fix disk cache datarace (#486)

There was a data race when multiple threads/processes found that the cache directory for a given params didn't exist as they both tried to:
- Create a tmp params file
- Rename the tmp params file

Both threads would create the tmp params file (one would overwrite the other).  Then one would rename the tmp file successfully, and the other would try to rename it again but would find it missing.

The fix involves using a file lock on the tmp file so that only one thread goes through the writing and renaming while others wait.  It's the same approach done for cached files.

src/cache/disk.rs

@@ -49,14 +49,27 @@ pub fn get<T: Serialize + DeserializeOwned, P: Serialize>(
 
     // Store the params.json if it doesn't exist for better debuggability
     let params_path = cache_dir.join("params.json");
+    loop {
         if !params_path.try_exists()? {
-        // First write the file to .tmp and then rename to avoid a corrupted file if we crash in
-        // the middle of the write.
+            // First write the file to .tmp and then rename to avoid having a corrupted file if we
+            // crash in the middle of the write.
             let params_path_tmp = cache_dir.join("params.json.tmp");
             let mut file = File::create(&params_path_tmp)?;
+            match file.try_lock() {
+                Ok(_) => (),
+                Err(TryLockError::WouldBlock) => {
+                    // Lock not acquired.  Another process is storing the params, let's
+                    // try again in 100 ms.
+                    thread::sleep(time::Duration::from_millis(100));
+                    continue;
+                }
+                Err(TryLockError::Error(err)) => return Err(Box::new(err)),
+            }
             file.write_all(params_json.as_bytes())?;
             rename(params_path_tmp, params_path)?;
         }
+        break;
+    }
 
     let cache_path = cache_dir.join(format!("{}.cbor", name));
     let cache_path_tmp = cache_dir.join(format!("{}.cbor.tmp", name));