Implement MSC4133 to support custom profile fields. (#17488)

Implementation of
[MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133)
to support custom profile fields. It is behind an experimental flag and
includes tests.


### Pull Request Checklist

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https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [x] [Code
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(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

1 files changed, 27 insertions, 1 deletions

diff --git a/tests/util/test_stringutils.py b/tests/util/test_stringutils.py

index 646fd2163e..34c2395ecf 100644
--- a/tests/util/test_stringutils.py
+++ b/tests/util/test_stringutils.py
@@ -20,7 +20,11 @@
 #
 
 from synapse.api.errors import SynapseError
-from synapse.util.stringutils import assert_valid_client_secret, base62_encode
+from synapse.util.stringutils import (
+    assert_valid_client_secret,
+    base62_encode,
+    is_namedspaced_grammar,
+)
 
 from .. import unittest
 
@@ -58,3 +62,25 @@ class StringUtilsTestCase(unittest.TestCase):
         self.assertEqual("10", base62_encode(62))
         self.assertEqual("1c", base62_encode(100))
         self.assertEqual("001c", base62_encode(100, minwidth=4))
+
+    def test_namespaced_identifier(self) -> None:
+        self.assertTrue(is_namedspaced_grammar("test"))
+        self.assertTrue(is_namedspaced_grammar("m.test"))
+        self.assertTrue(is_namedspaced_grammar("org.matrix.test"))
+        self.assertTrue(is_namedspaced_grammar("org.matrix.msc1234"))
+        self.assertTrue(is_namedspaced_grammar("test"))
+        self.assertTrue(is_namedspaced_grammar("t-e_s.t"))
+
+        # Must start with letter.
+        self.assertFalse(is_namedspaced_grammar("1test"))
+        self.assertFalse(is_namedspaced_grammar("-test"))
+        self.assertFalse(is_namedspaced_grammar("_test"))
+        self.assertFalse(is_namedspaced_grammar(".test"))
+
+        # Must contain only a-z, 0-9, -, _, ..
+        self.assertFalse(is_namedspaced_grammar("test/"))
+        self.assertFalse(is_namedspaced_grammar('test"'))
+        self.assertFalse(is_namedspaced_grammar("testö"))
+
+        # Must be < 255 characters.
+        self.assertFalse(is_namedspaced_grammar("t" * 256))