Add HTTP::Server.listening? and raise errors when running or closed

spec/std/http/server/server_spec.cr

@@ -258,6 +258,58 @@ module HTTP
 
       server.addresses.should eq addresses
     end
+
+    describe "#bind" do
+      it "fails after listen" do
+        server = Server.new {}
+        server.bind_unused_port
+        spawn { server.listen }
+        Fiber.yield
+        expect_raises(Exception, "Can't add socket to running server.") do
+          server.bind_unused_port
+        end
+        server.close
+      end
+
+      it "fails after close" do
+        server = Server.new {}
+        server.bind_unused_port
+        spawn { server.listen }
+        Fiber.yield
+        server.close
+        expect_raises(Exception, "Can't add socket to closed server.") do
+          server.bind_unused_port
+        end
+        server.close
+      end
+    end
+
+    describe "#listen" do
+      it "fails after listen" do
+        server = Server.new {}
+        server.bind_unused_port
+        spawn { server.listen }
+        Fiber.yield
+        server.listening?.should be_true
+        expect_raises(Exception, "Can't start running server.") do
+          server.listen
+        end
+        server.close
+      end
+
+      it "fails after close" do
+        server = Server.new {}
+        server.bind_unused_port
+        spawn { server.listen }
+        Fiber.yield
+        server.listening?.should be_true
+        server.close
+        server.listening?.should be_false
+        expect_raises(Exception, "Can't re-start closed server.") do
+          server.listen
+        end
+      end
+    end
   end
 
   describe HTTP::Server::RequestProcessor do

src/http/server.cr

@@ -102,6 +102,9 @@ class HTTP::Server
   @wants_close = false
   @sockets = [] of Socket::Server
 
+  # Returns `true` if this server is listening on it's sockets.
+  getter? listening : Bool = false
+
   # Creates a new HTTP server with the given block as handler.
   def self.new(&handler : HTTP::Handler::Proc) : self
     new(handler)
@@ -159,6 +162,9 @@ class HTTP::Server
 
   # Adds a `Socket::Server` *socket* to this server.
   def bind(socket : Socket::Server) : Nil
+    raise "Can't add socket to running server." if listening?
+    raise "Can't add socket to closed server." if @wants_close
+
     @sockets << socket
   end
 
@@ -199,8 +205,11 @@ class HTTP::Server
 
   # Starts the server. Blocks until the server is closed.
   def listen
-    raise "Can't start server with not sockets to listen to" if @sockets.empty?
+    raise "Can't re-start closed server." if @wants_close
+    raise "Can't start server with no sockets to listen to. Use HTTP::Server#bind first." if @sockets.empty?
+    raise "Can't start running server." if listening?
 
+    @listening = true
     done = Channel(Nil).new
 
     @sockets.each do |socket|
@@ -228,6 +237,7 @@ class HTTP::Server
       # ignore exception on close
     end
 
+    @listening = false
     @sockets.clear
   end