Do not GC the current active incremental session directory

compiler/rustc_incremental/src/persist/fs.rs

@@ -721,11 +721,37 @@ pub(crate) fn garbage_collect_session_directories(sess: &Session) -> io::Result<
         }
     }
 
+    let current_session_directory_name =
+        session_directory.file_name().expect("session directory is not `..`");
+
     // Now garbage collect the valid session directories.
     let deletion_candidates =
         lock_file_to_session_dir.items().filter_map(|(lock_file_name, directory_name)| {
             debug!("garbage_collect_session_directories() - inspecting: {}", directory_name);
 
+            if directory_name.as_str() == current_session_directory_name {
+                // Skipping our own directory is, unfortunately, important for correctness.
+                //
+                // To summarize #147821: we will try to lock directories before deciding they can be
+                // garbage collected, but the ability of `flock::Lock` to detect a lock held *by the
+                // same process* varies across file locking APIs. Then, if our own session directory
+                // has become old enough to be eligible for GC, we are beholden to platform-specific
+                // details about detecting the our own lock on the session directory.
+                //
+                // POSIX `fcntl(F_SETLK)`-style file locks are maintained across a process. On
+                // systems where this is the mechanism for `flock::Lock`, there is no way to
+                // discover if an `flock::Lock` has been created in the same process on the same
+                // file. Attempting to set a lock on the lockfile again will succeed, even if the
+                // lock was set by another thread, on another file descriptor. Then we would
+                // garbage collect our own live directory, unable to tell it was locked perhaps by
+                // this same thread.
+                //
+                // It's not clear that `flock::Lock` can be fixed for this in general, and our own
+                // incremental session directory is the only one which this process may own, so skip
+                // it here and avoid the problem. We know it's not garbage anyway: we're using it.
+                return None;
+            }
+
             let Ok(timestamp) = extract_timestamp_from_session_dir(directory_name) else {
                 debug!(
                     "found session-dir with malformed timestamp: {}",