fix(fspy): prevent recursive access handling (#540)

## Motivation

File-access handling can call another interposed function while resolving or reporting a path. Without a same-thread re-entry guard, nested bookkeeping can recurse until the process stack overflows. This change suppresses only nested handling on the current thread, while accesses from other threads continue to be recorded.

Author: wan9chi

crates/fspy_preload_unix/src/client/mod.rs

@@ -2,8 +2,8 @@ pub mod convert;
 pub mod raw_exec;
 
 use std::{
-    ffi::OsStr, fmt::Debug, num::NonZeroUsize, os::unix::ffi::OsStrExt as _, path::Path,
-    sync::OnceLock,
+    cell::Cell, ffi::OsStr, fmt::Debug, num::NonZeroUsize, os::unix::ffi::OsStrExt as _,
+    path::Path, sync::OnceLock,
 };
 
 use convert::{ToAbsolutePath, ToAccessMode};
@@ -125,15 +125,37 @@ impl Client {
 
 static CLIENT: OnceLock<Client> = OnceLock::new();
 
+// Resolving and reporting a file access can call another interposed function.
+// Suppress same-thread re-entry to prevent recursive access handling while
+// still recording accesses from other threads.
+thread_local! {
+    static HANDLING_OPEN: Cell<bool> = const { Cell::new(false) };
+}
+
+struct ResetHandling<'a>(&'a Cell<bool>);
+impl Drop for ResetHandling<'_> {
+    fn drop(&mut self) {
+        self.0.set(false);
+    }
+}
+
 pub fn global_client() -> Option<&'static Client> {
     CLIENT.get()
 }
 
 pub unsafe fn handle_open(path: impl ToAbsolutePath, mode: impl ToAccessMode) {
-    if let Some(client) = global_client() {
-        // SAFETY: path and mode contain valid pointers/values forwarded from the interposed function's caller
-        unsafe { client.try_handle_open(path, mode) }.unwrap();
-    }
+    HANDLING_OPEN.with(|handling| {
+        if handling.replace(true) {
+            return;
+        }
+
+        let _reset = ResetHandling(handling);
+
+        if let Some(client) = global_client() {
+            // SAFETY: path and mode contain valid pointers/values forwarded from the interposed function's caller
+            unsafe { client.try_handle_open(path, mode) }.unwrap();
+        }
+    });
 }
 
 #[cfg(not(test))]