Select: fix case when select send executed before select receive, fixed #3862

spec/std/concurrent/select_spec.cr

@@ -0,0 +1,96 @@
+require "spec"
+
+describe "select" do
+  it "select many receviers" do
+    ch1 = Channel(Int32).new
+    ch2 = Channel(Int32).new
+    res = [] of Int32
+    spawn do
+      10.times do |i|
+        (i % 2 == 0) ? ch1.send(i) : ch2.send(i)
+      end
+    end
+
+    10.times do
+      select
+      when x = ch1.receive
+        res << x
+      when y = ch2.receive
+        res << y
+      end
+    end
+    res.should eq (0...10).to_a
+  end
+
+  it "select many senders" do
+    ch1 = Channel(Int32).new
+    ch2 = Channel(Int32).new
+    res = [] of Int32
+    spawn do
+      5.times { res << ch1.receive }
+    end
+
+    spawn do
+      5.times { res << ch2.receive }
+    end
+
+    10.times do |i|
+      select
+      when ch1.send(i)
+      when ch2.send(i)
+      end
+    end
+    res.should eq (0...10).to_a
+  end
+
+  it "select many receivers, senders" do
+    ch1 = Channel(Int32).new
+    ch2 = Channel(Int32).new
+    res = [] of Int32
+    spawn do
+      10.times do |i|
+        select
+        when x = ch1.receive
+          res << x
+        when ch2.send(i)
+        end
+      end
+    end
+
+    10.times do |i|
+      select
+      when ch1.send(i)
+      when y = ch2.receive
+        res << y
+      end
+    end
+    res.should eq (0...10).to_a
+  end
+
+  it "select should work with send which started before receive, fixed #3862" do
+    ch1 = Channel(Int32).new
+    ch2 = Channel(Int32).new
+
+    spawn do
+      select
+      when ch1.send(1)
+      when ch2.receive
+      end
+    end
+
+    x = nil
+
+    spawn do
+      select
+      when a = ch1.receive
+        x = a
+      when b = ch2.receive
+        x = b
+      end
+    end
+
+    Fiber.yield
+
+    x.should eq 1
+  end
+end

src/concurrent/channel.cr

@@ -263,7 +263,7 @@ class Channel::Unbuffered(T) < Channel(T)
   end
 
   def empty?
-    !@has_value
+    !@has_value && @senders.empty?
   end
 
   def full?