Scheduler: don't resume dead fibers and the @current fiber, fixes cry…

…stal-lang#3900

spec/std/concurrent/select_spec.cr

@@ -100,4 +100,63 @@ describe "select" do
     sleep
     x.should eq 1
   end
+
+  it "select with buffered shouldn't segfault on resuming already completed fiber, fixed #3900" do
+    ch1 = Channel::Buffered(Int32).new(1)
+    ch2 = Channel::Buffered(Int32).new(1)
+    res = [] of Int32
+
+    spawn do
+      3.times do
+        select
+        when x = ch1.receive
+          res << x
+        when y = ch2.receive
+          res << y
+        end
+      end
+    end
+
+    spawn do
+      3.times do |i|
+        select
+        when ch1.send(1)
+        when ch2.send(2)
+        end
+      end
+    end
+
+    sleep 0
+    res.should eq([1, 2, 1])
+  end
+
+  it "select with buffered channels shouldn't segfault when current fiber resumes itself" do
+    ch1 = Channel::Buffered(Int32).new(1)
+    ch2 = Channel::Buffered(Int32).new(1)
+    res = [] of Int32
+
+    spawn do
+      3.times do
+        select
+        when x = ch1.receive
+          res << x
+        when y = ch2.receive
+          res << y
+        end
+      end
+    end
+
+    spawn do
+      3.times do |i|
+        select
+        when ch1.send(1)
+        when ch2.send(2)
+        end
+      end
+      sleep 0
+    end
+
+    sleep 0
+    res.should eq([1, 2, 1])
+  end
 end

src/crystal/scheduler.cr

@@ -63,7 +63,11 @@ class Crystal::Scheduler
   end
 
   protected def reschedule : Nil
-    if runnable = @runnables.shift?
+    while runnable = @runnables.shift?
+      next if runnable == @current
+      break if runnable.alive
+    end
+    if runnable
       runnable.resume
     else
       Crystal::EventLoop.resume

src/fiber.cr

@@ -62,6 +62,7 @@ class Fiber
 
   # :nodoc:
   property previous : Fiber?
+  property alive : Bool = true
 
   # :nodoc:
   def self.inactive(fiber : Fiber)
@@ -155,6 +156,7 @@ class Fiber
     ex.inspect_with_backtrace(STDERR)
     STDERR.flush
   ensure
+    @alive = false
     @@stack_pool << @stack
 
     # Remove the current fiber from the linked list