Store calculated room ID on top-level

Rather than validating and then re-calcualting it every time, let's just
store it on the top-level.

rust/src/events/formats/v1.rs

@@ -35,7 +35,7 @@ use crate::events::formats::EventCommonFields;
 pub struct EventFormatV1 {
     pub auth_events: Vec<(String, HashMap<String, String>)>,
     pub prev_events: Vec<(String, HashMap<String, String>)>,
-    pub room_id: Box<str>,
+    pub room_id: Arc<str>,
     pub event_id: Arc<str>,
 }
 

rust/src/events/formats/v2v3.rs

@@ -29,6 +29,8 @@
 //! [`SimpleAuthPrevEvents`] is shared with [`v4`](super::v4) since the
 //! flat-list encoding carries forward unchanged.
 
+use std::sync::Arc;
+
 use anyhow::{bail, Error};
 use serde::{Deserialize, Serialize};
 
@@ -37,7 +39,7 @@ use crate::events::formats::EventCommonFields;
 /// Version-specific fields for room versions 3-10.
 #[derive(Serialize, Deserialize)]
 pub struct EventFormatV2V3 {
-    pub room_id: Box<str>,
+    pub room_id: Arc<str>,
     pub auth_events: Vec<String>,
     pub prev_events: Vec<String>,
 }

rust/src/events/formats/v4.rs

@@ -27,7 +27,7 @@
 //! [`EventFormatV4::room_id`] and [`EventFormatV4::auth_event_ids`]
 //! expose the derived values to callers.
 
-use std::borrow::Cow;
+use std::sync::Arc;
 
 use anyhow::{bail, ensure, Error};
 use serde::{Deserialize, Serialize};
@@ -45,15 +45,13 @@ pub struct EventFormatV4 {
         with = "crate::json::allow_missing",
         skip_serializing_if = "AllowMissing::is_absent"
     )]
-    pub room_id: AllowMissing<Box<str>>,
+    pub room_id: AllowMissing<Arc<str>>,
     pub auth_events: Vec<String>,
     pub prev_events: Vec<String>,
 }
 
 impl EventFormatV4 {
     pub fn validate(&self, common_fields: &EventCommonFields) -> Result<(), Error> {
-        validate_optional_room_id(self.room_id.as_deref_opt(), common_fields)?;
-
         // Ensure that we don't have an event_id set.
         if common_fields.other_fields.contains_key("event_id") {
             bail!("v4 events must not have an explicit event_id");
@@ -66,8 +64,8 @@ impl EventFormatV4 {
         &self,
         event_id: &str,
         common_fields: &EventCommonFields,
-    ) -> Result<Cow<'_, str>, Error> {
-        get_room_id_for_optional_room_id(self.room_id.as_deref_opt(), event_id, common_fields)
+    ) -> Result<Arc<str>, Error> {
+        get_room_id_for_optional_room_id(self.room_id.as_ref_opt(), event_id, common_fields)
     }
 
     pub fn auth_event_ids(&self, common_fields: &EventCommonFields) -> Result<Vec<String>, Error> {
@@ -104,10 +102,10 @@ impl EventFormatV4 {
 /// Validation helper for v4+ events that can have an optional room ID.
 ///
 /// Returns the validated room ID (which will be `None` for create events).
-pub fn validate_optional_room_id<'a>(
-    room_id: Option<&'a str>,
+pub fn validate_optional_room_id(
+    room_id: Option<&Arc<str>>,
     common_fields: &'_ EventCommonFields,
-) -> Result<Option<&'a str>, Error> {
+) -> Result<Option<Arc<str>>, Error> {
     let is_create_event = common_fields.type_state_key_tuple() == Some((M_ROOM_CREATE, ""));
 
     match (is_create_event, room_id) {
@@ -123,7 +121,7 @@ pub fn validate_optional_room_id<'a>(
                 room_id
             );
 
-            Ok(Some(room_id))
+            Ok(Some(Arc::clone(room_id)))
         }
 
         // For create events, room_id must be absent.
@@ -134,13 +132,13 @@ pub fn validate_optional_room_id<'a>(
 
 /// Room ID derivation helper for v4+ events, which can have an optional room
 /// ID.
-pub fn get_room_id_for_optional_room_id<'a>(
-    room_id: Option<&'a str>,
+pub fn get_room_id_for_optional_room_id(
+    room_id: Option<&Arc<str>>,
     event_id: &str,
     common_fields: &EventCommonFields,
-) -> Result<Cow<'a, str>, Error> {
+) -> Result<Arc<str>, Error> {
     match validate_optional_room_id(room_id, common_fields)? {
-        Some(room_id) => Ok(room_id.into()),
+        Some(room_id) => Ok(room_id),
         None => {
             // This is the create event, where the room ID is derived from the
             // event ID by replacing the leading '$' with '!'.

rust/src/events/formats/vmsc4242.rs

@@ -24,7 +24,7 @@
 //!
 //! [MSC4242]: https://github.com/matrix-org/matrix-spec-proposals/pull/4242
 
