From 9edfec415743e272f44bb076a6513a82735ff776 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 16:00:02 +0300
Subject: [PATCH] sync: fix read from closed channel behavior as in Go (fixes
 #26796)

---
 vlib/sync/channel_select_3_test.v | 17 +++++++++++
 vlib/sync/channels.c.v            | 50 ++++++++++++++++++++++++-------
 vlib/v/gen/c/cgen.v               |  2 +-
 vlib/v/markused/walker.v          |  1 +
 4 files changed, 59 insertions(+), 11 deletions(-)

diff --git a/vlib/sync/channel_select_3_test.v b/vlib/sync/channel_select_3_test.v
index ef2b8eeaa..6b7609c3a 100644
--- a/vlib/sync/channel_select_3_test.v
+++ b/vlib/sync/channel_select_3_test.v
@@ -121,6 +121,23 @@ fn test_select_blocks() {
 	assert is_open == false
 }
 
+fn test_select_closed_receive_beats_timeout() {
+	ch := chan bool{}
+	ch.close()
+	mut got := true
+	mut timed_out := false
+	select {
+		got = <-ch {
+			assert got == false
+		}
+		10 * time.millisecond {
+			timed_out = true
+		}
+	}
+	assert got == false
+	assert timed_out == false
+}
+
 fn test_select_else_skips_closed_buffered_receive() {
 	ch := chan int{cap: 1}
 	ch <- 1
diff --git a/vlib/sync/channels.c.v b/vlib/sync/channels.c.v
index f944f4f5e..dbf7f59c2 100644
--- a/vlib/sync/channels.c.v
+++ b/vlib/sync/channels.c.v
@@ -547,20 +547,44 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) ChanState {
 	return .success
 }
 
-// Wait `timeout` on any of `channels[i]` until one of them can push (`is_push[i] = true`) or pop (`is_push[i] = false`)
-// object referenced by `objrefs[i]`. `timeout = time.infinite` means wait unlimited time. `timeout <= 0` means return
-// immediately if no transaction can be performed without waiting.
-// return value: the index of the channel on which a transaction has taken place
-//               -1 if waiting for a transaction has exceeded timeout
-//               -2 if all channels are closed
-
+// channel_select waits `timeout` on any of `channels[i]` until one of them can
+// push (`dir[i] == .push`) or pop (`dir[i] == .pop`) the object referenced by
+// `objrefs[i]`. `timeout = time.infinite` means wait unlimited time.
+// `timeout <= 0` means return immediately if no transaction can be performed
+// without waiting. It returns the selected channel index, `-1` on timeout, and
+// `-2` when all channels are closed.
 pub fn channel_select(mut channels []&Channel, dir []Direction, mut objrefs []voidptr, timeout time.Duration) int {
 	skip_closed_pop := timeout < 0
 	actual_timeout := if skip_closed_pop { time.Duration(0) } else { timeout }
-	return channel_select_priv(mut channels, dir, mut objrefs, actual_timeout, skip_closed_pop)
+	closed_pop_mode := if skip_closed_pop {
+		SelectClosedPopMode.skip
+	} else {
+		SelectClosedPopMode.closed
+	}
+	return channel_select_priv(mut channels, dir, mut objrefs, actual_timeout, closed_pop_mode)
+}
+
+enum SelectClosedPopMode {
+	closed
+	ready
+	skip
 }
 
-fn channel_select_priv(mut channels []&Channel, dir []Direction, mut objrefs []voidptr, timeout time.Duration, skip_closed_pop bool) int {
+// channel_select_lang is used by the language `select` implementation.
+// Closed receive cases stay selectable for blocking/timed selects to match
+// plain `<-ch` semantics, while `select ... else` still skips them.
+fn channel_select_lang(mut channels []&Channel, dir []Direction, mut objrefs []voidptr, timeout time.Duration) int {
+	skip_closed_pop := timeout < 0
+	actual_timeout := if skip_closed_pop { time.Duration(0) } else { timeout }
+	closed_pop_mode := if skip_closed_pop {
+		SelectClosedPopMode.skip
+	} else {
+		SelectClosedPopMode.ready
+	}
+	return channel_select_priv(mut channels, dir, mut objrefs, actual_timeout, closed_pop_mode)
+}
+
+fn channel_select_priv(mut channels []&Channel, dir []Direction, mut objrefs []voidptr, timeout time.Duration, closed_pop_mode SelectClosedPopMode) int {
 	$if debug_channels ? {
 		assert channels.len == dir.len
 		assert dir.len == objrefs.len
@@ -602,7 +626,7 @@ fn channel_select_priv(mut channels []&Channel, dir []Direction, mut objrefs []v
 			}
 			stat := if dir[i] == .push {
 				channels[i].try_push_priv(objrefs[i], true)
-			} else if skip_closed_pop {
+			} else if closed_pop_mode == .skip {
 				channels[i].try_pop_select_priv(objrefs[i])
 			} else {
 				channels[i].try_pop_priv(objrefs[i], true)
@@ -610,6 +634,12 @@ fn channel_select_priv(mut channels []&Channel, dir []Direction, mut objrefs []v
 			if stat == .success {
 				event_idx = i
 				break outer
+			} else if stat == .closed && dir[i] == .pop && closed_pop_mode == .ready {
+				unsafe {
+					C.memset(objrefs[i], 0, channels[i].objsize)
+				}
+				event_idx = i
+				break outer
 			} else if stat == .closed {
 				num_closed++
 			}
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 68cf40e11..717eeec5f 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7306,7 +7306,7 @@ fn (mut g Gen) select_expr(node ast.SelectExpr) {
 		g.writeln('}));\n')
 	}
 	select_result := g.new_tmp_var()
-	g.write('int ${select_result} = sync__channel_select(&${chan_array}, ${directions_array}, &${objs_array}, ')
+	g.write('int ${select_result} = sync__channel_select_lang(&${chan_array}, ${directions_array}, &${objs_array}, ')
 	if has_timeout {
 		g.expr(timeout_expr)
 	} else if has_else {
diff --git a/vlib/v/markused/walker.v b/vlib/v/markused/walker.v
index ad7ba5dfe..7fbddfd9b 100644
--- a/vlib/v/markused/walker.v
+++ b/vlib/v/markused/walker.v
@@ -2403,6 +2403,7 @@ fn (mut w Walker) mark_resource_dependencies() {
 	if w.uses_channel {
 		w.fn_by_name('sync.new_channel_st')
 		w.fn_by_name('sync.channel_select')
+		w.fn_by_name('sync.channel_select_lang')
 	}
 	if w.uses_lock {
 		w.mark_by_sym_name('sync.RwMutex')
-- 
2.39.5