fix: multi-process safe parallel filesystem capabilities probing (#1373)

This is achieved by making filenames unique so they won't clash.

gix-fs/Cargo.toml

@@ -21,5 +21,9 @@ gix-features = { version = "^0.38.1", path = "../gix-features", features = ["fs-
 gix-utils = { version = "^0.1.12", path = "../gix-utils" }
 serde = { version = "1.0.114", optional = true, default-features = false, features = ["std", "derive"] }
 
+# For `Capabilities` to assure parallel operation works.
+fastrand = { version = "2.1.0", default-features = false, features = ["std"] }
+
 [dev-dependencies]
 tempfile = "3.5.0"
+crossbeam-channel = "0.5.0"

gix-fs/src/capabilities.rs

@@ -60,7 +60,8 @@ impl Capabilities {
         use std::os::unix::fs::{MetadataExt, OpenOptionsExt};
 
         // test it exactly as we typically create executable files, not using chmod.
-        let test_path = root.join("_test_executable_bit");
+        let rand = fastrand::usize(..);
+        let test_path = root.join(format!("_test_executable_bit{rand}"));
         let res = std::fs::OpenOptions::new()
             .create_new(true)
             .write(true)
@@ -87,8 +88,9 @@ impl Capabilities {
     }
 
     fn probe_precompose_unicode(root: &Path) -> std::io::Result<bool> {
-        let precomposed = "ä";
-        let decomposed = "a\u{308}";
+        let rand = fastrand::usize(..);
+        let precomposed = format!("ä{rand}");
+        let decomposed = format!("a\u{308}{rand}");
 
         let precomposed = root.join(precomposed);
         std::fs::OpenOptions::new()
@@ -101,7 +103,8 @@ impl Capabilities {
     }
 
     fn probe_symlink(root: &Path) -> std::io::Result<bool> {
-        let link_path = root.join("__file_link");
+        let rand = fastrand::usize(..);
+        let link_path = root.join(format!("__file_link{rand}"));
         if crate::symlink::create("dangling".as_ref(), &link_path).is_err() {
             return Ok(false);
         }

gix-fs/tests/capabilities/mod.rs

@@ -20,3 +20,29 @@ fn probe() {
         assert!(caps.executable_bit, "Unix should always honor executable bits");
     }
 }
+
+#[test]
+fn parallel_probe() {
+    let dir = tempfile::tempdir().unwrap();
+    std::fs::File::create(dir.path().join("config")).unwrap();
+    let baseline = gix_fs::Capabilities::probe(dir.path());
+
+    let (tx, rx) = crossbeam_channel::unbounded::<()>();
+    let threads: Vec<_> = (0..10)
+        .map(|_id| {
+            std::thread::spawn({
+                let dir = dir.path().to_owned();
+                let rx = rx.clone();
+                move || {
+                    for _ in rx {}
+                    let actual = gix_fs::Capabilities::probe(&dir);
+                    assert_eq!(actual, baseline);
+                }
+            })
+        })
+        .collect();
+    drop((rx, tx));
+    for thread in threads {
+        thread.join().expect("no panic");
+    }
+}