7 files changed, 149 insertions, 1 deletions
diff --git a/changelog.d/13759.misc b/changelog.d/13759.misc
new file mode 100644
index 0000000000..f91c512483
--- /dev/null
+++ b/changelog.d/13759.misc
@@ -0,0 +1 @@
+Add a check for editable installs if the Rust library needs rebuilding.
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 0a9760cafc..deddf3cec2 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -19,3 +19,7 @@ name = "synapse.synapse_rust"
 
 [dependencies]
 pyo3 = { version = "0.16.5", features = ["extension-module", "macros", "abi3", "abi3-py37"] }
+
+[build-dependencies]
+blake2 = "0.10.4"
+hex = "0.4.3"
diff --git a/rust/build.rs b/rust/build.rs
new file mode 100644
index 0000000000..2117975e56
--- /dev/null
+++ b/rust/build.rs
@@ -0,0 +1,45 @@
+//! This build script calculates the hash of all files in the `src/`
+//! directory and adds it as an environment variable during build time.
+//!
+//! This is used so that the python code can detect when the built native module
+//! does not match the source in-tree, helping to detect the case where the
+//! source has been updated but the library hasn't been rebuilt.
+
+use std::path::PathBuf;
+
+use blake2::{Blake2b512, Digest};
+
+fn main() -> Result<(), std::io::Error> {
+    let mut dirs = vec![PathBuf::from("src")];
+
+    let mut paths = Vec::new();
+    while let Some(path) = dirs.pop() {
+        let mut entries = std::fs::read_dir(path)?
+            .map(|res| res.map(|e| e.path()))
+            .collect::<Result<Vec<_>, std::io::Error>>()?;
+
+        entries.sort();
+
+        for entry in entries {
+            if entry.is_dir() {
+                dirs.push(entry)
+            } else {
+                paths.push(entry.to_str().expect("valid rust paths").to_string());
+            }
+        }
+    }
+
+    paths.sort();
+
+    let mut hasher = Blake2b512::new();
+
+    for path in paths {
+        let bytes = std::fs::read(path)?;
+        hasher.update(bytes);
+    }
+
+    let hex_digest = hex::encode(hasher.finalize());
+    println!("cargo:rustc-env=SYNAPSE_RUST_DIGEST={hex_digest}");
+
+    Ok(())
+}
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index 142fc2ed93..ba42465fb8 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -1,5 +1,13 @@
 use pyo3::prelude::*;
 
+/// Returns the hash of all the rust source files at the time it was compiled.
+///
+/// Used by python to detect if the rust library is outdated.
+#[pyfunction]
+fn get_rust_file_digest() -> &'static str {
+    env!("SYNAPSE_RUST_DIGEST")
+}
+
 /// Formats the sum of two numbers as string.
 #[pyfunction]
 #[pyo3(text_signature = "(a, b, /)")]
@@ -11,6 +19,6 @@ fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
 #[pymodule]
 fn synapse_rust(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
-
+    m.add_function(wrap_pyfunction!(get_rust_file_digest, m)?)?;
     Ok(())
 }
diff --git a/stubs/synapse/synapse_rust.pyi b/stubs/synapse/synapse_rust.pyi
index 5b51ba05d7..8658d3138f 100644
--- a/stubs/synapse/synapse_rust.pyi
+++ b/stubs/synapse/synapse_rust.pyi
@@ -1 +1,2 @@
 def sum_as_string(a: int, b: int) -> str: ...
+def get_rust_file_digest() -> str: ...
diff --git a/synapse/__init__.py b/synapse/__init__.py
index b1369aca8f..1bed6393bd 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -20,6 +20,8 @@ import json
 import os
 import sys
 
+from synapse.util.rust import check_rust_lib_up_to_date
+
 # Check that we're not running on an unsupported Python version.
 if sys.version_info < (3, 7):
     print("Synapse requires Python 3.7 or above.")
@@ -78,3 +80,6 @@ if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     from synapse.util.patch_inline_callbacks import do_patch
 
     do_patch()
+
+
+check_rust_lib_up_to_date()
diff --git a/synapse/util/rust.py b/synapse/util/rust.py
new file mode 100644
index 0000000000..30ecb9ffd9
--- /dev/null
+++ b/synapse/util/rust.py
@@ -0,0 +1,84 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import sys
+from hashlib import blake2b
+
+import synapse
+from synapse.synapse_rust import get_rust_file_digest
+
+
+def check_rust_lib_up_to_date() -> None:
+    """For editable installs check if the rust library is outdated and needs to
+    be rebuilt.
+    """
+
+    if not _dist_is_editable():
+        return
+
+    synapse_dir = os.path.dirname(synapse.__file__)
+    synapse_root = os.path.abspath(os.path.join(synapse_dir, ".."))
+
+    # Double check we've not gone into site-packages...
+    if os.path.basename(synapse_root) == "site-packages":
+        return
+
+    # ... and it looks like the root of a python project.
+    if not os.path.exists("pyproject.toml"):
+        return
+
+    # Get the hash of all Rust source files
+    hash = _hash_rust_files_in_directory(os.path.join(synapse_root, "rust", "src"))
+
+    if hash != get_rust_file_digest():
+        raise Exception("Rust module outdated. Please rebuild using `poetry install`")
+
+
+def _hash_rust_files_in_directory(directory: str) -> str:
+    """Get the hash of all files in a directory (recursively)"""
+
+    directory = os.path.abspath(directory)
+
+    paths = []
+
+    dirs = [directory]
+    while dirs:
+        dir = dirs.pop()
+        with os.scandir(dir) as d:
+            for entry in d:
+                if entry.is_dir():
+                    dirs.append(entry.path)
+                else:
+                    paths.append(entry.path)
+
+    # We sort to make sure that we get a consistent and well-defined ordering.
+    paths.sort()
+
+    hasher = blake2b()
+
+    for path in paths:
+        with open(os.path.join(directory, path), "rb") as f:
+            hasher.update(f.read())
+
+    return hasher.hexdigest()
+
+
+def _dist_is_editable() -> bool:
+    """Is distribution an editable install?"""
+    for path_item in sys.path:
+        egg_link = os.path.join(path_item, "matrix-synapse.egg-link")
+        if os.path.isfile(egg_link):
+            return True
+    return False