(fields, cfg.fieldJoin(field))
@@ -851,8 +854,7 @@ func (cfg *Config) glob(base, pat string) ([]string, error) {
 		}
 		expr, err := pattern.Regexp(part, pattern.Filenames)
 		if err != nil {
-			// If any glob part is not a valid pattern, don't glob.
-			return nil, nil
+			return nil, err
 		}
 		rx := regexp.MustCompile("^" + expr + "$")
 		var newMatches []string

interp/interp_test.go

@@ -2581,6 +2581,12 @@ set +o pipefail
 		"shopt -s nullglob; touch existing-1; echo missing-* existing-*",
 		"existing-1\n",
 	},
+	// Ensure that setting nullglob does not return invalid globs as null
+	// strings.
+	{
+		"shopt -s nullglob; [ -n butter ] && echo bubbles",
+		"bubbles\n",
+	},
 	{
 		"cat <<EOF\n{foo_interp_missing,bar_interp_missing}\nEOF",
 		"{foo_interp_missing,bar_interp_missing}\n",

pattern/pattern.go

@@ -20,6 +20,15 @@ import (
 // Not all functions change their behavior with all of the options below.
 type Mode uint
 
+type SyntaxError struct {
+	msg string
+	err error
+}
+
+func (e SyntaxError) Error() string { return e.msg }
+
+func (e SyntaxError) Unwrap() error { return e.err }
+
 const (
 	Shortest  Mode = 1 << iota // prefer the shortest match.
 	Filenames                  // "*" and "?" don't match slashes; only "**" does
@@ -85,13 +94,13 @@ writeLoop:
 			}
 		case '\\':
 			if i++; i >= len(pat) {
-				return "", fmt.Errorf(`\ at end of pattern`)
+				return "", &SyntaxError{msg: `\ at end of pattern`}
 			}
 			buf.WriteString(regexp.QuoteMeta(string(pat[i])))
 		case '[':
 			name, err := charClass(pat[i:])
 			if err != nil {
-				return "", err
+				return "", &SyntaxError{msg: "charClass invalid", err: err}
 			}
 			if name != "" {
 				buf.WriteString(name)
@@ -110,19 +119,19 @@ writeLoop:
 			}
 			buf.WriteByte(c)
 			if i++; i >= len(pat) {
-				return "", fmt.Errorf("[ was not matched with a closing ]")
+				return "", &SyntaxError{msg: "[ was not matched with a closing ]"}
 			}
 			switch c = pat[i]; c {
 			case '!', '^':
 				buf.WriteByte('^')
 				if i++; i >= len(pat) {
-					return "", fmt.Errorf("[ was not matched with a closing ]")
+					return "", &SyntaxError{msg: "[ was not matched with a closing ]"}
 				}
 			}
 			if c = pat[i]; c == ']' {
 				buf.WriteByte(']')
 				if i++; i >= len(pat) {
-					return "", fmt.Errorf("[ was not matched with a closing ]")
+					return "", &SyntaxError{msg: "[ was not matched with a closing ]"}
 				}
 			}
 			rangeStart := byte(0)
@@ -140,7 +149,7 @@ writeLoop:
 					break loopBracket
 				}
 				if rangeStart != 0 && rangeStart > c {
-					return "", fmt.Errorf("invalid range: %c-%c", rangeStart, c)
+					return "", &SyntaxError{msg: fmt.Sprintf("invalid range: %c-%c", rangeStart, c)}
 				}
 				if c == '-' {
 					rangeStart = pat[i-1]
@@ -149,7 +158,7 @@ writeLoop:
 				}
 			}
 			if i >= len(pat) {
-				return "", fmt.Errorf("[ was not matched with a closing ]")
+				return "", &SyntaxError{msg: "[ was not matched with a closing ]"}
 			}
 		case '{':
 			if mode&Braces == 0 {
@@ -183,7 +192,7 @@ writeLoop:
 				start, err1 := strconv.Atoi(match[1])
 				end, err2 := strconv.Atoi(match[2])
 				if err1 != nil || err2 != nil || start > end {
-					return "", fmt.Errorf("invalid range: %q", match[0])
+					return "", &SyntaxError{msg: fmt.Sprintf("invalid range: %q", match[0])}
 				}
 				// TODO: can we do better here?
 				buf.WriteString("(?:")