fix: QwebJSON circular reference on non-serializable session_info values

The QwebJSON.dumps default handler returned non-serializable objects
unchanged, causing Python's json encoder to report "Circular reference
detected". Use odoo.tools.json.json_default() as fallback to properly
convert ReadonlyDict, lazy, datetime, bytes, Domain and other types.

🤖 assisted by claude

docs/QWEBJSON_CIRCULAR_REFERENCE.md

@@ -0,0 +1,73 @@
+# QwebJSON Circular Reference Bugfix
+
+## Problem
+
+When loading the Odoo 19 web client, the `web.webclient_bootstrap` QWeb template
+fails with:
+
+```
+ValueError: Circular reference detected
+```
+
+at the line:
+
+```xml
+<t t-out="json.dumps(session_info)"/>
+```
+
+## Root Cause
+
+The `QwebJSON.dumps()` default handler in `odoo/addons/base/models/ir_qweb.py`
+returned non-serializable objects unchanged:
+
+```python
+# Original (broken)
+class QwebJSON(json.JSON):
+    def dumps(self, *args, **kwargs):
+        prev_default = kwargs.pop('default', lambda obj: obj)
+        return super().dumps(*args, **kwargs, default=(
+            lambda obj: prev_default(str(obj) if isinstance(obj, QwebContent) else obj)
+        ))
+```
+
+When the JSON encoder encounters a non-serializable type (e.g. `ReadonlyDict`,
+`lazy`, `datetime`, `bytes`, `Domain`), it calls the `default` handler. The handler
+returns the object unchanged (since it's not `QwebContent`). The encoder tries to
+serialize it again, gets the same object back, and raises "Circular reference detected."
+
+This is a **latent bug in the Odoo 19 core**. In vanilla Odoo it rarely triggers
+because `session_info` values are typically JSON-serializable. However, any module
+(OCA or custom) that introduces a non-serializable type into `session_info` or any
+other QWeb JSON-serialized context will expose this bug.
+
+## Fix
+
+Use `odoo.tools.json.json_default()` as the fallback for non-QwebContent objects:
+
+```python
+# Fixed
+class QwebJSON(json.JSON):
+    def dumps(self, *args, **kwargs):
+        prev_default = kwargs.pop('default', lambda obj: obj)
+        return super().dumps(*args, **kwargs, default=(
+            lambda obj: prev_default(str(obj) if isinstance(obj, QwebContent) else json.json_default(obj))
+        ))
+```
+
+`json_default()` (from `odoo/tools/json.py`) properly handles:
+
+- `datetime` -> string via `fields.Datetime.to_string()`
+- `date` -> string via `fields.Date.to_string()`
+- `lazy` -> resolved value
+- `ReadonlyDict` -> regular `dict`
+- `bytes` -> decoded string
+- `Domain` -> list
+- anything else -> `str(obj)`
+
+## File Changed
+
+- `odoo-bringout-oca-ocb-base/odoo/addons/base/models/ir_qweb.py` (class `QwebJSON`)
+
+## Date
+
+2026-03-10

odoo-bringout-oca-ocb-base/odoo/addons/base/models/ir_qweb.py

@@ -656,7 +656,7 @@ class QwebJSON(json.JSON):
     def dumps(self, *args, **kwargs):
         prev_default = kwargs.pop('default', lambda obj: obj)
         return super().dumps(*args, **kwargs, default=(
-            lambda obj: prev_default(str(obj) if isinstance(obj, QwebContent) else obj)
+            lambda obj: prev_default(str(obj) if isinstance(obj, QwebContent) else json.json_default(obj))
         ))