address some edge cases in bind mount masking

* check files/links before mounting and ignore if the mountpoint already exists
* if the mountpoint of a directory exists and isn't a dir, we can't
  recurse and mount the subdir siblings

Author: r-burns

src/main.rs

@@ -47,41 +47,51 @@ impl<'a> RunChroot<'a> {
     fn bind_mount_directory(&self, entry: &fs::DirEntry) {
         let mountpoint = self.rootdir.join(entry.file_name());
 
-        // if there is already a dir here, recurse into it,
-        // and mount any subdirs which don't already exist
-        if mountpoint.is_dir() {
-            let dir = fs::read_dir(entry.path()).unwrap_or_else(|err| {
-                panic!("failed to list dir {}: {}", entry.path().display(), err)
-            });
-
-            let child = RunChroot::new(&mountpoint);
-            for entry in dir {
-                child.bind_mount_direntry(entry);
-            }
-        } else {
+        // if the destination doesn't exist we can proceed as normal
+        if !mountpoint.exists() {
             if let Err(e) = fs::create_dir(&mountpoint) {
                 if e.kind() != io::ErrorKind::AlreadyExists {
                     panic!("failed to create {}: {}", &mountpoint.display(), e);
                 }
             }
 
             bind_mount(&entry.path(), &mountpoint)
+        } else {
+            // otherwise, if the dest is also a dir, we can recurse into it
+            // and mount subdirectory siblings of existing paths
+            if mountpoint.is_dir() {
+                let dir = fs::read_dir(entry.path()).unwrap_or_else(|err| {
+                    panic!("failed to list dir {}: {}", entry.path().display(), err)
+                });
+
+                let child = RunChroot::new(&mountpoint);
+                for entry in dir {
+                    let entry = entry.expect("error while listing subdir");
+                    child.bind_mount_direntry(&entry);
+                }
+            }
         }
     }
 
     fn bind_mount_file(&self, entry: &fs::DirEntry) {
         let mountpoint = self.rootdir.join(entry.file_name());
+        if mountpoint.exists() {
+            return;
+        }
         fs::File::create(&mountpoint)
             .unwrap_or_else(|err| panic!("failed to create {}: {}", &mountpoint.display(), err));
 
         bind_mount(&entry.path(), &mountpoint)
     }
 
     fn mirror_symlink(&self, entry: &fs::DirEntry) {
+        let link_path = self.rootdir.join(entry.file_name());
+        if link_path.exists() {
+            return;
+        }
         let path = entry.path();
         let target = fs::read_link(&path)
             .unwrap_or_else(|err| panic!("failed to resolve symlink {}: {}", &path.display(), err));
-        let link_path = self.rootdir.join(entry.file_name());
         symlink(&target, &link_path).unwrap_or_else(|_| {
             panic!(
                 "failed to create symlink {} -> {}",
@@ -91,16 +101,12 @@ impl<'a> RunChroot<'a> {
         });
     }
 
-    fn bind_mount_direntry(&self, entry: io::Result<fs::DirEntry>) {
-        let entry = entry.expect("error while listing from /nix directory");
-        // do not bind mount an existing nix installation
-        if entry.file_name() == PathBuf::from("nix") {
-            return;
-        }
+    fn bind_mount_direntry(&self, entry: &fs::DirEntry) {
         let path = entry.path();
         let stat = entry
             .metadata()
             .unwrap_or_else(|err| panic!("cannot get stat of {}: {}", path.display(), err));
+
         if stat.is_dir() {
             self.bind_mount_directory(&entry);
         } else if stat.is_file() {
@@ -132,7 +138,12 @@ impl<'a> RunChroot<'a> {
         let nix_root = PathBuf::from("/");
         let dir = fs::read_dir(&nix_root).expect("failed to list /nix directory");
         for entry in dir {
-            self.bind_mount_direntry(entry);
+            let entry = entry.expect("error while listing from /nix directory");
+            // do not bind mount an existing nix installation
+            if entry.file_name() == PathBuf::from("nix") {
+                continue;
+            }
+            self.bind_mount_direntry(&entry);
         }
 
         // mount the store