Add shared garbage collection lock for dotslash execution

Summary:
We want to add garbage collection for dotslash cache (discussion [post](https://fb.workplace.com/groups/dotslash.users/permalink/2064356387419058/)). For safe removal we have to ensure artifact not being used before removing them

Introducing a shared gc file lock that multiple executors can acquire

Later GC process has to acquire the exclusive lock on the same file to be able to clean up the artifact

Without any GC process running this should have no impact on execution of dotslash so this diff itself should not introduce blocking/deadlock

GC testing will be in next diff

Reviewed By: dtolnay

Differential Revision: D73410163

fbshipit-source-id: 9c130a090c8ac1836892a07be42281687a4bad17

Author: khoi0107

src/download.rs

@@ -253,7 +253,7 @@ pub fn acquire_download_lock_for_artifact(
             match FileLock::acquire(&artifact_location.lock_path) {
                 Ok(file_lock) => return Ok(file_lock),
                 Err(err @ FileLockError::LockExclusive(..)) => return Err(err.into()),
-                Err(FileLockError::Create(..)) => {}
+                Err(FileLockError::Create(..) | FileLockError::LockShared(..)) => {}
             }
         }
     }

src/util/file_lock.rs

@@ -23,6 +23,9 @@ pub enum FileLockError {
 
     #[error("failed to get exclusive lock `{0}`")]
     LockExclusive(PathBuf, #[source] io::Error),
+
+    #[error("failed to get shared lock `{0}`")]
+    LockShared(PathBuf, #[source] io::Error),
 }
 
 #[derive(Debug, Default)]
@@ -54,6 +57,29 @@ impl FileLock {
         }
         inner(path.as_ref())
     }
+
+    pub fn acquire_shared_lock<P>(path: P) -> Result<FileLock, FileLockError>
+    where
+        P: AsRef<Path>,
+    {
+        fn inner(path: &Path) -> Result<FileLock, FileLockError> {
+            let lock_file = File::options()
+                .read(true)
+                .write(true)
+                .create(true)
+                .truncate(false)
+                .open(path)
+                .map_err(|e| FileLockError::Create(path.to_path_buf(), e))?;
+
+            fs2::FileExt::lock_shared(&lock_file)
+                .map_err(|e| FileLockError::LockShared(path.to_path_buf(), e))?;
+
+            Ok(FileLock {
+                file: Some(lock_file),
+            })
+        }
+        inner(path.as_ref())
+    }
 }
 
 impl Drop for FileLock {