1.142.0rc1 regression fix: Allow coercing a str to a FilePath in MasConfigModel (#19144)

changelog.d/19144.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in 1.142.0rc1 where any attempt to configure `matrix_authentication_service.secret_path` would prevent the homeserver from starting up.

synapse/config/mas.py

@@ -37,7 +37,8 @@ class MasConfigModel(ParseModel):
     enabled: StrictBool = False
     endpoint: AnyHttpUrl = AnyHttpUrl("http://localhost:8080")
     secret: Optional[StrictStr] = Field(default=None)
-    secret_path: Optional[FilePath] = Field(default=None)
+    # We set `strict=False` to allow `str` instances.
+    secret_path: Optional[FilePath] = Field(default=None, strict=False)
 
     @model_validator(mode="after")
     def verify_secret(self) -> Self:

synapse/util/pydantic_models.py

@@ -30,6 +30,13 @@ class ParseModel(BaseModel):
 
     but otherwise uses Pydantic's default behaviour.
 
+    Strict mode can adversely affect some types of fields, and should be disabled
+    for a field if:
+
+    - the field's type is a `Path` or `FilePath`. Strict mode will refuse to
+      coerce from `str` (likely what the yaml parser will produce) to `FilePath`,
+      raising a `ValidationError`.
+
     For now, ignore unknown fields. In the future, we could change this so that unknown
     config values cause a ValidationError, provided the error messages are meaningful to
     server operators.