Add Socket::IPAddress predicates and constants

spec/std/socket/address_spec.cr

@@ -97,6 +97,27 @@ describe Socket::IPAddress do
       end
     end
   end
+
+  it "#loopback?" do
+    Socket::IPAddress.new("127.0.0.1", 0).loopback?.should be_true
+    Socket::IPAddress.new("127.255.255.254", 0).loopback?.should be_true
+    Socket::IPAddress.new("128.0.0.1", 0).loopback?.should be_false
+    Socket::IPAddress.new("0.0.0.0", 0).loopback?.should be_false
+    Socket::IPAddress.new("::1", 0).loopback?.should be_true
+    Socket::IPAddress.new("0000:0000:0000:0000:0000:0000:0000:0001", 0).loopback?.should be_true
+    Socket::IPAddress.new("::2", 0).loopback?.should be_false
+    Socket::IPAddress.new(Socket::IPAddress::LOOPBACK, 0).loopback?.should be_true
+    Socket::IPAddress.new(Socket::IPAddress::LOOPBACK6, 0).loopback?.should be_true
+  end
+
+  it "#unspecified?" do
+    Socket::IPAddress.new("0.0.0.0", 0).unspecified?.should be_true
+    Socket::IPAddress.new("127.0.0.1", 0).unspecified?.should be_false
+    Socket::IPAddress.new("::", 0).unspecified?.should be_true
+    Socket::IPAddress.new("0000:0000:0000:0000:0000:0000:0000:0000", 0).unspecified?.should be_true
+    Socket::IPAddress.new(Socket::IPAddress::UNSPECIFIED, 0).unspecified?.should be_true
+    Socket::IPAddress.new(Socket::IPAddress::UNSPECIFIED6, 0).unspecified?.should be_true
+  end
 end
 
 describe Socket::UNIXAddress do

src/compiler/crystal/tools/playground/server.cr

@@ -528,17 +528,12 @@ module Crystal::Playground
 
       server = HTTP::Server.new handlers
 
-      host = @host
-      if host
-        address = server.bind_tcp host, @port
-      else
-        address = server.bind_tcp @port
-      end
+      address = server.bind_tcp @host || Socket::IPAddress::LOOPBACK, @port
       @port = address.port
 
       puts "Listening on http://#{address}"
-      if host == "0.0.0.0"
-        puts "WARNING running playground with 0.0.0.0 is unsecure."
+      if address.unspecified?
+        puts "WARNING running playground on #{address.address} is insecure."
       end
 
       begin

src/http/server.cr

@@ -154,7 +154,7 @@ class HTTP::Server
   # If *reuse_port* is `true`, it enables the `SO_REUSEPORT` socket option,
   # which allows multiple processes to bind to the same port.
   def bind_tcp(port : Int32, reuse_port : Bool = false) : Socket::IPAddress
-    bind_tcp "127.0.0.1", port, reuse_port
+    bind_tcp Socket::IPAddress::LOOPBACK, port, reuse_port
   end
 
   # Creates a `TCPServer` listenting on *address* and adds it as a socket, returning the local address
@@ -180,7 +180,7 @@ class HTTP::Server
   # server = HTTP::Server.new { }
   # server.bind_unused_port # => Socket::IPAddress.new("127.0.0.1", 12345)
   # ```
-  def bind_unused_port(host : String = "127.0.0.1", reuse_port : Bool = false) : Socket::IPAddress
+  def bind_unused_port(host : String = Socket::IPAddress::LOOPBACK, reuse_port : Bool = false) : Socket::IPAddress
     bind_tcp host, 0, reuse_port
   end
 

src/socket/address.cr

@@ -72,6 +72,13 @@ class Socket
   # resolve an IP, or don't know whether a `String` constains an IP or a domain
   # name, you should use `Addrinfo.resolve` instead.
   struct IPAddress < Address
+    UNSPECIFIED  = "0.0.0.0"
+    UNSPECIFIED6 = "::"
+    LOOPBACK     = "127.0.0.1"
+    LOOPBACK6    = "::1"
+    BROADCAST    = "255.255.255.255"
+    BROADCAST6   = "ff0X::1"
+
     getter port : Int32
 
     @address : String?
@@ -190,6 +197,43 @@ class Socket
       end
     end
 
+    # Returns `true` if this IP is a loopback address.
+    #
+    # In the IPv4 family, loopback addresses are all addresses in the subnet
+    # `127.0.0.0/24`. In IPv6 `::1` is the loopback address.
+    def loopback? : Bool
+      if addr = @addr4
+        addr.s_addr & 0x00000000ff_u32 == 0x0000007f_u32
+      elsif addr = @addr6
+        ipv6_addr8(addr) == StaticArray[0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 1_u8]
+      else
+        raise "unreachable!"
+      end
+    end
+
+    # Returns `true` if this IP is an unspecified address, either the IPv4 address `0.0.0.0` or the IPv6 address `::`.
+    def unspecified? : Bool
+      if addr = @addr4
+        addr.s_addr == 0_u32
+      elsif addr = @addr6
+        ipv6_addr8(addr) == StaticArray[0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8]
+      else
+        raise "unreachable!"
+      end
+    end
+
+    private def ipv6_addr8(addr : LibC::In6Addr)
+      {% if flag?(:darwin) || flag?(:openbsd) || flag?(:freebsd) %}
+        addr.__u6_addr.__u6_addr8
+      {% elsif flag?(:linux) && flag?(:musl) %}
+        addr.__in6_union.__s6_addr
+      {% elsif flag?(:linux) %}
+        addr.__in6_u.__u6_addr8
+      {% else %}
+        {% raise "Unsupported platform" %}
+      {% end %}
+    end
+
     def ==(other : IPAddress)
       family == other.family &&
         port == other.port &&