Support Unix domain sockets on Windows

Unix domain sockets were introduced in Insider Build 17063 [1]. There are some caveats, as only SOCK_STREAM is supported and abstract sockets are
not implemented. The header file <afunix.h> declares the `struct sockaddr_un` type, but hasn't been picked by mingw-w64. The struct definition is
embedded if the header wasn't found. The curl [3] project has taken that approach too.

It is expected that users get runtime errors if the system doesn't have support for Unix domain sockets.

Fixes ocaml#8863.

[1]: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/
[2]: microsoft/WSL#4240 (comment)
[3]: https://github.com/curl/curl/blob/curl-7_74_0/lib/config-win32.h#L725-L734

configure.ac

@@ -1330,6 +1330,12 @@ AC_CHECK_TYPE(
 
 AC_CHECK_FUNC([inet_aton], [AC_DEFINE([HAS_INET_ATON])])
 
+## Unix domain sockets support on Windows
+
+AS_CASE([$host],
+  [*-*-mingw32|*-pc-windows],
+    [AC_CHECK_HEADERS([afunix.h], [AC_DEFINE([HAS_AFUNIX_H])])])
+
 ## IPv6 support
 
 ipv6=true

manual/manual/library/libunix.etex

@@ -90,7 +90,7 @@ processes}
 \entree{"getpwnam", "getpwuid"}{always raise "Not_found"}
 \entree{"getgrnam", "getgrgid"}{always raise "Not_found"}
 \entree{type "socket_domain"}{"PF_INET" is fully supported;
-"PF_INET6" is fully supported (since 4.01.0); "PF_UNIX" is not supported }
+"PF_INET6" is fully supported (since 4.01.0); "PF_UNIX" is supported since Windows 10 17063.}
 \entree{"establish_server"}{not implemented; use threads}
 \entree{terminal functions ("tc*")}{not implemented}
 \entree{"setsid"}{not implemented}

otherlibs/unix/socketaddr.c

@@ -55,7 +55,6 @@ void get_sockaddr(value mladr,
                   socklen_param_type * adr_len /*out*/)
 {
   switch(Tag_val(mladr)) {
-#ifndef _WIN32
   case 0:                       /* ADDR_UNIX */
     { value path;
       mlsize_t len;
@@ -75,7 +74,6 @@ void get_sockaddr(value mladr,
         + len;
       break;
     }
-#endif
   case 1:                       /* ADDR_INET */
 #ifdef HAS_IPV6
     if (caml_string_length(Field(mladr, 0)) == 16) {
@@ -114,16 +112,13 @@ value alloc_sockaddr(union sock_addr_union * adr /*in*/,
                      socklen_param_type adr_len, int close_on_error)
 {
   value res;
-#ifndef _WIN32
   if (adr_len < offsetof(struct sockaddr, sa_data)) {
     // Only possible for an unnamed AF_UNIX socket, in
     // which case sa_family might be uninitialized.
     return alloc_unix_sockaddr(caml_alloc_string(0));
   }
-#endif
 
   switch(adr->s_gen.sa_family) {
-#ifndef _WIN32
   case AF_UNIX:
     { /* Based on recommendation in section BUGS of Linux unix(7). See
          http://man7.org/linux/man-pages/man7/unix.7.html. */
@@ -147,7 +142,6 @@ value alloc_sockaddr(union sock_addr_union * adr /*in*/,
       );
       break;
     }
-#endif
   case AF_INET:
     { value a = alloc_inet_addr(&adr->s_inet.sin_addr);
       Begin_root (a);

otherlibs/unix/socketaddr.h

@@ -17,7 +17,24 @@
 #define CAML_SOCKETADDR_H
 
 #include "caml/misc.h"
-#ifndef _WIN32
+
+#ifdef _WIN32
+#ifdef HAS_AFUNIX_H
+#include <afunix.h>
+#else
+#define UNIX_PATH_MAX 108
+
+typedef struct sockaddr_un
+{
+    ADDRESS_FAMILY sun_family;     /* AF_UNIX */
+    char sun_path[UNIX_PATH_MAX];  /* pathname */
+} SOCKADDR_UN, *PSOCKADDR_UN;
+
+#define SIO_AF_UNIX_GETPEERPID _WSAIOR(IOC_VENDOR, 256) // Returns ULONG PID of the connected peer process
+
+#endif
+
+#else
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/un.h>
@@ -27,9 +44,7 @@
 
