Fix mt issues (#178)

src/db/pool.cr

@@ -216,8 +216,10 @@ module DB
 
     private def build_resource : T
       resource = @factory.call
+      sync do
         @total << resource
         @idle << resource
+      end
       resource
     end
 

src/db/pool_prepared_statement.cr

@@ -7,6 +7,7 @@ module DB
   class PoolPreparedStatement < PoolStatement
     # connections where the statement was prepared
     @connections = Set(WeakRef(Connection)).new
+    @mutex = Mutex.new
 
     def initialize(db : Database, query : String)
       super
@@ -20,6 +21,7 @@ module DB
     end
 
     protected def do_close
+      @mutex.synchronize do
         # TODO close all statements on all connections.
         # currently statements are closed when the connection is closed.
 
@@ -27,23 +29,33 @@ module DB
         # deallocate itself when the connection is free.
         @connections.clear
       end
+    end
 
     # builds a statement over a real connection
     # the connection is registered in `@connections`
     private def build_statement : Statement
       clean_connections
-      conn, existing = @db.checkout_some(@connections)
+
+      conn, existing = @mutex.synchronize do
+        @db.checkout_some(@connections)
+      end
+
       begin
         stmt = conn.prepared.build(@query)
       rescue ex
         conn.release
         raise ex
       end
-      @connections << WeakRef.new(conn) unless existing
+      unless existing
+        @mutex.synchronize do
+          @connections << WeakRef.new(conn)
+        end
+      end
       stmt
     end
 
     private def clean_connections
+      @mutex.synchronize do
         # remove disposed or closed connections
         @connections.each do |ref|
           conn = ref.value
@@ -53,4 +65,5 @@ module DB
         end
       end
     end
+  end
 end

src/db/string_key_cache.cr

@@ -1,21 +1,28 @@
 module DB
   class StringKeyCache(T)
     @cache = {} of String => T
+    @mutex = Mutex.new
 
     def fetch(key : String) : T
+      @mutex.synchronize do
         value = @cache.fetch(key, nil)
         value = @cache[key] = yield unless value
         value
       end
+    end
 
     def each_value
+      @mutex.synchronize do
         @cache.each do |_, value|
           yield value
         end
       end
+    end
 
     def clear
+      @mutex.synchronize do
         @cache.clear
       end
     end
+  end
 end