[libglnx] glnx-errors.h: add a glnx_throw_prefix() variant

[Colin Walters <walters src gnome org>]

commit 32231fdb5273dd2a812c61549d6c0e681c0f5d59
Author: Jonathan Lebon <jlebon redhat com>
Date:   Thu May 11 10:48:30 2017 -0400

    glnx-errors.h: add a glnx_throw_prefix() variant
    
    For completeness. It just looks much cleaner than doing the `, FALSE`
    trick. It also takes care of appending the ': ' for you like its errno
    version.

 glnx-errors.c               |   24 ++++++++++++++++++------
 glnx-errors.h               |   37 ++++++++++++++++++++++++++++++++++---
 tests/test-libglnx-errors.c |    7 +++++++
 3 files changed, 59 insertions(+), 9 deletions(-)
---
diff --git a/glnx-errors.c b/glnx-errors.c
index edbce20..4800873 100644
--- a/glnx-errors.c
+++ b/glnx-errors.c
@@ -24,6 +24,22 @@
 #include <glnx-errors.h>
 
 void
+glnx_real_set_prefix_error_va (GError     *error,
+                               const char *format,
+                               va_list     args)
+{
+  if (error == NULL)
+    return;
+
+  g_autofree char *old_msg = g_steal_pointer (&error->message);
+  g_autoptr(GString) buf = g_string_new ("");
+  g_string_append_vprintf (buf, format, args);
+  g_string_append (buf, ": ");
+  g_string_append (buf, old_msg);
+  error->message = g_string_free (g_steal_pointer (&buf), FALSE);
+}
+
+void
 glnx_real_set_prefix_error_from_errno_va (GError     **error,
                                           gint         errsv,
                                           const char  *format,
@@ -32,13 +48,9 @@ glnx_real_set_prefix_error_from_errno_va (GError     **error,
   if (!error)
     return;
 
-  /* TODO - enhance GError to have a "set and take ownership" API */
-  g_autoptr(GString) buf = g_string_new ("");
-  g_string_append_vprintf (buf, format, args);
-  g_string_append (buf, ": ");
-  g_string_append (buf, g_strerror (errsv));
   g_set_error_literal (error,
                        G_IO_ERROR,
                        g_io_error_from_errno (errsv),
-                       buf->str);
+                       g_strerror (errsv));
+  glnx_real_set_prefix_error_va (*error, format, args);
 }
diff --git a/glnx-errors.h b/glnx-errors.h
index 2f6b962..e26a513 100644
--- a/glnx-errors.h
+++ b/glnx-errors.h
@@ -30,7 +30,7 @@ G_BEGIN_DECLS
  * This function returns %FALSE so it can be used conveniently in a single
  * statement:
  *
- * ``
+ * ```
  *   if (strcmp (foo, "somevalue") != 0)
  *     return glnx_throw (error, "key must be somevalue, not '%s'", foo);
  * ```
@@ -49,16 +49,47 @@ glnx_throw (GError **error, const char *fmt, ...)
   return FALSE;
 }
 
-/* Like glnx_throw(), but yields a NULL pointer. */
+/* Like `glnx_throw ()`, but returns %NULL. */
 #define glnx_null_throw(error, args...) \
   ({glnx_throw (error, args); NULL;})
 
+/* Implementation detail of glnx_throw_prefix() */
+void glnx_real_set_prefix_error_va (GError     *error,
+                                    const char *format,
+                                    va_list     args) G_GNUC_PRINTF (2,0);
+
+/* Prepend to @error's message by `$prefix: ` where `$prefix` is computed via
+ * printf @fmt. Returns %FALSE so it can be used conveniently in a single
+ * statement:
+ *
+ * ```
+ *   if (!function_that_fails (s, error))
+ *     return glnx_throw_prefix (error, "while handling '%s'", s);
+ * ```
+ * */
+static inline gboolean G_GNUC_PRINTF (2,3)
+glnx_prefix_error (GError **error, const char *fmt, ...)
+{
+  if (error == NULL)
+    return FALSE;
+
+  va_list args;
+  va_start (args, fmt);
+  glnx_real_set_prefix_error_va (*error, fmt, args);
+  va_end (args);
+  return FALSE;
+}
+
+/* Like `glnx_prefix_error ()`, but returns %NULL. */
+#define glnx_prefix_error_null(error, args...) \
+  ({glnx_prefix_error (error, args); NULL;})
+
 /* Set @error using the value of `g_strerror (errno)`.
  *
  * This function returns %FALSE so it can be used conveniently in a single
  * statement:
  *
- * ``
+ * ```
  *   if (unlinkat (fd, somepathname) < 0)
  *     return glnx_throw_errno (error);
  * ```
diff --git a/tests/test-libglnx-errors.c b/tests/test-libglnx-errors.c
index 7950244..721818b 100644
--- a/tests/test-libglnx-errors.c
+++ b/tests/test-libglnx-errors.c
@@ -60,6 +60,9 @@ test_error_errno (void)
     {
       g_assert (!glnx_throw_errno (&error));
       g_assert_error (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND);
+      g_assert (!glnx_prefix_error (&error, "myprefix"));
+      g_assert_error (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND);
+      g_assert (g_str_has_prefix (error->message, "myprefix: "));
       g_clear_error (&error);
     }
   else
@@ -71,6 +74,10 @@ test_error_errno (void)
       gpointer dummy = glnx_null_throw_errno (&error);
       g_assert (dummy == NULL);
       g_assert_error (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND);
+      dummy = glnx_prefix_error_null (&error, "myprefix");
+      g_assert (dummy == NULL);
+      g_assert_error (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND);
+      g_assert (g_str_has_prefix (error->message, "myprefix: "));
       g_clear_error (&error);
     }
   else