lib/gpg: Add helper to kill GPG agent

With GnuPG 2, any time you do basically any operation, a gpg-agent will
be spawned for the GPG home directory in use. The classic way to kill a
gpg-agent is to use `gpg-connect-agent` and send the `killagent` command
as is done in libtest.sh.

Closes: #1799
Approved by: cgwalters

src/libotutil/ot-gpg-utils.c

@@ -437,3 +437,29 @@ ot_gpgme_new_ctx (const char *homedir,
 
   return g_steal_pointer (&context);
 }
+
+void
+ot_gpgme_kill_agent (const char *homedir)
+{
+  g_return_if_fail (homedir != NULL);
+
+  /* Run gpg-connect-agent killagent /bye */
+  g_autoptr(GError) local_error = NULL;
+  g_autoptr(GSubprocess) proc = g_subprocess_new(G_SUBPROCESS_FLAGS_STDOUT_SILENCE,
+                                                 &local_error,
+                                                 "gpg-connect-agent",
+                                                 "--homedir",
+                                                 homedir,
+                                                 "killagent",
+                                                 "/bye",
+                                                 NULL);
+  if (proc == NULL) {
+    g_debug ("Spawning gpg-connect-agent failed: %s", local_error->message);
+    return;
+  }
+  if (!g_subprocess_wait_check (proc, NULL, &local_error)) {
+    g_debug ("Killing GPG agent with gpg-connect-agent failed: %s",
+             local_error->message);
+    return;
+  }
+}

src/libotutil/ot-gpg-utils.h

@@ -46,4 +46,6 @@ gpgme_data_t ot_gpgme_data_output (GOutputStream *output_stream);
 gpgme_ctx_t ot_gpgme_new_ctx (const char *homedir,
                               GError    **error);
 
+void ot_gpgme_kill_agent (const char *homedir);
+
 G_END_DECLS