thread_sync.c: Clarify and document the behavior of timeout == 0

[Feature #18982]

Instead of introducing an `exception: false` argument to have `non_block`
return nil rather than raise, we can clearly document that a timeout of 0
immediately returns.

The code is refactored a bit to avoid doing a time calculation in
such case.

2 files changed, 14 insertions, 1 deletions

diff --git a/spec/ruby/shared/queue/deque.rb b/spec/ruby/shared/queue/deque.rb
index ed32bd29c8..616e56ec8a 100644
--- a/spec/ruby/shared/queue/deque.rb
+++ b/spec/ruby/shared/queue/deque.rb
@@ -87,6 +87,13 @@ describe :queue_deq, shared: true do
         t.join
       end
 
+      it "immediately returns nil if no item is available and the timeout is 0" do
+        q = @object.call
+        q << 1
+        q.send(@method, timeout: 0).should == 1
+        q.send(@method, timeout: 0).should == nil
+      end
+
       it "raise TypeError if timeout is not a valid numeric" do
         q = @object.call
         -> { q.send(@method, timeout: "1") }.should raise_error(
diff --git a/spec/ruby/shared/sizedqueue/enque.rb b/spec/ruby/shared/sizedqueue/enque.rb
index 126470594a..059f1025a7 100644
--- a/spec/ruby/shared/sizedqueue/enque.rb
+++ b/spec/ruby/shared/sizedqueue/enque.rb
@@ -73,7 +73,13 @@ describe :sizedqueue_enq, shared: true do
         t.join
       end
 
-      it "returns nil if no item is available in time" do
+      it "returns nil if no space is avialable and timeout is 0" do
+        q = @object.call(1)
+        q.send(@method, 1, timeout: 0).should == q
+        q.send(@method, 2, timeout: 0).should == nil
+      end
+
+      it "returns nil if no space is available in time" do
         q = @object.call(1)
         q << 1
         t = Thread.new {