Log infinite symlink offender (#20985)

Gave this a stab since I have infinite free time now, ~~but I have no
idea how to see the warning output in tests~~.

Figured out the (seemingly) only way to see log output in tests is by
using the `env_logger` crate and running with the `RUST_LOG=warn` env
var set. The following command spits out the subsequent log messages in
the tests: `RUST_LOG=warn ./cargo test -p fs -- --nocapture`

```
[2024-06-04T17:37:12Z WARN  fs::directory] Exceeded the maximum link depth while traversing link `self` to path ".". Stopping traversal.
[2024-06-04T17:37:12Z WARN  fs::directory] Exceeded the maximum link depth while traversing link `self` to path ".". Stopping traversal.
```

---------

Co-authored-by: Huon Wilson <huon@exoflare.io>

src/rust/engine/fs/Cargo.toml

@@ -31,6 +31,7 @@ workunit_store = { path = "../workunit_store" }
 [dev-dependencies]
 tempfile = { workspace = true }
 tokio = { workspace = true, features = ["rt", "macros"] }
+env_logger = { workspace = true }
 
 [lints]
 workspace = true

src/rust/engine/fs/src/directory.rs

@@ -650,7 +650,15 @@ impl DigestTrie {
                     link_depth += 1;
                     if s.target == Component::CurDir.as_os_str() {
                         if link_depth >= MAX_LINK_DEPTH {
-                            warn!("Exceeded the maximum link depth while traversing links. Stopping traversal.");
+                            let mut logical_path = PathBuf::new();
+                            for component in path_so_far.components() {
+                                let cs = component.as_os_str();
+                                logical_path.push(cs);
+                                if cs == entry.name().as_str() {
+                                    break;
+                                }
+                            }
+                            warn!("Exceeded the maximum link depth while traversing link {:#?} to path {:#?}. Stopping traversal.", logical_path, s.target);
                             return;
                         }
                         self.walk_helper(root, path.clone(), symlink_behavior, link_depth, f);
@@ -958,7 +966,7 @@ impl DigestTrie {
 
             if let Some(Entry::Symlink(s)) = maybe_matching_entry {
                 if link_depth >= MAX_LINK_DEPTH {
-                    warn!("Exceeded the maximum link depth while traversing links. Stopping traversal.");
+                    warn!("Exceeded the maximum link depth while traversing link {:#?} to path {:#?}. Stopping traversal.", logical_path, s.target);
                     return Ok(None);
                 }