MDEV-15655: Add Linux abstract socket support

The functionality of the socket system variable is extended
here such that a preciding '@' indicates that the socket
will be an abstract socket. Thie socket name wil be
the remainder of the name after the '@'. This is consistent
with the approached used by systemd in socket activation.

Thanks to Sergey Vojtovich:

On OS X sockaddr_un is defined as:

struct sockaddr_un
{
  u_char sun_len;
  u_char sun_family;
  char  sun_path[104];
};

There is a comment in man 7 unix (on linux):

"
On Linux, the above offsetof() expression equates to the same value as sizeof(sa_family_t),
but some other implementations include other fields before sun_path, so the offsetof()
expression more portably describes the size of the address structure.
"

As such, use the offsetof for Linux and use the previous sizeof(UNIXaddr)
for non-unix platforms as that's what worked before and they don't
support abstract sockets so there's no compatibility problem..

strace -fe trace=networking mysqld --skip-networking --socket @abc ...
...
[pid 10578] socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0) = 22
[pid 10578] setsockopt(22, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
[pid 10578] bind(22, {sa_family=AF_UNIX, sun_path=@"abc"}, 6) = 0
[pid 10578] listen(22, 80)              = 0
...
Version: '10.3.6-MariaDB-log'  socket: '@abc'  port: 0  Source distribution

$ lsof -p 10578
mysqld  10578  dan   22u  unix 0x00000000087e688c       0t0 4787815 @abc type=STREAM

client/mysqltest.cc

@@ -6010,9 +6010,10 @@ void do_connect(struct st_command *command)
   {
     /*
       If the socket is specified just as a name without path
+      or an abstract socket indicator ('@'), then
       append tmpdir in front
     */
-    if (*ds_sock.str != FN_LIBCHAR)
+    if (*ds_sock.str != FN_LIBCHAR && *ds_sock.str != '@')
     {
       char buff[FN_REFLEN];
       fn_format(buff, ds_sock.str, TMPDIR, "", 0);

mysql-test/main/connect-abstract.cnf

@@ -0,0 +1,9 @@
+
+!include include/default_my.cnf
+
+[mysqld.1]
+socket=       @ENV.ABSTRACT_SOCKET
+
+# Using @OPT.port here for uniqueness
+[ENV]
+ABSTRACT_SOCKET= @mtr-test-abstract-socket-@OPT.port

mysql-test/main/connect-abstract.result

@@ -0,0 +1,5 @@
+connect con1,localhost,root,,test,,$ABSTRACT_SOCKET;
+select 1;
+1
+1
+disconnect con1;

mysql-test/main/connect-abstract.test

@@ -0,0 +1,6 @@
+--source include/linux.inc
+--source include/not_embedded.inc
+
+connect(con1,localhost,root,,test,,$ABSTRACT_SOCKET);
+select 1;
+disconnect con1;

sql/mysqld.cc

@@ -2784,9 +2784,10 @@ static void network_init(void)
   */
   if (mysqld_unix_port[0] && !opt_bootstrap)
   {
+    size_t port_len;
     DBUG_PRINT("general",("UNIX Socket is %s",mysqld_unix_port));
 
-    if (strlen(mysqld_unix_port) > (sizeof(UNIXaddr.sun_path) - 1))
+    if ((port_len=strlen(mysqld_unix_port)) > (sizeof(UNIXaddr.sun_path) - 1))
     {
       sql_print_error("The socket file path is too long (> %u): %s",
                       (uint) sizeof(UNIXaddr.sun_path) - 1, mysqld_unix_port);
@@ -2804,14 +2805,25 @@ static void network_init(void)
     bzero((char*) &UNIXaddr, sizeof(UNIXaddr));
     UNIXaddr.sun_family = AF_UNIX;
     strmov(UNIXaddr.sun_path, mysqld_unix_port);
-    (void) unlink(mysqld_unix_port);
+#if defined(__linux__)
+    /* Abstract socket */
+    if (mysqld_unix_port[0] == '@')
+      UNIXaddr.sun_path[0]= '\0';
+    else
+#endif
+      (void) unlink(mysqld_unix_port);
     arg= 1;
     (void) mysql_socket_setsockopt(unix_sock,SOL_SOCKET,SO_REUSEADDR,
                                    (char*)&arg, sizeof(arg));
     umask(0);
     if (mysql_socket_bind(unix_sock,
                           reinterpret_cast<struct sockaddr *>(&UNIXaddr),
-                          sizeof(UNIXaddr)) < 0)
+#if defined(__linux__)
+                          offsetof(struct sockaddr_un, sun_path) + port_len
+#else
+                          sizeof(UNIXaddr)
+#endif
+                         ) < 0)
     {
       sql_perror("Can't start server : Bind on unix socket"); /* purecov: tested */
       sql_print_error("Do you already have another mysqld server running on socket: %s ?",mysqld_unix_port);