interp: support lowercase volume names in $PATH

On Windows, "C:" is a valid volume name, just like "c:". We were
incorrectly assuming that the letter had to be uppercase.

Add a simple regression test for Windows, which sets up a PATH with a
script in a directory, then tries to execute it via such a PATH with a
lowercase volume name.

Fixes mvdan#355.

interp/interp.go

@@ -1209,6 +1209,10 @@ func (r *Runner) findExecutable(file string, exts []string) string {
 	return ""
 }
 
+func driveLetter(c byte) bool {
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
+}
+
 // splitList is like filepath.SplitList, but always using the unix path
 // list separator ':'. On Windows, it also makes sure not to split
 // [A-Z]:[/\].
@@ -1225,8 +1229,7 @@ func splitList(path string) []string {
 	for i := 0; i < len(list); i++ {
 		s := list[i]
 		switch {
-		case len(s) != 1, s[0] < 'A', s[0] > 'Z':
-			// not a disk name
+		case len(s) != 1, !driveLetter(s[0]):
 		case i+1 >= len(list):
 			// last element
 		case strings.IndexAny(list[i+1], `/\`) != 0:

interp/interp_test.go

@@ -2728,3 +2728,35 @@ func TestRunnerEnvNoModify(t *testing.T) {
 		t.Fatalf("\nwant: %q\ngot:  %q", want, got)
 	}
 }
+
+func TestMalformedPathOnWindows(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Skipping windows test on non-windows GOOS")
+	}
+	dir, err := ioutil.TempDir("", "interp-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	path := filepath.Join(dir, "test.cmd")
+	script := []byte("@echo foo")
+	if err := ioutil.WriteFile(path, script, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// set PATH to c:\tmp\dir instead of C:\tmp\dir
+	volume := filepath.VolumeName(dir)
+	pathList := strings.ToLower(volume) + dir[len(volume):]
+
+	file, _ := syntax.NewParser().Parse(strings.NewReader("test.cmd"), "")
+	var cb concBuffer
+	r, _ := New(Env(expand.ListEnviron("PATH="+pathList)), StdIO(nil, &cb, &cb))
+	if err := r.Run(context.Background(), file); err != nil {
+		t.Fatal(err)
+	}
+	want := "foo\r\n"
+	if got := cb.String(); got != want {
+		t.Fatalf("wrong output:\nwant: %q\ngot:  %q", want, got)
+	}
+}