run: Create a shared XDG_RUNTIME_DIR for each app-ID

Like $XDG_RUNTIME_DIR/app/$FLATPAK_ID, this is shared between all
instances of the app, except for subsandboxed instances created by
flatpak-spawn --sandbox or equivalent. Unlike
$XDG_RUNTIME_DIR/app/$FLATPAK_ID, it does not exist at an equivalent
path on the host and in the sandboxed app.

Resolves: https://github.com/flatpak/flatpak/issues/4120
Signed-off-by: Simon McVittie <smcv@collabora.com>

app/flatpak-builtins-build.c

@@ -580,7 +580,7 @@ flatpak_builtin_build (int argc, char **argv, GCancellable *cancellable, GError
                               NULL);
     }
 
-  flatpak_bwrap_populate_runtime_dir (bwrap);
+  flatpak_bwrap_populate_runtime_dir (bwrap, NULL);
 
   flatpak_bwrap_envp_to_args (bwrap);
 

common/flatpak-bwrap-private.h

@@ -85,7 +85,8 @@ gboolean      flatpak_bwrap_bundle_args (FlatpakBwrap *bwrap,
                                          GError      **error);
 void          flatpak_bwrap_add_runtime_dir_member (FlatpakBwrap *bwrap,
                                                     const char *name);
-void          flatpak_bwrap_populate_runtime_dir (FlatpakBwrap *bwrap);
+void          flatpak_bwrap_populate_runtime_dir (FlatpakBwrap *bwrap,
+                                                  const char *shared_xdg_runtime_dir);
 
 void          flatpak_bwrap_child_setup_cb (gpointer user_data);
 void          flatpak_bwrap_child_setup (GArray *fd_array,

common/flatpak-bwrap.c

@@ -417,12 +417,51 @@ flatpak_bwrap_add_runtime_dir_member (FlatpakBwrap *bwrap,
   g_ptr_array_add (bwrap->runtime_dir_members, g_strdup (name));
 }
 
-void
-flatpak_bwrap_populate_runtime_dir (FlatpakBwrap *bwrap)
+static void
+expect_symlink (const char *host_path,
+                const char *target)
 {
-  flatpak_bwrap_add_arg (bwrap, "--symlink");
-  flatpak_bwrap_add_arg (bwrap, "../../../.flatpak-info");
-  flatpak_bwrap_add_arg_printf (bwrap, "/run/user/%d/flatpak-info", getuid ());
+  /* This shouldn't fail in practice, so there's not much point in
+   * translating the warning */
+  if (symlink (target, host_path) < 0 && errno != EEXIST)
+    {
+      g_warning ("Unable to create symlink at %s: %s",
+                 host_path, g_strerror (errno));
+    }
+  else
+    {
+      g_autoptr(GError) local_error = NULL;
+      g_autofree char *got = glnx_readlinkat_malloc (AT_FDCWD,
+                                                     host_path,
+                                                     NULL,
+                                                     &local_error);
+
+      if (got == NULL)
+        g_warning ("%s is not a symlink to \"%s\" as expected: %s",
+                   host_path, target, local_error->message);
+      else if (strcmp (got, target) != 0)
+        g_warning ("%s is a symlink to \"%s\", not \"%s\" as expected",
+                   host_path, got, target);
+    }
+}
+
+void
+flatpak_bwrap_populate_runtime_dir (FlatpakBwrap *bwrap,
+                                    const char *shared_xdg_runtime_dir)
+{
+  if (shared_xdg_runtime_dir != NULL)
+    {
+      g_autofree char *host_path = g_build_filename (shared_xdg_runtime_dir,
+                                                     "flatpak-info", NULL);
+
+      expect_symlink (host_path, "../../../.flatpak-info");
+    }
+  else
+    {
+      flatpak_bwrap_add_arg (bwrap, "--symlink");
+      flatpak_bwrap_add_arg (bwrap, "../../../.flatpak-info");
+      flatpak_bwrap_add_arg_printf (bwrap, "/run/user/%d/flatpak-info", getuid ());
+    }
 
   if (bwrap->runtime_dir_members != NULL)
     {
@@ -431,10 +470,21 @@ flatpak_bwrap_populate_runtime_dir (FlatpakBwrap *bwrap)
       for (i = 0; i < bwrap->runtime_dir_members->len; i++)
         {
           const char *member = g_ptr_array_index (bwrap->runtime_dir_members, i);
+          g_autofree char *target = g_strdup_printf ("../../flatpak/%s", member);
 
-          flatpak_bwrap_add_arg (bwrap, "--symlink");
-          flatpak_bwrap_add_arg_printf (bwrap, "../../flatpak/%s", member);
-          flatpak_bwrap_add_arg_printf (bwrap, "/run/user/%d/%s", getuid (), member);
+          if (shared_xdg_runtime_dir != NULL)
+            {
+              g_autofree char *host_path = g_build_filename (shared_xdg_runtime_dir,
+                                                             member, NULL);
+
+              expect_symlink (host_path, target);
+            }
+          else
+            {
+              flatpak_bwrap_add_arg (bwrap, "--symlink");
+              flatpak_bwrap_add_arg (bwrap, target);
+              flatpak_bwrap_add_arg_printf (bwrap, "/run/user/%d/%s", getuid (), member);
+            }
         }
     }
 }

common/flatpak-dir.c

@@ -7857,7 +7857,7 @@ apply_extra_data (FlatpakDir   *self,
                                          NULL, cancellable, error))
     return FALSE;
 
