Enqueue senders in Channel#close (#5880)

Fixes #5875

Author: carlhoerberg

spec/std/channel_spec.cr

@@ -137,6 +137,29 @@ describe Channel::Unbuffered do
     spawn { ch.send 123 }
     ch.receive?.should eq(123)
   end
+
+  it "wakes up the sender fiber when channel is closed" do
+    ch = Channel::Unbuffered(Nil).new
+    sender_closed = false
+    spawn do
+      ch.send nil
+      ch.send nil
+    rescue Channel::ClosedError
+      sender_closed = true
+    end
+    receiver_closed = false
+    spawn do
+      Fiber.yield
+      ch.receive
+    rescue Channel::ClosedError
+      receiver_closed = true
+    end
+    Fiber.yield
+    ch.close
+    Fiber.yield
+    sender_closed.should be_true
+    receiver_closed.should be_true
+  end
 end
 
 describe Channel::Buffered do

src/concurrent/channel.cr

@@ -30,6 +30,8 @@ abstract class Channel(T)
 
   def close
     @closed = true
+    Scheduler.enqueue @senders
+    @senders.clear
     Scheduler.enqueue @receivers
     @receivers.clear
     nil
@@ -259,6 +261,7 @@ class Channel::Unbuffered(T) < Channel(T)
     @value.tap do
       @has_value = false
       Scheduler.enqueue @sender.not_nil!
+      @sender = nil
     end
   end
 
@@ -269,4 +272,12 @@ class Channel::Unbuffered(T) < Channel(T)
   def full?
     @has_value || @receivers.empty?
   end
+
+  def close
+    super
+    if sender = @sender
+      Scheduler.enqueue sender
+      @sender = nil
+    end
+  end
 end