eval: learn serverstart, list, and close().

Implement functions for spawning, destroying, and listing active
servers, and add server_address_list() to msgpack_rpc/server.c for the
serverlist() vimL function.

runtime/doc/eval.txt

@@ -5373,13 +5373,25 @@ server2client( {clientid}, {string})			*server2client()*
 			:echo server2client(expand("<client>"), "HELLO")
 <
 serverlist()					*serverlist()*
-		Return a list of available server names, one per line.
-		When there are no servers or the information is not available
-		an empty string is returned.  See also |clientserver|.
-		{only available when compiled with the |+clientserver| feature}
+		Returns a list of available server names in a list.
+		When there are no servers an empty string is returned.
 		Example: >
 			:echo serverlist()
 <
+serverlisten([{address}])				*serverlisten()*
+		Opens a Unix or TCP socket at {address} for clients to connect
+		to and returns {address}. If no address is given, it is
+		equivalent to >
+			:call serverlisten(tempname())
+< 		If |$NVIM_LISTEN_ADDRESS| is not set, it will be set to
+		{address}.
+
+serverstop({address})					*serverstop()*
+		Closes the Unix or TCP socket at {address}. Does nothing if
+		{address} is empty, or does not refer to a server. If
+		{address} equals |$NVIM_LISTEN_ADDRESS|, the listen address
+		will be unset.
+
 setbufvar({expr}, {varname}, {val})			*setbufvar()*
 		Set option or local variable {varname} in buffer {expr} to
 		{val}.

src/nvim/eval.c

@@ -87,6 +87,7 @@
 #include "nvim/os/rstream_defs.h"
 #include "nvim/os/time.h"
 #include "nvim/msgpack_rpc/channel.h"
+#include "nvim/msgpack_rpc/server.h"
 #include "nvim/api/private/helpers.h"
 #include "nvim/api/vim.h"
 #include "nvim/os/dl.h"
@@ -6605,6 +6606,9 @@ static struct fst {
   {"searchpair",      3, 7, f_searchpair},
   {"searchpairpos",   3, 7, f_searchpairpos},
   {"searchpos",       1, 4, f_searchpos},
+  {"serverlist",      0, 0, f_serverlist},
+  {"serverstart",     0, 1, f_serverstart},
+  {"serverstop",      1, 1, f_serverstop},
   {"setbufvar",       3, 3, f_setbufvar},
   {"setcmdpos",       1, 1, f_setcmdpos},
   {"setline",         2, 2, f_setline},
@@ -13292,6 +13296,69 @@ static void f_searchpos(typval_T *argvars, typval_T *rettv)
     list_append_number(rettv->vval.v_list, (varnumber_T)n);
 }
 
+/// "serverlist()" function
+static void f_serverlist(typval_T *argvars, typval_T *rettv)
+{
+  size_t n;
+  char **addrs = server_address_list(&n);
+
+  // Copy addrs into a linked list.
+  list_T *l = rettv_list_alloc(rettv);
+  for (size_t i = 0; i < n; i++) {
+    listitem_T *li = listitem_alloc();
+    li->li_tv.v_type = VAR_STRING;
+    li->li_tv.v_lock = 0;
+    li->li_tv.vval.v_string = (char_u *) addrs[i];
+    list_append(l, li);
+  }
+  xfree(addrs);
+}
+
+/// "serverstart()" function
+static void f_serverstart(typval_T *argvars, typval_T *rettv)
+{
+  rettv->v_type = VAR_STRING;
+  rettv->vval.v_string = NULL;  // Will hold the address of the new server.
+
+  if (check_restricted() || check_secure()) {
+    return;
+  }
+
+  // If the user supplied an address, use it, otherwise use a temp.
+  if (argvars[0].v_type != VAR_UNKNOWN) {
+    if (argvars[0].v_type != VAR_STRING) {
+      EMSG(_(e_invarg));
+      return;
+    } else {
+      rettv->vval.v_string = vim_strsave(get_tv_string(argvars));
+    }
+  } else {
+    rettv->vval.v_string = vim_tempname();
+  }
+
+  int result = server_start((char *) rettv->vval.v_string);
+  if (result != 0) {
+    EMSG2("Failed to start server: %s", uv_strerror(result));
+  }
+}
+
+/// "serverstop()" function
+static void f_serverstop(typval_T *argvars, typval_T *rettv)
+{
+  if (check_restricted() || check_secure()) {
+    return;
+  }
+
+  if (argvars[0].v_type == VAR_UNKNOWN || argvars[0].v_type != VAR_STRING) {
+    EMSG(_(e_invarg));
+    return;
+  }
+
+  if (argvars[0].vval.v_string) {
+    server_stop((char *) argvars[0].vval.v_string);
+  }
+}
+
 /*
  * "setbufvar()" function
  */

src/nvim/msgpack_rpc/server.c

@@ -56,13 +56,18 @@ bool server_init(void)
 {
   ga_init(&servers, sizeof(Server *), 1);
 
-  if (!os_getenv(LISTEN_ADDRESS_ENV_VAR)) {
-    char *listen_address = (char *)vim_tempname();
-    os_setenv(LISTEN_ADDRESS_ENV_VAR, listen_address, 1);
-    xfree(listen_address);
+  bool must_free = false;
+  const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
+  if (listen_address == NULL || *listen_address == NUL) {
+    must_free = true;
+    listen_address = (char *)vim_tempname();
   }
 
-  return server_start((char *)os_getenv(LISTEN_ADDRESS_ENV_VAR)) == 0;
+  bool ok = (server_start(listen_address) == 0);
+  if (must_free) {
+    xfree((char *) listen_address);
+  }
+  return ok;
 }
 
 /// Retrieve the file handle from a server.
@@ -199,6 +204,12 @@ int server_start(const char *endpoint)
     return result;
   }
 
+  // Update $NVIM_LISTEN_ADDRESS, if not set.
+  const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
+  if (listen_address == NULL || *listen_address == NUL) {
+    os_setenv(LISTEN_ADDRESS_ENV_VAR, addr, 1);
+  }
+
   server->type = server_type;
 
   // Add the server to the list.
@@ -232,6 +243,12 @@ void server_stop(char *endpoint)
     return;
   }
 
+  // If we are invalidating the listen address, unset it.
+  const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
+  if (listen_address && strcmp(addr, listen_address) == 0) {
+    os_unsetenv(LISTEN_ADDRESS_ENV_VAR);
+  }
+
   uv_close(server_handle(server), free_server);
 
   // Remove this server from the list by swapping it with the last item.
@@ -242,6 +259,22 @@ void server_stop(char *endpoint)
   servers.ga_len--;
 }
 
+/// Returns an allocated array of server addresses.
+/// @param[out] size The size of the returned array.
+char **server_address_list(size_t *size)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if ((*size = (size_t) servers.ga_len) == 0) {
+    return NULL;
+  }
+
+  char **addrs = xcalloc((size_t) servers.ga_len, sizeof(const char **));
+  for (int i = 0; i < servers.ga_len; i++) {
+    addrs[i] = xstrdup(((Server **)servers.ga_data)[i]->addr);
+  }
+  return addrs;
+}
+
 static void connection_cb(uv_stream_t *server, int status)
 {
   int result;