BPY: Implement get_transform and set_transform for runtime-defined RNA properties.

Improve handling of runtime defined python RNA properties. Mainly:
* Add `get_transform` and `set_transform` new callbacks.
  These allow to edit the value, while still using the default
  (IDProperty-based) storage system.
* Read-only properties should now be defined using a new `options` flag,
  `READ_ONLY`.
* `get`/`set` should only be used when storing data outside of the
  default system now.
  * Having a `get` without a `set` defined forces property to be
    read-only (same behavior as before).
  * Having a `set` without a `get` is now an error.
* Just like with existing `get/set` callbacks, `get_/set_transform`
  callbacks must always generate values matching the constraints defined
  by their `bpy.props` property definition (same type, within required
  range, same dimensions/sizes for the `Vector` properties, etc.).
* To simplify handling of non-statically sized strings, the relevant
  RNA API has been modified, to use `std::string` instead of
  (allocated) char arrays.

Relevant unittests and benchmarking have been added or updated as part
of this project.

Note: From initial benchmarking, 'transform' versions of get/set are
several times faster than 'real' get/set.

Implements #141042.

Pull Request: https://projects.blender.org/blender/blender/pulls/141303

tests/python/bl_pyapi_idprop.py

@@ -7,6 +7,8 @@ import bpy
 import rna_prop_ui
 import idprop
 
+import io
+import sys
 import unittest
 from array import array
 
@@ -266,6 +268,12 @@ class TestIdPropertyDynamicRNA(TestHelper, unittest.TestCase):
         string_prop: bpy.props.StringProperty()
         int_prop: bpy.props.IntProperty()
         float_array_prop: bpy.props.FloatVectorProperty(size=[3])
+        # Basic get/set transform.
+        string_prop_transform: bpy.props.StringProperty(
+            default="test",
+            maxlen=10,
+            get_transform=lambda self, storage_val, is_set: storage_val + "!!",
+            set_transform=lambda self, new_val, storage_val, is_set: storage_val + "!!" + new_val)
 
     def setUp(self):
         super().setUp()
@@ -348,6 +356,40 @@ class TestIdPropertyDynamicRNA(TestHelper, unittest.TestCase):
         with self.assertRaises(TypeError):
             self.id.bl_system_properties_get()['dynrna_prop']['float_array_prop'] = [1.0, 10.0, 100.0, 0.1]
 
+    def test_get_set_transform(self):
+        self.assertEqual(len(self.id.dynrna_prop.string_prop_transform), 6)
+        self.assertEqual(self.id.dynrna_prop.string_prop_transform, "test!!")
+        # Default value only, was not yet set.
+        self.assertFalse('string_prop_transform' in self.id.bl_system_properties_get()['dynrna_prop'])
+
+        self.id.dynrna_prop.string_prop_transform = "-"
+        self.assertEqual(self.id.dynrna_prop.string_prop_transform, "test!!-!!")
+        self.assertEqual(self.id.bl_system_properties_get()['dynrna_prop']['string_prop_transform'], "test!!-")
+
+        # Raw-set exactly maxlen - 1 char.
+        self.id.bl_system_properties_get()['dynrna_prop']['string_prop_transform'] = "test!!tes"
+        # get_transform will produce an 11-char results, which should trigger an error.
+        # These asserts are not raised currently, but only print in `stderr`...
+        # But the returned string is the 'storage' one, not the result from get_transform.
+        stderr, sys.stderr = sys.stderr, io.StringIO()
+        self.assertEqual(self.id.dynrna_prop.string_prop_transform, "test!!tes")
+        self.assertTrue("ValueError" in sys.stderr.getvalue() and
+                        "10" in sys.stderr.getvalue() and "11" in sys.stderr.getvalue())
+        sys.stderr.close()
+        sys.stderr = stderr
+
+        # Raw-set back to default value.
+        self.id.bl_system_properties_get()['dynrna_prop']['string_prop_transform'] = "test"
+        # Now set_transform will return 12-char string, wich is also invalid and discarded.
+        stderr, sys.stderr = sys.stderr, io.StringIO()
+        self.id.dynrna_prop.string_prop_transform = "test!!"
+        self.assertTrue("ValueError" in sys.stderr.getvalue() and
+                        "10" in sys.stderr.getvalue() and "12" in sys.stderr.getvalue())
+        sys.stderr.close()
+        sys.stderr = stderr
+        self.assertEqual(self.id.bl_system_properties_get()['dynrna_prop']['string_prop_transform'], "test")
+        self.assertEqual(self.id.dynrna_prop.string_prop_transform, "test!!")
+
 
 class TestIdPropertyGroupView(TestHelper, unittest.TestCase):