structure: Remove quadratic behaviour from gst_structure_fixate()

It was iterating over each field and after fixating its value was again
iterating over every field to find where to store the value.

Instead directly overwrite the value after validating it.

Also actually check that the structure is writable before modifying its fields
by using gst_structure_map_in_place() instead of gst_structure_fixate().

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7420>

subprojects/gstreamer/gst/gststructure.c

@@ -1001,6 +1001,59 @@ gst_structure_new_id (GQuark name_quark, GQuark field_quark, ...)
 #define GIT_G_WARNING GST_WARNING
 #endif
 
+static gboolean
+gst_structure_validate_field_value (const GstStructure * structure,
+    GQuark field, const GValue * value)
+{
+  GType field_value_type;
+
+  field_value_type = G_VALUE_TYPE (value);
+  if (field_value_type == G_TYPE_STRING) {
+    const gchar *s;
+
+    s = g_value_get_string (value);
+    /* only check for NULL strings in taglists, as they are allowed in message
+     * structs, e.g. error message debug strings */
+    if (G_UNLIKELY (IS_TAGLIST (structure) && (s == NULL || *s == '\0'))) {
+      if (s == NULL) {
+        GIT_G_WARNING ("Trying to set NULL string on field '%s' on taglist. "
+            "Please file a bug.", g_quark_to_string (field));
+        return FALSE;
+      } else {
+        /* empty strings never make sense */
+        GIT_G_WARNING ("Trying to set empty string on taglist field '%s'. "
+            "Please file a bug.", g_quark_to_string (field));
+        return FALSE;
+      }
+    } else if (G_UNLIKELY (s != NULL && !g_utf8_validate (s, -1, NULL))) {
+      g_warning ("Trying to set string on %s field '%s', but string is not "
+          "valid UTF-8. Please file a bug.",
+          IS_TAGLIST (structure) ? "taglist" : "structure",
+          g_quark_to_string (field));
+      return FALSE;
+    }
+  } else if (G_UNLIKELY (field_value_type == G_TYPE_DATE)) {
+    const GDate *d;
+
+    d = g_value_get_boxed (value);
+    /* only check for NULL GDates in taglists, as they might make sense
+     * in other, generic structs */
+    if (G_UNLIKELY ((IS_TAGLIST (structure) && d == NULL))) {
+      GIT_G_WARNING ("Trying to set NULL GDate on field '%s' on taglist. "
+          "Please file a bug.", g_quark_to_string (field));
+      return FALSE;
+    } else if (G_UNLIKELY (d != NULL && !g_date_valid (d))) {
+      g_warning
+          ("Trying to set invalid GDate on %s field '%s'. Please file a bug.",
+          IS_TAGLIST (structure) ? "taglist" : "structure",
+          g_quark_to_string (field));
+      return FALSE;
+    }
+  }
+
+  return TRUE;
+}
+
 /* If the structure currently contains a field with the same name, it is
  * replaced with the provided field. Otherwise, the field is added to the
  * structure. The field's value is not deeply copied.
@@ -1009,60 +1062,15 @@ static void
 gst_structure_set_field (GstStructure * structure, GstStructureField * field)
 {
   GstStructureField *f;
-  GType field_value_type;
   guint i, len;
 
+  if (!gst_structure_validate_field_value (structure, field->name,
+          &field->value)) {
+    g_value_unset (&field->value);
+    return;
+  }
+
   len = GST_STRUCTURE_LEN (structure);
-
-  field_value_type = G_VALUE_TYPE (&field->value);
-  if (field_value_type == G_TYPE_STRING) {
-    const gchar *s;
-
-    s = g_value_get_string (&field->value);
-    /* only check for NULL strings in taglists, as they are allowed in message
-     * structs, e.g. error message debug strings */
-    if (G_UNLIKELY (IS_TAGLIST (structure) && (s == NULL || *s == '\0'))) {
-      if (s == NULL) {
-        GIT_G_WARNING ("Trying to set NULL string on field '%s' on taglist. "
-            "Please file a bug.", g_quark_to_string (field->name));
-        g_value_unset (&field->value);
-        return;
-      } else {
-        /* empty strings never make sense */
-        GIT_G_WARNING ("Trying to set empty string on taglist field '%s'. "
-            "Please file a bug.", g_quark_to_string (field->name));
-        g_value_unset (&field->value);
-        return;
-      }
-    } else if (G_UNLIKELY (s != NULL && !g_utf8_validate (s, -1, NULL))) {
-      g_warning ("Trying to set string on %s field '%s', but string is not "
-          "valid UTF-8. Please file a bug.",
-          IS_TAGLIST (structure) ? "taglist" : "structure",
-          g_quark_to_string (field->name));
-      g_value_unset (&field->value);
-      return;
-    }
-  } else if (G_UNLIKELY (field_value_type == G_TYPE_DATE)) {
-    const GDate *d;
-
-    d = g_value_get_boxed (&field->value);
-    /* only check for NULL GDates in taglists, as they might make sense
-     * in other, generic structs */
-    if (G_UNLIKELY ((IS_TAGLIST (structure) && d == NULL))) {
-      GIT_G_WARNING ("Trying to set NULL GDate on field '%s' on taglist. "
-          "Please file a bug.", g_quark_to_string (field->name));
-      g_value_unset (&field->value);
-      return;
-    } else if (G_UNLIKELY (d != NULL && !g_date_valid (d))) {
-      g_warning
-          ("Trying to set invalid GDate on %s field '%s'. Please file a bug.",
-          IS_TAGLIST (structure) ? "taglist" : "structure",
-          g_quark_to_string (field->name));
-      g_value_unset (&field->value);
-      return;
-    }
-  }
-
   for (i = 0; i < len; i++) {
     f = GST_STRUCTURE_FIELD (structure, i);
 
@@ -2845,13 +2853,16 @@ gst_structure_fixate_field_nearest_fraction (GstStructure * structure,
 }
 
 static gboolean
-default_fixate (GQuark field_id, const GValue * value, gpointer data)
+default_fixate (GQuark field_id, GValue * value, gpointer data)
 {
   GstStructure *s = data;
   GValue v = { 0 };
 
   if (gst_value_fixate (&v, value)) {
-    gst_structure_id_take_value (s, field_id, &v);
+    if (gst_structure_validate_field_value (s, field_id, value)) {
+      g_value_unset (value);
+      *value = v;
+    }
   }
   return TRUE;
 }
@@ -3371,7 +3382,7 @@ gst_structure_fixate (GstStructure * structure)
 {
   g_return_if_fail (GST_IS_STRUCTURE (structure));
 
-  gst_structure_foreach (structure, default_fixate, structure);
+  gst_structure_map_in_place (structure, default_fixate, structure);
 }
 
 static gboolean