Merge pull request #234 from bobrik/ivan/fastpath

Enable oppportunistic fd counting fast path

src/process/mod.rs

@@ -1084,6 +1084,15 @@ impl Process {
     ///
     /// Calling this function is more efficient than calling `fd().unwrap().count()`
     pub fn fd_count(&self) -> ProcResult<usize> {
+        // Use fast path if available (Linux v6.2): /~https://github.com/torvalds/linux/commit/f1f1f2569901
+        let stat = wrap_io_error!(
+            self.root.join("fd"),
+            rustix::fs::statat(&self.fd, "fd", AtFlags::SYMLINK_NOFOLLOW)
+        )?;
+        if stat.st_size > 0 {
+            return Ok(stat.st_size as usize);
+        }
+
         let fds = wrap_io_error!(
             self.root.join("fd"),
             rustix::fs::openat(

src/process/tests.rs

@@ -336,6 +336,33 @@ fn test_proc_fds() {
     }
 }
 
+#[test]
+fn test_proc_fd_count_runsinglethread() {
+    let myself = Process::myself().unwrap();
+
+    let before = myself.fd_count().unwrap();
+
+    let one = File::open("/proc/self").unwrap();
+    let two = File::open("/proc/self/status").unwrap();
+
+    let after = myself.fd_count().unwrap();
+
+    assert_eq!(
+        before + 2,
+        after,
+        "opened two files and expected {} open fds, saw {}",
+        before + 2,
+        after
+    );
+
+    drop(one);
+    drop(two);
+
+    let after_closing = myself.fd_count().unwrap();
+
+    assert_eq!(before, after_closing);
+}
+
 #[test]
 fn test_proc_fd() {
     let myself = Process::myself().unwrap();