-  flatpak_bwrap_populate_runtime_dir (bwrap);
+  flatpak_bwrap_populate_runtime_dir (bwrap, NULL);
 
   flatpak_bwrap_envp_to_args (bwrap);
 

common/flatpak-instance-private.h

@@ -52,5 +52,9 @@ gboolean flatpak_instance_ensure_per_app_tmp (const char *app_id,
                                               int per_app_dir_lock_fd,
                                               char **shared_tmp_out,
                                               GError **error);
+gboolean flatpak_instance_ensure_per_app_xdg_runtime_dir (const char *app_id,
+                                                          int per_app_dir_lock_fd,
+                                                          char **shared_dir_out,
+                                                          GError **error);
 
 #endif /* __FLATPAK_INSTANCE_PRIVATE_H__ */

common/flatpak-instance.c

@@ -671,6 +671,47 @@ flatpak_instance_ensure_per_app_tmp (const char *app_id,
   return TRUE;
 }
 
+/*
+ * @app_id: $FLATPAK_ID
+ * @per_app_dir_lock_fd: Used to prove that we have already taken out
+ *  a per-app non-exclusive lock to stop this directory from being
+ *  garbage-collected
+ * @shared_dir_out: (out) (not optional) (not nullable): Used to return
+ *  the path to the shared $XDG_RUNTIME_DIR
+ *
+ * Create a per-app $XDG_RUNTIME_DIR.
+ */
+gboolean
+flatpak_instance_ensure_per_app_xdg_runtime_dir (const char *app_id,
+                                                 int per_app_dir_lock_fd,
+                                                 char **shared_dir_out,
+                                                 GError **error)
+{
+  g_autofree char *per_app_parent = NULL;
+  g_autofree char *shared_dir = NULL;
+
+  g_return_val_if_fail (app_id != NULL, FALSE);
+  g_return_val_if_fail (shared_dir_out != NULL, FALSE);
+  g_return_val_if_fail (*shared_dir_out == NULL, FALSE);
+  g_return_val_if_fail (error == NULL || *error == NULL, FALSE);
+
+  /* We don't actually do anything with this, we just pass it in here as
+   * proof that we already set up the directory that contains the lock
+   * file for per-app things, to force us to get the sequence right */
+  g_return_val_if_fail (per_app_dir_lock_fd >= 0, FALSE);
+
+  per_app_parent = flatpak_instance_get_apps_directory ();
+  shared_dir = g_build_filename (per_app_parent, app_id, "xdg-run", NULL);
+
+  if (g_mkdir_with_parents (shared_dir, 0700) != 0)
+    return glnx_throw_errno_prefix (error,
+                                    _("Unable to create directory %s"),
+                                    shared_dir);
+
+  *shared_dir_out = g_steal_pointer (&shared_dir);
+  return TRUE;
+}
+
 /*
  * @host_dir_out: (not optional): used to return the directory on the host
  *  system representing this instance

common/flatpak-run.c

@@ -3834,6 +3834,7 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
   g_autofree char *checksum = NULL;
   glnx_autofd int per_app_dir_lock_fd = -1;
   g_autofree char *per_app_dir_lock_path = NULL;
+  g_autofree char *shared_xdg_runtime_dir = NULL;
   int ld_so_fd = -1;
   g_autoptr(GFile) runtime_ld_so_conf = NULL;
   gboolean generate_ld_so_conf = TRUE;
@@ -4261,6 +4262,16 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
                                                 &per_app_dir_lock_path,
                                                 error))
         return FALSE;
+
+      if (!flatpak_instance_ensure_per_app_xdg_runtime_dir (app_id,
+                                                            per_app_dir_lock_fd,
+                                                            &shared_xdg_runtime_dir,
+                                                            error))
+        return FALSE;
+
+      flatpak_bwrap_add_arg (bwrap, "--bind");
+      flatpak_bwrap_add_arg (bwrap, shared_xdg_runtime_dir);
+      flatpak_bwrap_add_arg_printf (bwrap, "/run/user/%d", getuid ());
     }
 
   if (!flatpak_run_add_dconf_args (bwrap, app_id, metakey, error))
@@ -4330,7 +4341,7 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
         flatpak_bwrap_add_args_data_fd (bwrap, "--pidns", pidns_fd, NULL);
     }
 
-  flatpak_bwrap_populate_runtime_dir (bwrap);
+  flatpak_bwrap_populate_runtime_dir (bwrap, shared_xdg_runtime_dir);
 
   if (custom_command)
     {