Implement basic in-memory session store

Sessions are stored in a non-persistent Hash.  Only String values are allowed.  A reaper fiber regularly removes any sessions which expire due to inactivity.

spec/session_spec.cr

@@ -0,0 +1,57 @@
+require "./spec_helper"
+
+describe "Session" do
+  it "can establish a session" do
+    sid = nil
+    existing = nil
+    get "/" do |env|
+      sess = env.session
+      existing = sess["token"]?
+      sid = sess.id
+      sess["token"] = "abc"
+      "Hello"
+    end
+
+    # make first request without any cookies/session
+    request = HTTP::Request.new("GET", "/")
+    response = call_request_on_app(request)
+    # dup headers due to Crystal#2920
+    headers = response.headers.dup
+
+    # verify we got a cookie and session ID
+    cookie = response.headers["Set-Cookie"]?
+    cookie.should_not be_nil
+    response.cookies[Kemal::Sessions::NAME].value.should eq(sid)
+    lastsid = sid
+    existing.should be_nil
+
+    # make second request with cookies to get session
+    request = HTTP::Request.new("GET", "/", headers)
+    response = call_request_on_app(request)
+
+    # verify we got cookies and we could see values set
+    # in the previous request
+    cookie2 = response.headers["Set-Cookie"]?
+    cookie2.should_not be_nil
+    cookie2.should eq(cookie)
+    response.cookies[Kemal::Sessions::NAME].value.should eq(lastsid)
+    existing.should eq("abc")
+  end
+
+  it "can prune old sessions" do
+    s = Kemal::Sessions::STORE
+    s.clear
+
+    Kemal::Sessions.prune!
+
+    id = "foo"
+    s[id] = Kemal::Sessions::Session.new(id)
+    s.size.should eq(1)
+    Kemal::Sessions.prune!
+    s.size.should eq(1)
+
+    s[id].last_access_at = (Time.now - 1.week).epoch_ms
+    Kemal::Sessions.prune!
+    s.size.should eq(0)
+  end
+end

src/kemal.cr

@@ -12,6 +12,8 @@ module Kemal
     config.server = HTTP::Server.new(config.host_binding.not_nil!, config.port, config.handlers)
     config.server.not_nil!.tls = config.ssl
 
+    Kemal::Sessions.run_reaper!
+
     unless Kemal.config.error_handlers.has_key?(404)
       error 404 do |env|
         render_404

src/kemal/context.cr

@@ -19,5 +19,10 @@ class HTTP::Server
     def route_defined?
       route_lookup.found?
     end
+
+    def session
+      @session ||= Kemal::Sessions.new(self)
+      @session.not_nil!
+    end
   end
 end

src/kemal/session.cr

@@ -0,0 +1,96 @@
+require "secure_random"
+
+module Kemal
+  # Kemal's default session is in-memory only and holds simple String values only.
+  # The client-side cookie stores a random ID.
+  class Sessions
+    NAME = "SessionId"
+
+    # I hate websites which require daily login so the default
+    # inactivity timeout is 48 hours.
+    TTL  = 48.hours
+
+    # In-memory, ephemeral datastore only.
+    #
+    # Implementing Redis or Memcached as a datastore
+    # is left as an exercise to another reader.
+    #
+    # Note that the only thing we store on the client-side
+    # is an opaque, random String.  If we actually wanted to
+    # store any data, we'd need to implement encryption, key
+    # rotation, tamper-detection and that whole iceberg.
+    STORE = Hash(String, Session).new
+
+    class Session
+      getter! id : String
+      property! last_access_at : Int64
+
+      def initialize(@id)
+        @last_access_at = Time.new.epoch_ms
+        @store = Hash(String, String).new
+      end
+
+      def [](key : String)
+        @last_access_at = Time.now.epoch_ms
+        @store[key]
+      end
+
+      def []?(key : String)
+        @last_access_at = Time.now.epoch_ms
+        @store[key]?
+      end
+
+      def []=(key : String, value : String)
+        @last_access_at = Time.now.epoch_ms
+        @store[key] = value
+      end
+    end
+
+    getter! id : String
+
+    def initialize(ctx : HTTP::Server::Context)
+      id = ctx.request.cookies[NAME]?.try &.value
+      if id && id.size == 32
+        # valid
+      else
+        # new or invalid
+        id = SecureRandom.hex
+      end
+
+      ctx.response.cookies[NAME] = id
+      @id = id
+    end
+
+    def []=(key : String, value : String)
+      store = STORE[id]? || begin
+        STORE[id] = Session.new(id)
+      end
+      store[key] = value
+    end
+
+    def [](key : String)
+      STORE[@id][key]
+    end
+
+    def []?(key : String)
+      STORE[@id]?.try &.[key]?
+    end
+
+    def self.prune!(before = (Time.now - Kemal::Sessions::TTL).epoch_ms)
+      Kemal::Sessions::STORE.delete_if { |id, entry| entry.last_access_at < before }
+      nil
+    end
+
+    # This is an hourly job to prune the in-memory hash of any
+    # sessions which have expired due to inactivity, otherwise
+    # we'll have a slow memory leak and possible DDoS vector.
+    def self.run_reaper!
+      spawn do
+        loop do
+          prune!
+          sleep 3600
+        end
+      end
+    end
+  end
+end