Allow TCPSocket to bind to specific local address

spec/std/socket_spec.cr

@@ -386,6 +386,27 @@ describe TCPSocket do
       TCPSocket.new("localhostttttt", 12345)
     end
   end
+
+  it "binds to a local port" do
+    port = TCPServer.open("::", 0) do |server|
+      server.local_address.port
+    end
+    port.should be > 0
+
+    TCPServer.open("::", port) do |server|
+      server.local_address.family.should eq(Socket::Family::INET6)
+      server.local_address.port.should eq(port)
+      server.local_address.address.should eq("::")
+
+      # test sync flag propagation after accept
+      server.sync = !server.sync?
+
+      TCPSocket.open("::", server.local_address.port, "::", 54321) do |client|
+        sock = server.accept
+        sock.sync?.should eq(server.sync?)
+      end
+    end
+  end
 end
 
 describe UDPSocket do

src/socket/tcp_socket.cr

@@ -17,7 +17,36 @@ class TCPSocket < IPSocket
   # must be in seconds (integers or floats).
   #
   # Note that `dns_timeout` is currently ignored.
-  def initialize(host, port, dns_timeout = nil, connect_timeout = nil)
+  def initialize(host, port, local_host, local_port, dns_timeout : Time::Span | Number | Nil = nil, connect_timeout : Time::Span | Number | Nil = nil)
+    getaddrinfo(host, port, nil, Type::STREAM, Protocol::TCP, timeout: dns_timeout) do |addrinfo|
+      super create_socket(addrinfo.ai_family, addrinfo.ai_socktype, addrinfo.ai_protocol)
+
+      getaddrinfo(local_host, local_port, nil, Type::STREAM, Protocol::TCP, timeout: dns_timeout) do |localaddrinfo|
+        ret =
+          {% if flag?(:freebsd) || flag?(:openbsd) %}
+            LibC.bind(fd, localaddrinfo.ai_addr.as(LibC::Sockaddr*), localaddrinfo.ai_addrlen)
+          {% else %}
+            LibC.bind(fd, localaddrinfo.ai_addr, localaddrinfo.ai_addrlen)
+          {% end %}
+        unless ret == 0
+          next false if localaddrinfo.ai_next
+          raise Errno.new("Error binding TCP socket at #{local_host}:#{local_port}")
+        end
+
+        if err = nonblocking_connect(host, port, addrinfo, timeout: connect_timeout)
+          close
+          next false if addrinfo.ai_next
+          raise err
+        end
+
+        true
+      end
+
+      true
+    end
+  end
+
+  def initialize(host, port, dns_timeout : Time::Span | Number | Nil = nil, connect_timeout : Time::Span | Number | Nil = nil)
     getaddrinfo(host, port, nil, Type::STREAM, Protocol::TCP, timeout: dns_timeout) do |addrinfo|
       super create_socket(addrinfo.ai_family, addrinfo.ai_socktype, addrinfo.ai_protocol)
 
@@ -38,9 +67,10 @@ class TCPSocket < IPSocket
   # Opens a TCP socket to a remote TCP server, yields it to the block, then
   # eventually closes the socket when the block returns.
   #
+  # Forwards all params to TCPSocket#new.
   # Returns the value of the block.
-  def self.open(host, port)
-    sock = new(host, port)
+  def self.open(*args, **kwargs)
+    sock = new(*args, **kwargs)
     begin
       yield sock
     ensure