Print a warning if unlinking an ephemeral file fails.

Unlink failure isn't serious on its own, we were about to remove the
file anyway, but it shouldn't happen and could be a symptom of
something more serious.

We just saw "No such file or directory" errors happening from
ephemeral file writeback in staging, and I suspect if we had this
warning in place, we would have seen these warnings too, if the
problem was that the ephemeral file was removed before dropping the
EphemeralFile struct. Next time it happens, we'll have more
information.

pageserver/src/layered_repository/ephemeral_file.rs

@@ -16,6 +16,7 @@ use std::io::{Error, ErrorKind};
 use std::ops::DerefMut;
 use std::path::PathBuf;
 use std::sync::{Arc, RwLock};
+use tracing::*;
 use zenith_utils::zid::ZTenantId;
 use zenith_utils::zid::ZTimelineId;
 
@@ -244,9 +245,15 @@ impl Drop for EphemeralFile {
         // remove entry from the hash map
         EPHEMERAL_FILES.write().unwrap().files.remove(&self.file_id);
 
-        // unlink file
-        // FIXME: print error
-        let _ = std::fs::remove_file(&self.file.path);
+        // unlink the file
+        let res = std::fs::remove_file(&self.file.path);
+        if let Err(e) = res {
+            warn!(
+                "could not remove ephemeral file '{}': {}",
+                self.file.path.display(),
+                e
+            );
+        }
     }
 }