allow DB to use a connection pool.

allow Driver to parse connection pool options for extensibility.
fix waiting_resource counter after a timeout was generated.

spec/db_spec.cr

@@ -61,7 +61,7 @@ describe DB do
   it "should raise if the sole connection is been used" do
     with_dummy do |db|
       db.query "1" do |rs|
-        expect_raises Exception, /DB Pool Exhausted/ do
+        expect_raises DB::PoolTimeout do
           db.scalar "2"
         end
       end

spec/dummy_driver.cr

@@ -187,7 +187,7 @@ end
 def with_dummy
   DummyDriver::DummyConnection.clear_connections
 
-  DB.open "dummy://host" do |db|
+  DB.open "dummy://host?checkout_timeout=0.5" do |db|
     yield db
   end
 end

spec/pool_spec.cr

@@ -131,6 +131,13 @@ describe DB::Pool do
     end
   end
 
+  it "should be able to release after a timeout" do
+    pool = DB::Pool.new(->{ Closable.new }, max_pool_size: 1, checkout_timeout: 0.1)
+    a = pool.checkout
+    pool.checkout rescue nil
+    pool.release a
+  end
+
   it "should close if max idle amount is reached" do
     all = [] of Closable
     pool = DB::Pool.new(->{ Closable.new.tap { |c| all << c } }, max_pool_size: 3, max_idle_pool_size: 1)

src/db.cr

@@ -6,8 +6,7 @@ require "uri"
 #
 # Drivers implementors check `Driver` class.
 #
-# Currently a *single connection* to the database is stablished.
-# In the future a connection pool and transaction support will be available.
+# DB manage a connection pool. The connection pool can be configured by `URI` query. See `Database`.
 #
 # ### Usage
 #

src/db/database.cr

@@ -1,7 +1,14 @@
+require "http/params"
+
 module DB
   # Acts as an entry point for database access.
-  # Currently it creates a single connection to the database.
-  # Eventually a connection pool will be handled.
+  # Connections are managed by a pool.
+  # The connection pool can be configured from URI parameters:
+  #
+  #   - initial_pool_size (default 1)
+  #   - max_pool_size (default 1)
+  #   - max_idle_pool_size (default 1)
+  #   - checkout_timeout (default 5.0)
   #
   # It should be created from DB module. See `DB#open`.
   #
@@ -13,19 +20,21 @@ module DB
     # Returns the uri with the connection settings to the database
     getter uri
 
-    @connection : Connection?
+    @pool : Pool(Connection)
 
     # :nodoc:
     def initialize(@driver : Driver, @uri : URI)
-      @in_pool = true
-      @connection = @driver.build_connection(self)
+      # TODO: PR HTTP::Params.new -> HTTP::Params.new(Hash(String, Array(String)).new)
+      params = (query = uri.query) ? HTTP::Params.parse(query) : HTTP::Params.new(Hash(String, Array(String)).new)
+      pool_options = @driver.connection_pool_options(params)
+
+      @pool = uninitialized Pool(Connection) # in order to use self in the factory proc
+      @pool = Pool.new(->{ @driver.build_connection(self).as(Connection) }, **pool_options)
     end
 
     # Closes all connection to the database.
     def close
-      @connection.try &.close
-      # prevent GC Warning: Finalization cycle involving discovered by mysql implementation
-      @connection = nil
+      @pool.close
     end
 
     # :nodoc:
@@ -41,14 +50,12 @@ module DB
 
     # :nodoc:
     def get_from_pool
-      raise "DB Pool Exhausted" unless @in_pool
-      @in_pool = false
-      @connection.not_nil!
+      @pool.checkout
     end
 
     # :nodoc:
     def return_to_pool(connection)
-      @in_pool = true
+      @pool.release connection
     end
 
     # yields a connection from the pool

src/db/driver.cr

@@ -27,5 +27,14 @@ module DB
   # driver implementation instructions.
   abstract class Driver
     abstract def build_connection(db : Database) : Connection
+
+    def connection_pool_options(params : HTTP::Params)
+      {
+        initial_pool_size:  params.fetch("initial_pool_size", 1).to_i,
+        max_pool_size:      params.fetch("max_pool_size", 1).to_i,
+        max_idle_pool_size: params.fetch("max_idle_pool_size", 1).to_i,
+        checkout_timeout:   params.fetch("checkout_timeout", 5.0).to_f,
+      }
+    end
   end
 end

src/db/pool.cr

@@ -72,12 +72,18 @@ module DB
       timeout.start
       # if there are no available resources, sleep until one is available
       @availability_channel.receive
-      timeout.raise_if_reached
+      if timeout.timeout_reached?
+        dec_waiting_resource
+        raise DB::PoolTimeout.new
+      end
 
       # double check there is something available to be checkedout
       while @available.empty?
         @availability_channel.receive
-        timeout.raise_if_reached
+        if timeout.timeout_reached?
+          dec_waiting_resource
+          raise DB::PoolTimeout.new
+        end
       end
 
       timeout.cancel
@@ -125,10 +131,6 @@ module DB
       def timeout_reached?
         @should_timeout
       end
-
-      def raise_if_reached
-        raise DB::PoolTimeout.new if timeout_reached?
-      end
     end
   end
 end