address review: backup group bits, skip no-op fchown

crates/engine/src/atomic_write.rs

@@ -71,6 +71,8 @@ pub fn write_atomic_with(path: &Path, permissions: Option<Permissions>, fill: im
     let (mut file, tmp) = create_temp(&dir, permissions.as_ref())?;
     let write = (|| {
         if let Some(permissions) = permissions {
+            #[cfg(unix)]
+            let permissions = keep_group(&file, path, permissions);
             file.set_permissions(permissions)?;
         }
         fill(&mut file)?;
@@ -90,6 +92,20 @@ pub fn write_atomic_with(path: &Path, permissions: Option<Permissions>, fill: im
     Ok(())
 }
 
+/// `permissions` for the new file when `path` already exists: the group bits mean "this group" only
+/// if the file is still in the old file's group, so the group is restored first. When it can't be
+/// (the user is not a member of it), the group bits are dropped rather than handed to another group.
+#[cfg(unix)]
+fn keep_group(file: &File, path: &Path, permissions: Permissions) -> Permissions {
+    use std::os::unix::fs::{MetadataExt, PermissionsExt, fchown};
+    let Ok(old) = std::fs::symlink_metadata(path) else { return permissions };
+    // A file already in the old group needs no change (and a restrictive syscall policy may refuse a no-op fchown).
+    if !old.is_file() || file.metadata().is_ok_and(|m| m.gid() == old.gid()) || fchown(file, None, Some(old.gid())).is_ok() {
+        return permissions;
+    }
+    Permissions::from_mode(permissions.mode() & !0o070)
+}
+
 /// Flush `dir` so a rename just made in it survives a crash or power loss (POSIX records a
 /// rename in the directory, not in the file). Best effort: some file systems (network shares)
 /// refuse to sync a directory, and the file itself is already on disk.
@@ -231,6 +247,33 @@ mod tests {
         std::fs::remove_dir_all(&dir).ok();
     }
 
+    /// The replacement keeps the old file's group (when the user is in another group, as `id -G`
+    /// says) together with its mode, so the group bits never name a different group.
+    #[cfg(unix)]
+    #[test]
+    fn keeps_the_group_and_mode_of_the_file_it_replaces() {
+        use std::os::unix::fs::{MetadataExt, PermissionsExt, chown};
+        let dir = std::env::temp_dir().join(format!("cadcraft-aw-grp-{}-{}", std::process::id(), SEQ.fetch_add(1, Ordering::Relaxed)));
+        std::fs::create_dir_all(&dir).unwrap();
+        let dest = dir.join("out.bin");
+        std::fs::write(&dest, b"old").unwrap();
+        std::fs::set_permissions(&dest, Permissions::from_mode(0o640)).unwrap();
+        let own = std::fs::metadata(&dest).unwrap().gid();
+        let other = std::process::Command::new("id")
+            .arg("-G")
+            .output()
+            .ok()
+            .and_then(|o| String::from_utf8_lossy(&o.stdout).split_whitespace().filter_map(|g| g.parse::<u32>().ok()).find(|g| *g != own));
+        if let Some(gid) = other {
+            chown(&dest, None, Some(gid)).unwrap();
+        }
+        let want = std::fs::metadata(&dest).unwrap();
+        write_atomic(&dest, b"new").unwrap();
+        let got = std::fs::metadata(&dest).unwrap();
+        assert_eq!((got.gid(), got.mode() & 0o777), (want.gid(), 0o640));
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
     #[cfg(unix)]
     #[test]
     fn does_not_follow_a_planted_temp_symlink() {

crates/engine/src/cmd/file.rs

@@ -601,6 +601,21 @@ pub fn save_keeps_current(_path: &std::path::Path) -> bool {
     false
 }
 
+/// The mode of a new backup: no more open than the drawing (`src_mode`, group `src_gid`) nor than the
+/// `.bak` it replaces (`old`: mode and group). The backup keeps the old `.bak`'s group, so when that
+/// differs from the drawing's group the group bits are dropped: that group could not read the drawing.
+#[cfg(unix)]
+fn backup_mode(src_mode: u32, src_gid: u32, old: Option<(u32, u32)>) -> u32 {
+    let mut mode = src_mode & 0o777;
+    if let Some((old_mode, old_gid)) = old {
+        mode &= old_mode;
+        if old_gid != src_gid {
+            mode &= !0o070;
+        }
+    }
+    mode
+}
+
 /// Before `path` is replaced: copy it to its `.bak`, streamed and written atomically too
 /// ([`crate::atomic_write::write_atomic_with`]). Nothing
 /// to keep when there is no file yet. A file that doesn't look like a drawing
@@ -625,14 +640,10 @@ fn backup(path: &std::path::Path) -> std::io::Result<()> {
     // written through.
     #[cfg(unix)]
     let permissions = {
-        use std::os::unix::fs::PermissionsExt;
-        let mut mode = src.metadata()?.permissions().mode() & 0o777;
-        if let Ok(m) = std::fs::symlink_metadata(&bak)
-            && m.is_file()
-        {
-            mode &= m.permissions().mode();
-        }
-        Some(std::fs::Permissions::from_mode(mode))
+        use std::os::unix::fs::{MetadataExt, PermissionsExt};
+        let sm = src.metadata()?;
+        let old = std::fs::symlink_metadata(&bak).ok().filter(|m| m.is_file());
+        Some(std::fs::Permissions::from_mode(backup_mode(sm.mode(), sm.gid(), old.map(|m| (m.mode(), m.gid())))))
     };
     #[cfg(not(unix))]
     let permissions = None;
@@ -993,6 +1004,19 @@ mod tests {
     use crate::Session;
     use crate::temp_dir::TempDir;
 
+    /// A backup is never more open than the drawing or the old `.bak`, and loses the group bits when
+    /// it would keep the old `.bak`'s group while the drawing is in another.
+    #[cfg(unix)]
+    #[test]
+    fn backup_mode_drops_group_bits_for_a_different_group() {
+        use super::backup_mode;
+        assert_eq!(backup_mode(0o640, 10, None), 0o640);
+        assert_eq!(backup_mode(0o640, 10, Some((0o664, 10))), 0o640);
+        assert_eq!(backup_mode(0o640, 10, Some((0o664, 20))), 0o600);
+        assert_eq!(backup_mode(0o644, 10, Some((0o644, 20))), 0o604);
+        assert_eq!(backup_mode(0o600, 10, Some((0o664, 10))), 0o600);
+    }
+
     fn active_title(s: &Session) -> String {
         s.state().unwrap().title.clone()
     }