Allow DB::Pool to be used a generic connection pool

examples/http_client.cr

@@ -0,0 +1,8 @@
+require "http"
+require "../src/db/pool"
+
+pool = DB::Pool.new { HTTP::Client.new(URI.parse("https://google.com")) }
+
+pool.checkout do |http|
+  pp http.get("/")
+end

examples/pool.cr

@@ -0,0 +1,30 @@
+require "../src/db/pool"
+require "uuid"
+
+class DummyIO < IO
+  def read(slice : Bytes)
+    sleep rand.seconds
+  end
+
+  def write(slice : Bytes) : Nil
+    sleep rand.seconds # simulate I/O yielding the CPU
+    STDOUT.puts "wrote #{String.new(slice)}"
+  end
+end
+
+pool = DB::Pool.new { DummyIO.new }
+channel = Channel(Nil).new
+count = 10
+
+count.times do
+  spawn do
+    pool.checkout do |io|
+      io << "hello"
+    end
+    channel.send nil
+  end
+end
+
+count.times { channel.receive }
+
+pool.close

src/db/error.cr

@@ -14,10 +14,10 @@ module DB
   # Raised when an established connection is lost
   # probably due to socket/network issues.
   # It is used by the connection pool retry logic.
-  class ConnectionLost < Error
-    getter connection : Connection
+  class ConnectionLost(T) < Error
+    getter connection
 
-    def initialize(@connection)
+    def initialize(@connection : T)
     end
   end
 

src/db/pool.cr

@@ -1,5 +1,7 @@
 require "weak_ref"
 
+require "./error"
+
 module DB
   class Pool(T)
     # Pool configuration
@@ -52,12 +54,19 @@ module DB
       @idle.clear
     end
 
-    record Stats, open_connections : Int32
+    record Stats,
+      open_connections : Int32,
+      idle_connections : Int32,
+      in_flight_connections : Int32,
+      max_connections : Int32
 
     # Returns stats of the pool
     def stats
       Stats.new(
-        open_connections: @total.size
+        open_connections: @total.size,
+        idle_connections: @idle.size,
+        in_flight_connections: @inflight,
+        max_connections: @max_pool_size,
       )
     end
 
@@ -91,10 +100,19 @@ module DB
         resource
       end
 
-      res.before_checkout
+      if res.responds_to?(:before_checkout)
+        res.before_checkout
+      end
       res
     end
 
+    def checkout(&block : T ->)
+      connection = checkout
+      yield connection
+    ensure
+      release connection if connection
+    end
+
     # ```
     # selected, is_candidate = pool.checkout_some(candidates)
     # ```
@@ -122,7 +140,9 @@ module DB
       sync do
         if can_increase_idle_pool
           @idle << resource
-          resource.after_release
+          if resource.responds_to?(:after_release)
+            resource.after_release
+          end
           idle_pushed = true
         else
           resource.close
@@ -215,7 +235,7 @@ module DB
           sync_dec_waiting_resource
         when timeout(@checkout_timeout.seconds)
           sync_dec_waiting_resource
-          raise DB::PoolTimeout.new
+          raise DB::PoolTimeout.new("Could not check out a connection in #{@checkout_timeout} seconds")
         end
       end
     {% else %}
@@ -232,7 +252,7 @@ module DB
           sync_dec_waiting_resource
         when 1
           sync_dec_waiting_resource
-          raise DB::PoolTimeout.new
+          raise DB::PoolTimeout.new("Could not check out a connection in #{@checkout_timeout} seconds")
         else
           raise DB::Error.new
         end