core: add "tls-ca-path" option

Some organizations will want to use private Certificate Authorities to
serve content to their clients.  While it's possible to add the CA
to the system-wide CA store, that has two drawbacks:

1) Compromise of that cert means it can be used for other web traffic
2) All of ca-certificates is trusted

This patch allows a much stronger scenario where *only* the CAs in
tls-ca-path are used for verification from the given repository.

https://bugzilla.gnome.org/show_bug.cgi?id=726256

doc/ostree.repo-config.xml

@@ -152,6 +152,11 @@ Boston, MA 02111-1307, USA.
         <term><varname>tls-client-key-path</varname></term>
         <listitem><para>Path to file containing client-side certificate key, to present when making requests to this repository.</para></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>tls-ca-path</varname></term>
+        <listitem><para>Path to file containing trusted anchors instead of the system CA database.</para></listitem>
+      </varlistentry>
     </variablelist>
 
   </refsect1>

src/libostree/ostree-fetcher.c

@@ -233,6 +233,16 @@ _ostree_fetcher_set_client_cert (OstreeFetcher *fetcher,
     }
 }
 
+void
+_ostree_fetcher_set_tls_database (OstreeFetcher *self,
+                                  GTlsDatabase  *db)
+{
+  if (db)
+    g_object_set ((GObject*)self->session, "tls-database", db, NULL);
+  else
+    g_object_set ((GObject*)self->session, "ssl-use-system-ca-file", TRUE, NULL);
+}
+
 static void
 on_request_sent (GObject        *object, GAsyncResult   *result, gpointer        user_data);
 

src/libostree/ostree-fetcher.h

@@ -57,6 +57,9 @@ OstreeFetcher *_ostree_fetcher_new (GFile                     *tmpdir,
 void _ostree_fetcher_set_client_cert (OstreeFetcher *fetcher,
                                      GTlsCertificate *cert);
 
+void _ostree_fetcher_set_tls_database (OstreeFetcher *self,
+                                       GTlsDatabase *db);
+
 char * _ostree_fetcher_query_state_text (OstreeFetcher              *self);
 
 guint64 _ostree_fetcher_bytes_transferred (OstreeFetcher       *self);

src/libostree/ostree-repo-pull.c

@@ -1129,6 +1129,22 @@ ostree_repo_pull (OstreeRepo               *self,
       }
   }
 
+  {
+    gs_free char *tls_ca_path = NULL;
+    gs_unref_object GTlsDatabase *db = NULL;
+
+    if (!ot_keyfile_get_value_with_default (config, remote_key,
+                                            "tls-ca-path",
+                                            NULL, &tls_ca_path, error))
+      goto out;
+
+    db = g_tls_file_database_new (tls_ca_path, error);
+    if (!db)
+      goto out;
+
+    _ostree_fetcher_set_tls_database (pull_data->fetcher, db);
+  }
+
   if (!pull_data->base_uri)
     {
       g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,