Add a salt to the model output hash

backend/Dockerfile

@@ -52,8 +52,13 @@ RUN cd tinystan && \
     emmake make test_models/bernoulli/bernoulli.js -j$(nproc) && \
     emstrip test_models/bernoulli/bernoulli.wasm
 
-COPY stan-wasm-server /stan-wasm-server
-WORKDIR /stan-wasm-server
+COPY stan-wasm-server /app/stan-wasm-server
+
+# compute a hash of our app to make the model cache use unique URLS
+# (to avoid browsers caching across different versions)
+RUN find /app/ -type f -print0 | sort -z | xargs -0 sha1sum | sha1sum | cut -d' ' -f1 | tee /app/stan-wasm-server/hash-salt.txt
+
+WORKDIR /app/stan-wasm-server
 
 ENV SWS_PASSCODE=1234
 ENV SWS_LOG_LEVEL=debug

backend/stan-wasm-server/src/app/logic/compilation.py

@@ -1,6 +1,7 @@
 import asyncio
 import logging
 import time
+from functools import lru_cache
 from hashlib import sha1
 from pathlib import Path
 from shutil import copy2
@@ -18,10 +19,27 @@
 logger = logging.getLogger(__name__)
 
 
+@lru_cache
+def _get_salt() -> bytes:
+    """
+    Returns a salt to use when hashing Stan programs if one was set during the Docker build.
+    This helps ensure that models don't get cached by the browser across different deployments.
+    """
+    salt_file = Path(__file__).parent.parent.parent.parent / "hash-salt.txt"
+    if not salt_file.exists():
+        logger.warning("No hash salt file found at %s", salt_file)
+        salt = b""
+    else:
+        salt = salt_file.read_bytes().strip()
+        logger.info("Using hash salt from %s: %s", salt_file, repr(salt))
+    return salt
+
+
 def _compute_stan_program_hash(program_file: Path) -> str:
     stan_program = program_file.read_text()
-    # MAYBE: replace stan_program with a canonical form?
-    return sha1(stan_program.encode()).hexdigest()
+    hasher = sha1(_get_salt())
+    hasher.update(stan_program.encode())
+    return hasher.hexdigest()
 
 
 def make_canonical_model_dir(src_file: Path, built_model_dir: Path) -> Path:

backend/stan-wasm-server/src/app/main.py

@@ -138,18 +138,28 @@ async def compile_stan(
     return {"model_id": model_dir.name}
 
 
+def send_interrupt() -> None:
+    """
+    Send an interrupt signal to the parent process.
+    uvicorn interprets this like Ctrl-C, and gracefully shuts down.
+    The orchestrator then restarts the server.
+    """
+    import os
+    import signal
+
+    os.kill(os.getppid(), signal.SIGINT)
+
+
 @app.post("/restart")
 async def restart(
-    settings: DependsOnSettings, authorization: str = Header(None)
-) -> None:
+    settings: DependsOnSettings,
+    background_tasks: BackgroundTasks,
+    authorization: str = Header(None),
+) -> DictResponse:
     if settings.restart_token is None:
         raise StanPlaygroundAuthenticationException("Restart token not set at startup")
     check_authorization(authorization, settings.restart_token)
 
-    import os
-    import signal
+    background_tasks.add_task(send_interrupt)
 
-    # send an interrupt signal to the parent process
-    # uvicorn interprets this like Ctrl-C, and gracefully shuts down
-    os.kill(os.getppid(), signal.SIGINT)
-    # actual restart is handled by the orchestrator
+    return {"status": "restarting"}