 union sock_addr_union {
   struct sockaddr s_gen;
-#ifndef _WIN32
   struct sockaddr_un s_unix;
-#endif
   struct sockaddr_in s_inet;
 #ifdef HAS_IPV6
   struct sockaddr_in6 s_inet6;

otherlibs/unix/unix.mli

@@ -1383,7 +1383,7 @@ type socket_domain =
 (** The type of socket domains.  Not all platforms support
     IPv6 sockets (type [PF_INET6]).
 
-    On Windows: [PF_UNIX] not implemented.  *)
+    On Windows: [PF_UNIX] supported since Windows 10 17063.  *)
 
 type socket_type =
     SOCK_STREAM                 (** Stream socket *)

otherlibs/unix/unixLabels.mli

@@ -1383,7 +1383,7 @@ type socket_domain = Unix.socket_domain =
 (** The type of socket domains.  Not all platforms support
     IPv6 sockets (type [PF_INET6]).
 
-    On Windows: [PF_UNIX] not implemented.  *)
+    On Windows: [PF_UNIX] supported since Windows 10 17063.  *)
 
 type socket_type = Unix.socket_type =
     SOCK_STREAM                 (** Stream socket *)

runtime/caml/s.h.in

@@ -73,6 +73,10 @@
 /* Define HAS_SOCKLEN_T if the type socklen_t is defined in
    /usr/include/sys/socket.h. */
 
+#undef HAS_AFUNIX_H
+
+/* Define HAS_AFUNIX_H if you have <afunix.h>. */
+
 #undef HAS_INET_ATON
 
 #undef HAS_IPV6

runtime/debugger.c

@@ -68,6 +68,16 @@ CAMLexport void caml_debugger_cleanup_fork(void)
 #define ATOM ATOM_WS
 #include <winsock.h>
 #undef ATOM
+#ifdef HAS_AFUNIX_H
+#include <afunix.h>
+#else
+struct sockaddr_un
+{
+    ADDRESS_FAMILY sun_family;     /* AF_UNIX */
+    char sun_path[108];            /* pathname */
+};
+#endif /* HAS_AFUNIX_H */
+#endif /* _WIN32 */
 #include <process.h>
 #endif
 
@@ -85,9 +95,7 @@ static value marshal_flags = Val_emptylist;
 static int sock_domain;         /* Socket domain for the debugger */
 static union {                  /* Socket address for the debugger */
   struct sockaddr s_gen;
-#ifndef _WIN32
   struct sockaddr_un s_unix;
-#endif
   struct sockaddr_in s_inet;
 } sock_addr;
 static int sock_addr_len;       /* Length of sock_addr */
@@ -181,7 +189,6 @@ void caml_debugger_init(void)
 {
   char * address;
   char_os * a;
-  size_t a_len;
   char * port, * p;
   struct hostent * host;
   int n;
@@ -216,7 +223,7 @@ void caml_debugger_init(void)
     if (*p == ':') { *p = 0; port = p+1; break; }
   }
   if (port == NULL) {
-#ifndef _WIN32
+    size_t a_len;
     /* Unix domain */
     sock_domain = PF_UNIX;
     sock_addr.s_unix.sun_family = AF_UNIX;
@@ -233,9 +240,6 @@ void caml_debugger_init(void)
     sock_addr_len =
       ((char *)&(sock_addr.s_unix.sun_path) - (char *)&(sock_addr.s_unix))
         + a_len;
-#else
-    caml_fatal_error("unix sockets not supported");
-#endif
   } else {
     /* Internet domain */
     sock_domain = PF_INET;