Extract fstat and fchmod logic to a helper

This extracts the logic that calls `fstat`, transforms the mode
with `let_readers_execute`, and writes it back through the same
file descriptor with `fchmod`, introducing a new helper function,
`set_executable`, for it.

The reason to do this is that it makes the issue of what kind of
`Error` appears in the returned `Result` clearer. This may be
slightly clearer to humans. It's significnatly clearer to the Rust
compiler, allowing both `map_err(std::io::Error::from)` calls to
be eliminated without having to write anything confusing or
inelegant.

(This is separate from the reason the `let_readers_execute`
function, which does not access the filesystem, is its own
function. That is so it's easy to test the mode transformation
logic.)

Author: EliahKagan

gix-worktree-state/src/checkout/entry.rs

@@ -285,9 +285,7 @@ pub(crate) fn finalize_entry(
     // For possibly existing, overwritten files, we must change the file mode explicitly.
     #[cfg(unix)]
     if let Some(path) = set_executable_after_creation {
-        let old_raw_mode = rustix::fs::fstat(&file).map_err(std::io::Error::from)?.st_mode;
-        let new_mode = let_readers_execute(old_raw_mode);
-        rustix::fs::fchmod(&file, new_mode).map_err(std::io::Error::from)?;
+        set_executable(&file)?;
     }
     // NOTE: we don't call `file.sync_all()` here knowing that some filesystems don't handle this well.
     //       revisit this once there is a bug to fix.
@@ -296,6 +294,17 @@ pub(crate) fn finalize_entry(
     Ok(())
 }
 
+/// Use `fstat` and `fchmod` on a file descriptor to make a regular file executable.
+///
+/// See `let_readers_execute` for the exact details of how the mode is transformed.
+#[cfg(unix)]
+fn set_executable(file: &std::fs::File) -> Result<(), std::io::Error> {
+    let old_raw_mode = rustix::fs::fstat(&file)?.st_mode;
+    let new_mode = let_readers_execute(old_raw_mode);
+    rustix::fs::fchmod(&file, new_mode)?;
+    Ok(())
+}
+
 /// Given the st_mode of a regular file, compute the mode with executable bits safely added.
 ///
 /// Currently this adds executable bits for whoever has read bits already. It doesn't use the umask.