improve the symlink probing by simplifying it

Less IO operations, and less that can go wrong.

Author: Byron

Diff for: gix-fs/src/capabilities.rs

@@ -101,22 +101,13 @@ impl Capabilities {
     }
 
     fn probe_symlink(root: &Path) -> std::io::Result<bool> {
-        let src_path = root.join("__link_src_file");
-        std::fs::OpenOptions::new()
-            .create_new(true)
-            .write(true)
-            .open(&src_path)?;
         let link_path = root.join("__file_link");
-        if crate::symlink::create(&src_path, &link_path).is_err() {
-            std::fs::remove_file(&src_path)?;
+        if crate::symlink::create("dangling".as_ref(), &link_path).is_err() {
             return Ok(false);
         }
 
         let res = std::fs::symlink_metadata(&link_path).map(|m| m.file_type().is_symlink());
-
-        let cleanup = crate::symlink::remove(&link_path).or_else(|_| std::fs::remove_file(&link_path));
-        std::fs::remove_file(&src_path).and(cleanup)?;
-
+        crate::symlink::remove(&link_path).or_else(|_| std::fs::remove_file(&link_path))?;
         res
     }
 }

Diff for: gix-fs/src/symlink.rs

@@ -2,6 +2,8 @@ use std::{io, io::ErrorKind::AlreadyExists, path::Path};
 
 #[cfg(not(windows))]
 /// Create a new symlink at `link` which points to `original`.
+///
+/// Note that `original` doesn't have to exist.
 pub fn create(original: &Path, link: &Path) -> io::Result<()> {
     std::os::unix::fs::symlink(original, link)
 }

Diff for: gix-fs/tests/capabilities/mod.rs

@@ -2,7 +2,7 @@
 fn probe() {
     let dir = tempfile::tempdir().unwrap();
     std::fs::File::create(dir.path().join("config")).unwrap();
-    gix_fs::Capabilities::probe(dir.path());
+    let caps = gix_fs::Capabilities::probe(dir.path());
 
     let entries: Vec<_> = std::fs::read_dir(dir.path())
         .unwrap()
@@ -15,4 +15,8 @@ fn probe() {
         0,
         "there should be no left-over files after probing, found {entries:?}"
     );
+    if cfg!(unix) {
+        assert!(caps.symlink, "Unix should always be able to create symlinks");
+        assert!(caps.executable_bit, "Unix should always honor executable bits");
+    }
 }