interp: handle the last exit status code better

In some edge cases, such as commands with no arguments or background
commands, we didn't properly reset it to 0.

Doing that breaks expansions like "$?"; to properly handle all cases, we
need to keep track of the current and last status codes separately, at
least while we execute a statement.

As a bonus, we can deduplicate a few places where we set r.exit = 0.

Fixes #506.

interp/builtin.go

@@ -54,7 +54,7 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 		r.exitShell = true
 		switch len(args) {
 		case 0:
-			return r.exit
+			return r.lastExit
 		case 1:
 			n, err := strconv.Atoi(args[0])
 			if err != nil {

interp/interp.go

@@ -435,7 +435,8 @@ type Runner struct {
 	noErrExit bool
 
 	err       error // current shell exit code or fatal error
-	exit      int   // current (last) exit status code
+	exit      int   // current exit status code
+	lastExit  int   // last exit status code
 	exitShell bool  // whether the shell needs to exit
 
 	bgShells errgroup.Group
@@ -752,6 +753,7 @@ func (r *Runner) stmt(ctx context.Context, st *syntax.Stmt) {
 	if r.stop(ctx) {
 		return
 	}
+	r.exit = 0
 	if st.Background {
 		r2 := r.Subshell()
 		st2 := *st
@@ -762,6 +764,7 @@ func (r *Runner) stmt(ctx context.Context, st *syntax.Stmt) {
 	} else {
 		r.stmtSync(ctx, st)
 	}
+	r.lastExit = r.exit
 }
 
 func (r *Runner) stmtSync(ctx context.Context, st *syntax.Stmt) {
@@ -777,9 +780,7 @@ func (r *Runner) stmtSync(ctx context.Context, st *syntax.Stmt) {
 			defer cls.Close()
 		}
 	}
-	if st.Cmd == nil {
-		r.exit = 0
-	} else {
+	if st.Cmd != nil {
 		r.cmd(ctx, st.Cmd)
 	}
 	if st.Negated {
@@ -825,6 +826,7 @@ func (r *Runner) Subshell() *Runner {
 		opts:        r.opts,
 		usedNew:     r.usedNew,
 		exit:        r.exit,
+		lastExit:    r.lastExit,
 
 		origStdout: r.origStdout, // used for process substitutions
 	}
@@ -1023,7 +1025,6 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 			}
 		}
 	case *syntax.TestClause:
-		r.exit = 0
 		if r.bashTest(ctx, x.X, false) == "" && r.exit == 0 {
 			// to preserve exit status code 2 for regex errors, etc
 			r.exit = 1

interp/interp_test.go

@@ -232,6 +232,8 @@ var runTests = []runTest{
 		"for i in 1; do break a; done",
 		"usage: break [n]\nexit status 2 #JUSTERR",
 	},
+	{"false; a=b", ""},
+	{"false; false &", ""},
 
 	// we don't need to follow bash error strings
 	{"exit a", "invalid exit status code: \"a\"\nexit status 2 #JUSTERR"},

interp/vars.go

@@ -60,7 +60,7 @@ func (r *Runner) lookupVar(name string) expand.Variable {
 	case "@", "*":
 		vr.Kind, vr.List = expand.Indexed, r.Params
 	case "?":
-		vr.Kind, vr.Str = expand.String, strconv.Itoa(r.exit)
+		vr.Kind, vr.Str = expand.String, strconv.Itoa(r.lastExit)
 	case "$":
 		vr.Kind, vr.Str = expand.String, strconv.Itoa(os.Getpid())
 	case "PPID":