[libglnx] errors: Add GLNX_AUTO_PREFIX_ERROR

[Colin Walters <walters src gnome org>]

commit 607f1775bb1c626cae1875a957a34802daebe81c
Author: Colin Walters <walters verbum org>
Date:   Wed Jul 12 21:11:02 2017 -0400

    errors: Add GLNX_AUTO_PREFIX_ERROR
    
    In a lot of places in ostree, we end up prefixing errors in the *caller*.
    Often we only have 1-2 callers, and doing the error prefixing isn't
    too duplicative.  But there are definitely cases where it's cleaner
    to do the prefixing in the callee.  We have functions that aren't
    ported to new style for this reason (they still do the prefixing
    in `out:`).
    
    Introduce a cleanup-oriented version of error prefixing so we can port those
    functions too.

 glnx-errors.h               |   30 ++++++++++++++++++++++++++++++
 tests/test-libglnx-errors.c |   43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 0 deletions(-)
---
diff --git a/glnx-errors.h b/glnx-errors.h
index e26a513..ecf7e85 100644
--- a/glnx-errors.h
+++ b/glnx-errors.h
@@ -84,6 +84,36 @@ glnx_prefix_error (GError **error, const char *fmt, ...)
 #define glnx_prefix_error_null(error, args...) \
   ({glnx_prefix_error (error, args); NULL;})
 
+/**
+ * GLNX_AUTO_PREFIX_ERROR:
+ *
+ * An autocleanup-based macro to automatically call `g_prefix_error()` (also with a colon+space `: `)
+ * when it goes out of scope.  This is useful when one wants error strings built up by the callee
+ * function, not all callers.
+ *
+ * ```
+ * gboolean start_http_request (..., GError **error)
+ * {
+ *   GLNX_AUTO_PREFIX_ERROR("HTTP request", error)
+ *
+ *   if (!libhttp_request_start (..., error))
+ *     return FALSE;
+ *   ...
+ *   return TRUE;
+ * ```
+ */
+typedef struct {
+  const char *prefix;
+  GError **error;
+} GLnxAutoErrorPrefix;
+static inline void
+glnx_cleanup_auto_prefix_error (GLnxAutoErrorPrefix *prefix)
+{
+  g_prefix_error (prefix->error, "%s: ", prefix->prefix);
+}
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(GLnxAutoErrorPrefix, glnx_cleanup_auto_prefix_error)
+#define GLNX_AUTO_PREFIX_ERROR(text, error) g_auto(GLnxAutoErrorPrefix) 
_GLNX_MAKE_ANONYMOUS(_glnxautoprefixerror_) = { text, error }
+
 /* Set @error using the value of `g_strerror (errno)`.
  *
  * This function returns %FALSE so it can be used conveniently in a single
diff --git a/tests/test-libglnx-errors.c b/tests/test-libglnx-errors.c
index 721818b..4e91e02 100644
--- a/tests/test-libglnx-errors.c
+++ b/tests/test-libglnx-errors.c
@@ -125,6 +125,48 @@ test_error_errno (void)
     g_assert_cmpint (fd, ==, -1);
 }
 
+static void
+test_error_auto_nothrow (GError **error)
+{
+  GLNX_AUTO_PREFIX_ERROR("foo", error);
+  /* Side effect to avoid otherwise empty function */
+  g_assert_no_error (*error);
+}
+
+static void
+test_error_auto_throw (GError **error)
+{
+  GLNX_AUTO_PREFIX_ERROR("foo", error);
+  (void) glnx_throw (error, "oops");
+}
+
+static void
+test_error_auto_throw_recurse (GError **error)
+{
+  GLNX_AUTO_PREFIX_ERROR("foo", error);
+
+  if (TRUE)
+    {
+      GLNX_AUTO_PREFIX_ERROR("bar", error);
+      (void) glnx_throw (error, "oops");
+    }
+}
+
+static void
+test_error_auto (void)
+{
+  g_autoptr(GError) error = NULL;
+  test_error_auto_nothrow (&error);
+  g_assert_no_error (error);
+  test_error_auto_throw (&error);
+  g_assert_nonnull (error);
+  g_assert_cmpstr (error->message, ==, "foo: oops");
+  g_clear_error (&error);
+  test_error_auto_throw_recurse (&error);
+  g_assert_nonnull (error);
+  g_assert_cmpstr (error->message, ==, "foo: bar: oops");
+}
+
 int main (int argc, char **argv)
 {
   int ret;
@@ -133,6 +175,7 @@ int main (int argc, char **argv)
 
   g_test_add_func ("/error-throw", test_error_throw);
   g_test_add_func ("/error-errno", test_error_errno);
+  g_test_add_func ("/error-auto", test_error_auto);
 
   ret = g_test_run();