syntax: extend FuzzQuote to check with our parser

That is, verify that our parser also parses the quoted string
as a single word in a simple command.

The updated fuzzer quickly found that we forgot about the empty string.
An empty string must be quoted, as otherwise it expands to zero fields.
The printf verification did not catch this.

syntax/fuzz_test.go

@@ -12,6 +12,8 @@ import (
 	"os/exec"
 	"strings"
 	"testing"
+
+	qt "github.com/frankban/quicktest"
 )
 
 func FuzzQuote(f *testing.F) {
@@ -57,8 +59,14 @@ func FuzzQuote(f *testing.F) {
 			panic(fmt.Sprintf("unknown lang variant: %d", lang))
 		}
 
-		// TODO: Also double-check with our parser.
-		// That should allow us to fuzz Bats too, for instance.
+		f, err := NewParser(Variant(lang)).Parse(strings.NewReader(quoted), "")
+		if err != nil {
+			t.Fatalf("parse error on %q quoted as %s: %v", s, quoted, err)
+		}
+		qt.Assert(t, len(f.Stmts), qt.Equals, 1, qt.Commentf("in: %q, quoted: %s", s, quoted))
+		call, ok := f.Stmts[0].Cmd.(*CallExpr)
+		qt.Assert(t, ok, qt.IsTrue, qt.Commentf("in: %q, quoted: %s", s, quoted))
+		qt.Assert(t, len(call.Args), qt.Equals, 1, qt.Commentf("in: %q, quoted: %s", s, quoted))
 
 		// Beware that this might run arbitrary code
 		// if Quote is too naive and allows ';' or '$'.

syntax/quote.go

@@ -46,6 +46,11 @@ const (
 // Some strings do not require any quoting and are returned unchanged.
 // Those strings can be directly surrounded in single quotes as well.
 func Quote(s string, lang LangVariant) (string, error) {
+	if s == "" {
+		// Special case; an empty string must always be quoted,
+		// as otherwise it expands to zero fields.
+		return "''", nil
+	}
 	shellChars := false
 	nonPrintable := false
 	offs := 0

syntax/quote_test.go

@@ -16,7 +16,7 @@ func TestQuote(t *testing.T) {
 		lang LangVariant
 		want interface{}
 	}{
-		{"", LangBash, ``},
+		{"", LangBash, `''`},
 		{"\a", LangBash, `$'\a'`},
 		{"\b", LangBash, `$'\b'`},
 		{"\f", LangBash, `$'\f'`},