glib: add internal g_datalist_id_update_atomic() function

glib/gdataset.c

@@ -827,6 +827,107 @@ g_datalist_id_remove_no_notify (GData	**datalist,
   return ret_data;
 }
 
+/**
+ * g_datalist_id_update_atomic:
+ * @datalist: the data list
+ * @key_id: they key to add.
+ * @callback: callback to update (set, remove, steal, update) the
+ *   data.
+ * @user_data: the user data for @callback.
+ *
+ * Will call @callback while holding the lock on @datalist. Be careful, to not
+ * end up calling into another data-list function, because the lock is not reentrant
+ * and deadlock will happen.
+ *
+ * The callback receives the current data and destroy function. If the key
+ * is currently not in @datalist, they will be %NULL.
+ * The callback can update those pointers, and @datalist will be updated.
+ * Note that if callback modifies a received data, then it MUST steal it
+ * and take ownership on it. Possibly freeing it with the provided destroy function.
+ *
+ * The point is to atomically update the entry, while holding a lock.
+ *
+ * Returns: the data that @callback returns.
+ */
+gpointer
+g_datalist_id_update_atomic (GData **datalist,
+                             GQuark key_id,
+                             GDataListUpdateAtomicFunc callback,
+                             gpointer user_data)
+{
+  GData *d;
+  GDataElt *data;
+  gpointer new_data;
+  GDestroyNotify new_destroy;
+  guint32 idx;
+  gboolean to_unlock = TRUE;
+
+  d = g_datalist_lock_and_get (datalist);
+
+  data = datalist_find (d, key_id, &idx);
+
+  if (data)
+    {
+      new_data = data->data;
+      new_destroy = data->destroy;
+    }
+  else
+    {
+      new_data = NULL;
+      new_destroy = NULL;
+    }
+
+  callback (key_id, &new_data, &new_destroy, user_data);
+
+  if (data && !new_data)
+    {
+      /* Remove.
+       *
+       * The old data->data was already stolen by callback(). */
+      if (idx != d->len - 1u)
+        *data = d->data[d->len - 1u];
+      d->len--;
+
+      /* We don't bother to shrink, but if all data are now gone
+       * we at least free the memory
+       */
+      if (d->len == 0)
+        {
+          /* datalist may be situated in dataset, so must not be
+           * unlocked when we free it
+           */
+          g_datalist_unlock_and_set (datalist, NULL);
+          g_free (d);
+          to_unlock = FALSE;
+        }
+    }
+  else if (data)
+    {
+      /* Update.
+       *
+       * The old data was stolen by callback(). We only update the pointers and are done. */
+      data->data = new_data;
+      data->destroy = new_destroy;
+    }
+  else if (!data && !new_data)
+    {
+      /* Absent, no change. We had no data, and nothing to add. */
+    }
+  else
+    {
+      /* Add */
+      if (datalist_append (&d, key_id, new_data, new_destroy))
+        {
+          g_datalist_unlock_and_set (datalist, d);
+          to_unlock = FALSE;
+        }
+    }
+
+  if (to_unlock)
+    g_datalist_unlock (datalist);
+  return new_data;
+}
+
 /**
  * g_dataset_id_get_data:
  * @dataset_location: (not nullable): the location identifying the dataset.

glib/gdatasetprivate.h

@@ -38,6 +38,15 @@ G_BEGIN_DECLS
 #define G_DATALIST_GET_FLAGS(datalist)				\
   ((gsize) g_atomic_pointer_get (datalist) & G_DATALIST_FLAGS_MASK)
 
+typedef gpointer (*GDataListUpdateAtomicFunc) (GQuark key_id,
+                                               gpointer *data,
+                                               GDestroyNotify *destroy_notify,
+                                               gpointer user_data);
+
+gpointer g_datalist_id_update_atomic (GData **datalist,
+                                      GQuark key_id,
+                                      GDataListUpdateAtomicFunc callback,
+                                      gpointer user_data);
 
 G_END_DECLS
 

glib/glib-private.c

@@ -24,6 +24,7 @@
 #include "glib-private.h"
 #include "glib-init.h"
 #include "gutilsprivate.h"
+#include "gdatasetprivate.h"
 
 #ifdef USE_INVALID_PARAMETER_HANDLER
 #include <crtdbg.h>
@@ -74,6 +75,8 @@ glib__private__ (void)
     g_uri_get_default_scheme_port,
 
     g_set_prgname_once,
+
+    g_datalist_id_update_atomic,
   };
 
   return &table;

glib/glib-private.h

@@ -75,6 +75,11 @@ void __lsan_ignore_object (const void *p) __attribute__ ((weak));
  */
 #define G_CONTAINER_OF(ptr, type, field) ((type *) G_STRUCT_MEMBER_P (ptr, -G_STRUCT_OFFSET (type, field)))
 
+typedef gpointer (*GDataListUpdateAtomicFunc) (GQuark key_id,
+                                               gpointer *data,
+                                               GDestroyNotify *destroy_notify,
+                                               gpointer user_data);
+
 /*
  * g_leak_sanitizer_is_supported:
  *
@@ -291,6 +296,11 @@ typedef struct {
   /* See gutils.c */
   gboolean (* g_set_prgname_once) (const gchar *prgname);
 
+  gpointer (*g_datalist_id_update_atomic) (GData **datalist,
+                                           GQuark key_id,
+                                           GDataListUpdateAtomicFunc callback,
+                                           gpointer user_data);
+
   /* Add other private functions here, initialize them in glib-private.c */
 } GLibPrivateVTable;