-use std::borrow::Cow;
+use std::sync::Arc;
 
 use anyhow::bail;
 use anyhow::Error;
@@ -34,7 +34,6 @@ use serde::{Deserialize, Serialize};
 
 use crate::events::constants::event_type::M_ROOM_CREATE;
 use crate::events::formats::v4::get_room_id_for_optional_room_id;
-use crate::events::formats::v4::validate_optional_room_id;
 use crate::events::formats::EventCommonFields;
 use crate::events::Event;
 use crate::json::AllowMissing;
@@ -49,13 +48,11 @@ pub struct EventFormatVMSC4242 {
         with = "crate::json::allow_missing",
         skip_serializing_if = "AllowMissing::is_absent"
     )]
-    pub room_id: AllowMissing<Box<str>>,
+    pub room_id: AllowMissing<Arc<str>>,
 }
 
 impl EventFormatVMSC4242 {
     pub fn validate(&self, common_fields: &EventCommonFields) -> Result<(), Error> {
-        validate_optional_room_id(self.room_id.as_deref_opt(), common_fields)?;
-
         // Ensure that we don't have any `auth_events` or `event_id` fields
         // set.
         if common_fields.other_fields.contains_key("auth_events") {
@@ -72,8 +69,8 @@ impl EventFormatVMSC4242 {
         &self,
         event_id: &str,
         common_fields: &EventCommonFields,
-    ) -> Result<Cow<'_, str>, Error> {
-        get_room_id_for_optional_room_id(self.room_id.as_deref_opt(), event_id, common_fields)
+    ) -> Result<Arc<str>, Error> {
+        get_room_id_for_optional_room_id(self.room_id.as_ref_opt(), event_id, common_fields)
     }
 
     pub fn auth_event_ids(&self, event: &Event) -> PyResult<Vec<String>> {

rust/src/events/mod.rs

@@ -48,7 +48,7 @@
 //!   string describing why auth rejected the event.
 //!
 
-use std::{borrow::Cow, sync::Arc};
+use std::sync::Arc;
 
 use pyo3::{
     exceptions::{PyAttributeError, PyKeyError, PyValueError},
@@ -137,6 +137,12 @@ pub struct Event {
     /// construction time and cached here.
     event_id: Arc<str>,
 
+    /// The calculated room ID.
+    ///
+    /// For some room versions this may be derived, e.g. for create events in
+    /// v4.
+    room_id: Arc<str>,
+
     /// Synapse-internal per-event state that lives outside the federated
     /// JSON (e.g. outlier flag, soft-failure, stream positions).
     #[pyo3(get)]
@@ -205,10 +211,32 @@ impl Event {
             }
         };
 
+        let room_id = match &*parsed_event.specific_fields {
+            EventFormatEnum::V1(format) => Arc::clone(&format.room_id),
+            EventFormatEnum::V2V3(format) => Arc::clone(&format.room_id),
+            EventFormatEnum::V4(format) => format
+                .room_id(&event_id, &parsed_event.common_fields)
+                .map_err(|err| {
+                PyValueError::new_err(format!(
+                    "Failed to calculate room_id for event {}: {}",
+                    event_id, err
+                ))
+            })?,
+            EventFormatEnum::VMSC4242(format) => format
+                .room_id(&event_id, &parsed_event.common_fields)
+                .map_err(|err| {
+                    PyValueError::new_err(format!(
+                        "Failed to calculate room_id for event {}: {}",
+                        event_id, err
+                    ))
+                })?,
+        };
+
         Ok(Self {
             parsed_event,
 
             event_id,
+            room_id,
             room_version,
             rejected_reason,
             internal_metadata,
@@ -341,6 +369,7 @@ impl Event {
             room_version: self.room_version,
             rejected_reason: self.rejected_reason.clone(),
             event_id: self.event_id.clone(),
+            room_id: self.room_id.clone(),
         };
         Ok(new_event)
     }
@@ -432,17 +461,8 @@ impl Event {
     }
 
     #[getter]
-    fn room_id(&self) -> PyResult<Cow<'_, str>> {
-        match &*self.parsed_event.specific_fields {
-            EventFormatEnum::V1(format) => Ok(format.room_id.as_ref().into()),
-            EventFormatEnum::V2V3(format) => Ok(format.room_id.as_ref().into()),
-            EventFormatEnum::V4(format) => {
-                Ok(format.room_id(&self.event_id, &self.parsed_event.common_fields)?)
-            }
-            EventFormatEnum::VMSC4242(format) => {
-                Ok(format.room_id(&self.event_id, &self.parsed_event.common_fields)?)
-            }
-        }
+    fn room_id(&self) -> &str {
+        &self.room_id
     }
 
     #[getter]

rust/src/json.rs

@@ -64,6 +64,14 @@ impl<T> AllowMissing<T> {
             AllowMissing::Absent => None,
         }
     }
+
+    /// Converts to `Option<&T>`.
+    pub fn as_ref_opt(&self) -> Option<&T> {
+        match self {
+            AllowMissing::Some(inner) => Some(inner),
+            AllowMissing::Absent => None,
+        }
+    }
 }
 
 /// A module that provides the serialization and deserialization logic for