stack: return cached file if possible in get_or_update

When we aren't replacing a file, prefer to return the cached file
after a call to `put`: if we lost the race and we can find a new
cached file, it's usually better to return what's now persisted on
disk, as though we had found it before attempting to fill the miss.

src/stack.rs

@@ -373,11 +373,15 @@ impl Cache {
 
         // Grab a read-only return value before publishing the file.
         let path = tmp.path();
-        let ret = File::open(path)?;
+        let mut ret = File::open(path)?;
         if replace {
             cache.set(key, path)?;
         } else {
             cache.put(key, path)?;
+            // Return the now-cached file, if we can get it.
+            if let Ok(Some(file)) = cache.get(key) {
+                ret = file;
+            }
         }
 
         Ok(ret)