lib/sysroot: Add journal-msg signal

This will allow us to drop the awful hack in rpm-ostree where we watch our own
stdout. In general, libraries shouldn't write to stdout.

Also we can kill the systemd journal wrapper code. There's some duplication at
each call site now...but it's easier than trying to write a `sd_journal_send()`
wrapper.

I was originally going to have this emit all of the structured data too as a
`GVariant` but decided it wasn't worth it right now.

Closes: #1052
Approved by: jlebon

src/libostree/ostree-sysroot.c

@@ -58,8 +58,18 @@ find_booted_deployment (OstreeSysroot       *self,
  */
 typedef struct {
   GObjectClass parent_class;
+
+  /* Signals */
+  void (*journal_msg) (OstreeSysroot *sysroot,
+                       const char    *msg);
 } OstreeSysrootClass;
 
+enum {
+  JOURNAL_MSG_SIGNAL,
+  LAST_SIGNAL,
+};
+static guint signals[LAST_SIGNAL] = { 0 };
+
 enum {
   PROP_0,
 
@@ -159,6 +169,27 @@ ostree_sysroot_class_init (OstreeSysrootClass *klass)
                                                         "",
                                                         G_TYPE_FILE,
                                                         G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY));
+
+  /**
+   * OstreeSysroot::journal-msg:
+   * @self: Self
+   * @msg: Human-readable string (should not contain newlines)
+   *
+   * libostree will log to the journal various events, such as the /etc merge
+   * status, and transaction completion. Connect to this signal to also
+   * synchronously receive the text for those messages. This is intended to be
+   * used by command line tools which link to libostree as a library.
+   *
+   * Currently, the structured data is only available via the systemd journal.
+   *
+   * Since: 2017.10
+   */
+  signals[JOURNAL_MSG_SIGNAL] =
+    g_signal_new ("journal-msg",
+                  G_OBJECT_CLASS_TYPE (object_class),
+                  G_SIGNAL_RUN_LAST,
+                  G_STRUCT_OFFSET (OstreeSysrootClass, journal_msg),
+                  NULL, NULL, NULL, G_TYPE_NONE, 1, G_TYPE_STRING);
 }
 
 static void
@@ -319,6 +350,13 @@ ostree_sysroot_ensure_initialized (OstreeSysroot  *self,
   return TRUE;
 }
 
+void
+_ostree_sysroot_emit_journal_msg (OstreeSysroot  *self,
+                                  const char     *msg)
+{
+  g_signal_emit (self, signals[JOURNAL_MSG_SIGNAL], 0, msg);
+}
+
 gboolean
 _ostree_sysroot_parse_deploy_path_name (const char *name,
                                         char      **out_csum,