gmessages: Factor out _g_fd_is_journal into its own translation unit

I want to use this in gio-launch-desktop, but gio-launch-desktop
doesn't depend on GLib, so I can't just call g_log_writer_is_journald().

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/gjournal-private.c

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2016 Red Hat, Inc.
+ * Copyright 2016-2022 Collabora Ltd.
+ * Copyright 2017-2022 Endless OS Foundation, LLC
+ * Copyright 2018 Will Thompson
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "gjournal-private.h"
+
+#if defined(__linux__) && !defined(__BIONIC__)
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+
+/*
+ * Reimplementation of g_str_has_prefix(), necessary when compiled into
+ * gio-launch-desktop.
+ */
+static int
+str_has_prefix (const char *str,
+                const char *prefix)
+{
+  return strncmp (str, prefix, strlen (prefix)) == 0;
+}
+
+/*
+ * _g_fd_is_journal:
+ * @output_fd: output file descriptor to check
+ *
+ * Same as g_log_writer_is_journald(), but with no GLib dependencies.
+ *
+ * Returns: 1 if @output_fd points to the journal, 0 otherwise
+ */
+int
+_g_fd_is_journal (int output_fd)
+{
+  /* FIXME: Use the new journal API for detecting whether we’re writing to the
+   * journal. See: https://github.com/systemd/systemd/issues/2473
+   */
+  union {
+    struct sockaddr_storage storage;
+    struct sockaddr sa;
+    struct sockaddr_un un;
+  } addr;
+  socklen_t addr_len;
+  int err;
+
+  if (output_fd < 0)
+    return 0;
+
+  /* Namespaced journals start with `/run/systemd/journal.${name}/` (see
+   * `RuntimeDirectory=systemd/journal.%i` in `systemd-journald@.service`. The
+   * default journal starts with `/run/systemd/journal/`. */
+  memset (&addr, 0, sizeof (addr));
+  addr_len = sizeof(addr);
+  err = getpeername (output_fd, &addr.sa, &addr_len);
+  if (err == 0 && addr.storage.ss_family == AF_UNIX)
+    return (str_has_prefix (addr.un.sun_path, "/run/systemd/journal/") ||
+            str_has_prefix (addr.un.sun_path, "/run/systemd/journal."));
+
+  return 0;
+}
+#endif

glib/gjournal-private.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2016-2022 Collabora Ltd.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GLIB_JOURNAL_PRIVATE_H__
+#define __GLIB_JOURNAL_PRIVATE_H__
+
+int _g_fd_is_journal (int output_fd);
+
+#endif

glib/gmessages.c

@@ -202,6 +202,10 @@
 #include "gpattern.h"
 #include "gthreadprivate.h"
 
+#if defined(__linux__) && !defined(__BIONIC__)
+#include "gjournal-private.h"
+#endif
+
 #ifdef G_OS_UNIX
 #include <unistd.h>
 #endif
@@ -2223,32 +2227,10 @@ gboolean
 g_log_writer_is_journald (gint output_fd)
 {
 #if defined(__linux__) && !defined(__BIONIC__)
-  /* FIXME: Use the new journal API for detecting whether we’re writing to the
-   * journal. See: https://github.com/systemd/systemd/issues/2473
-   */
-  union {
-    struct sockaddr_storage storage;
-    struct sockaddr sa;
-    struct sockaddr_un un;
-  } addr;
-  socklen_t addr_len;
-  int err;
-
-  if (output_fd < 0)
-    return FALSE;
-
-  /* Namespaced journals start with `/run/systemd/journal.${name}/` (see
-   * `RuntimeDirectory=systemd/journal.%i` in `systemd-journald@.service`. The
-   * default journal starts with `/run/systemd/journal/`. */
-  memset (&addr, 0, sizeof (addr));
-  addr_len = sizeof(addr);
-  err = getpeername (output_fd, &addr.sa, &addr_len);
-  if (err == 0 && addr.storage.ss_family == AF_UNIX)
-    return (g_str_has_prefix (addr.un.sun_path, "/run/systemd/journal/") ||
-            g_str_has_prefix (addr.un.sun_path, "/run/systemd/journal."));
-#endif
-
+  return _g_fd_is_journal (output_fd);
+#else
   return FALSE;
+#endif
 }
 
 static void escape_string (GString *string);

glib/meson.build

@@ -336,6 +336,10 @@ else
   platform_deps = []
 endif
 
+if host_system == 'linux'
+  glib_sources += files('gjournal-private.c')
+endif
+
 if glib_have_cocoa
   glib_sources += files('gosxutils.m')
   framework_dep = dependency('appleframeworks', modules : ['Foundation', 'CoreFoundation', 'AppKit'])