gmessages: Error if g_log_set_writer_func() is called multiple times

Inspired by https://gitlab.gnome.org/GNOME/glib/-/issues/2638.

This requires moving the tests to subprocesses, so that
`g_log_set_writer_func()` is only called once per process.

It also adds a test for the new error.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

glib/gmessages.c

@@ -2059,9 +2059,18 @@ g_log_set_writer_func (GLogWriterFunc func,
   g_return_if_fail (func != NULL);
 
   g_mutex_lock (&g_messages_lock);
+
+  if (log_writer_func != g_log_writer_default)
+    {
+      g_mutex_unlock (&g_messages_lock);
+      g_error ("g_log_set_writer_func() called multiple times");
+      return;
+    }
+
   log_writer_func = func;
   log_writer_user_data = user_data;
   log_writer_user_data_free = user_data_free;
+
   g_mutex_unlock (&g_messages_lock);
 }
 

glib/tests/logging.c

@@ -445,6 +445,10 @@ expect_log_writer (GLogLevelFlags   log_level,
 static void
 test_structured_logging_no_state (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       gpointer some_pointer = GUINT_TO_POINTER (0x100);
       guint some_integer = 123;
 
@@ -458,11 +462,21 @@ test_structured_logging_no_state (void)
                         some_pointer, some_integer);
 
       g_assert_cmpint (log_count, ==, 1);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_some_state (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       gpointer state_object = NULL;  /* this must not be dereferenced */
       const GLogField fields[] = {
         { "MESSAGE", "This is a debug message.", -1 },
@@ -477,11 +491,21 @@ test_structured_logging_some_state (void)
       g_log_structured_array (G_LOG_LEVEL_DEBUG, fields, G_N_ELEMENTS (fields));
 
       g_assert_cmpint (log_count, ==, 1);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_robustness (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       log_count = 0;
       g_log_set_writer_func (null_log_writer, NULL, NULL);
 
@@ -489,12 +513,22 @@ test_structured_logging_robustness (void)
       g_log (NULL, G_LOG_LEVEL_MESSAGE, "Test");
       g_log_structured (NULL, G_LOG_LEVEL_MESSAGE, "MESSAGE", "Test");
 
-  g_assert_cmpint (log_count, ==, 1);
+      g_assert_cmpint (log_count, ==, 2);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_roundtrip1 (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       gpointer some_pointer = GUINT_TO_POINTER (0x100);
       gint some_integer = 123;
       gchar message[200];
@@ -531,11 +565,21 @@ test_structured_logging_roundtrip1 (void)
           g_test_fail_printf ("Unexpected message: %s", str);
           g_free (str);
         }
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_roundtrip2 (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       const gchar *some_string = "abc";
       const GLogField fields[] = {
         { "GLIB_DOMAIN", "some-domain", -1 },
@@ -556,11 +600,21 @@ test_structured_logging_roundtrip2 (void)
                         some_string);
 
       g_assert (expected_messages == NULL);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_roundtrip3 (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       const GLogField fields[] = {
         { "GLIB_DOMAIN", "some-domain", -1 },
         { "PRIORITY", "4", -1 },
@@ -575,6 +629,12 @@ test_structured_logging_roundtrip3 (void)
                         "MESSAGE", "Test test test.");
 
       g_assert (expected_messages == NULL);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static GVariant *
@@ -597,6 +657,10 @@ create_variant_fields (void)
 static void
 test_structured_logging_variant1 (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       GVariant *v = create_variant_fields ();
 
       log_count = 0;
@@ -605,11 +669,21 @@ test_structured_logging_variant1 (void)
       g_log_variant ("some-domain", G_LOG_LEVEL_MESSAGE, v);
       g_variant_unref (v);
       g_assert_cmpint (log_count, ==, 1);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
 }
 
 static void
 test_structured_logging_variant2 (void)
 {
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func(), which
+   * can only be called once per process. */
+  if (g_test_subprocess ())
+    {
       const GLogField fields[] = {
         { "GLIB_DOMAIN", "some-domain", -1 },
         { "PRIORITY", "5", -1 },
@@ -627,6 +701,29 @@ test_structured_logging_variant2 (void)
       g_log_variant ("some-domain", G_LOG_LEVEL_MESSAGE, v);
       g_variant_unref (v);
       g_assert (expected_messages == NULL);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_passed ();
+    }
+}
+
+static void
+test_structured_logging_set_writer_func_twice (void)
+{
+  /* Test has to run in a subprocess as it calls g_log_set_writer_func() and
+   * causes an error. */
+  if (g_test_subprocess ())
+    {
+      g_log_set_writer_func (null_log_writer, NULL, NULL);
+      g_log_set_writer_func (expect_log_writer, NULL, NULL);
+    }
+  else
+    {
+      g_test_trap_subprocess (NULL, 0, 0);
+      g_test_trap_assert_failed ();
+    }
 }
 
 int
@@ -663,6 +760,7 @@ main (int argc, char *argv[])
   g_test_add_func ("/structured-logging/roundtrip3", test_structured_logging_roundtrip3);
   g_test_add_func ("/structured-logging/variant1", test_structured_logging_variant1);
   g_test_add_func ("/structured-logging/variant2", test_structured_logging_variant2);
+  g_test_add_func ("/structured-logging/set-writer-func-twice", test_structured_logging_set_writer_func_twice);
 
   return g_test_run ();
 }