Handle multiple fibers waiting for more resources than available

src/db/pool.cr

@@ -63,6 +63,9 @@ module DB
 
     def checkout : T
       res = sync do
+        resource = nil
+
+        until resource
           resource = if @idle.empty?
                        if can_increase_pool?
                          @inflight += 1
@@ -71,11 +74,17 @@ module DB
                          r
                        else
                          unsync { wait_for_available }
+                         # The wait for available can unlock
+                         # multiple fibers waiting for a resource.
+                         # Although only one will pick it due to the lock
+                         # in the end of the unsync, the pick_available
+                         # will return nil
                          pick_available
                        end
                      else
                        pick_available
                      end
+        end
 
         @idle.delete resource
 
@@ -194,7 +203,7 @@ module DB
     end
 
     private def pick_available
-      @idle.first
+      @idle.first?
     end
 
     private def wait_for_available