Check if Rust lib needs rebuilding. (#13759)

This protects against the common mistake of failing to remember to rebuild Rust code after making changes.

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"

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(())
+}

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+/// Used by python to detect if the rust library is outdated.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(())
 }