gen: fix bug where unreached defer is executed (#8594)

vlib/v/ast/ast.v

@@ -333,6 +333,7 @@ pub:
 	skip_gen        bool // this function doesn't need to be generated (for example [if foo])
 pub mut:
 	stmts         []Stmt
+	defer_stmts   []DeferStmt
 	return_type   table.Type
 	has_return    bool
 	comments      []Comment // comments *after* the header, but *before* `{`; used for InterfaceDecl
@@ -891,7 +892,8 @@ pub:
 	stmts []Stmt
 	pos   token.Position
 pub mut:
-	ifdef string
+	ifdef     string
+	idx_in_fn int = -1 // index in FnDecl.defer_stmts
 }
 
 // `(3+4)`

vlib/v/checker/checker.v

@@ -3130,6 +3130,10 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			c.inside_const = false
 		}
 		ast.DeferStmt {
+			if node.idx_in_fn < 0 {
+				node.idx_in_fn = c.cur_fn.defer_stmts.len
+				c.cur_fn.defer_stmts << &node
+			}
 			c.stmts(node.stmts)
 		}
 		ast.EnumDecl {

vlib/v/gen/c/cgen.v

@@ -994,6 +994,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 		ast.DeferStmt {
 			mut defer_stmt := node
 			defer_stmt.ifdef = g.defer_ifdef
+			g.writeln('${g.defer_flag_var(defer_stmt)} = true;')
 			g.defer_stmts << defer_stmt
 		}
 		ast.EnumDecl {
@@ -1280,6 +1281,8 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 fn (mut g Gen) write_defer_stmts() {
 	for defer_stmt in g.defer_stmts {
 		g.writeln('// Defer begin')
+		g.writeln('if (${g.defer_flag_var(defer_stmt)} == true) {')
+		g.indent++
 		if defer_stmt.ifdef.len > 0 {
 			g.writeln(defer_stmt.ifdef)
 			g.stmts(defer_stmt.stmts)
@@ -1290,6 +1293,8 @@ fn (mut g Gen) write_defer_stmts() {
 			g.stmts(defer_stmt.stmts)
 			g.indent++
 		}
+		g.indent--
+		g.writeln('}')
 		g.writeln('// Defer end')
 	}
 }

vlib/v/gen/c/fn.v

@@ -65,6 +65,10 @@ fn (mut g Gen) gen_fn_decl(node ast.FnDecl, skip bool) {
 		g.cur_generic_types = []
 		return
 	}
+	cur_fn_save := g.cur_fn
+	defer {
+		g.cur_fn = cur_fn_save
+	}
 	g.cur_fn = node
 	fn_start_pos := g.out.len
 	g.write_v_source_line_info(node.pos)
@@ -131,6 +135,10 @@ fn (mut g Gen) gen_fn_decl(node ast.FnDecl, skip bool) {
 	if is_live_wrap {
 		impl_fn_name = 'impl_live_$name'
 	}
+	last_fn_c_name_save := g.last_fn_c_name
+	defer {
+		g.last_fn_c_name = last_fn_c_name_save
+	}
 	g.last_fn_c_name = impl_fn_name
 	//
 	if is_live_wrap {
@@ -179,6 +187,9 @@ fn (mut g Gen) gen_fn_decl(node ast.FnDecl, skip bool) {
 	}
 	g.definitions.writeln(');')
 	g.writeln(') {')
+	for defer_stmt in node.defer_stmts {
+		g.writeln('bool ${g.defer_flag_var(defer_stmt)} = false;')
+	}
 	if is_live_wrap {
 		// The live function just calls its implementation dual, while ensuring
 		// that the call is wrapped by the mutex lock & unlock calls.
@@ -258,6 +269,10 @@ fn (mut g Gen) gen_fn_decl(node ast.FnDecl, skip bool) {
 	}
 }
 
+fn (g &Gen) defer_flag_var(stmt &ast.DeferStmt) string {
+	return '${g.last_fn_c_name}_defer_$stmt.idx_in_fn'
+}
+
 fn (mut g Gen) write_defer_stmts_when_needed() {
 	if g.defer_stmts.len > 0 {
 		g.write_defer_stmts()

vlib/v/tests/defer_test.v

@@ -27,8 +27,7 @@ fn set_num(i int, mut n Num) {
 	println('Hi')
 	if i < 5 {
 		return
-	}
-	else {
+	} else {
 		n.val++
 	}
 }
@@ -60,15 +59,18 @@ fn test_defer_early_exit() {
 
 fn test_defer_option() {
 	mut ok := Num{0}
-	set_num_opt(mut ok) or {}
+	set_num_opt(mut ok) or { }
 	assert ok.val == 1
 }
 
 fn test_defer_with_anon_fn() {
-	mut f := &Num{val: 110}
+	mut f := &Num{
+		val: 110
+	}
 	defer {
 		assert f.add(1) == 111
 	}
+
 	go fn () {
 		defer {
 			println('deferred 1')
@@ -83,3 +85,19 @@ fn test_defer_with_anon_fn() {
 	x()
 	return
 }
+
+fn set_num_if(mut n Num, v int, cond bool) {
+	if cond {
+		defer {
+			n.val = v
+		}
+	}
+}
+
+fn test_defer_with_if() {
+	mut n := Num{0}
+	set_num_if(mut n, 10, true)
+	assert n.val == 10
+	set_num_if(mut n, 20, false)
+	assert n.val == 10
+}