mononoke: blobstore: optimize is_present()

Summary:
This patch adds optimized implementations of is_present() for FileBlob (which
we use in tests), and ManifoldBlob (which AFAIK, we don't use: I believe we use
ManifoldThriftBlob instead).

The underlying goal is to make sure we don't perform accidental reads of LFS
blobs when all we meant was to check a blob existed when committing a
changeset.

For ManifoldBlob, this is implemented by querying for an empty range, which
appears to be explicitly supported as per the Rust client's source code (it
sets `?withPayload=0` as a query parameter).

Reviewed By: StanislavGlebik

Differential Revision: D16262273

fbshipit-source-id: c055f63d114d40be2c448b8c8e34eb82d0e641b5

blobstore/fileblob/src/lib.rs

@@ -84,4 +84,19 @@ impl Blobstore for Fileblob {
         })
         .boxify()
     }
+
+    fn is_present(&self, _ctx: CoreContext, key: String) -> BoxFuture<bool, Error> {
+        let p = self.path(&key);
+
+        poll_fn(move || {
+            let ret = match File::open(&p) {
+                Err(ref e) if e.kind() == io::ErrorKind::NotFound => false,
+                Err(e) => return Err(e),
+                Ok(_) => true,
+            };
+            Ok(Async::Ready(ret))
+        })
+        .from_err()
+        .boxify()
+    }
 }