pxar: implement allow_existing_dirs for pxar decoder

By default, restoring an archive will fail if files with the same filename
already exist in the target directory.

By setting the allow_existing_dirs flag, the restore will not fail if an
existing directory is encountered.
The metadata (permissions, acls, ...) of the existing directory will be set
to the ones from the archive.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

src/pxar/sequential_decoder.rs

@@ -34,6 +34,7 @@ use crate::tools::xattr;
 pub struct SequentialDecoder<'a, R: Read, F: Fn(&Path) -> Result<(), Error>> {
     reader: &'a mut R,
     feature_flags: u64,
+    allow_existing_dirs: bool,
     skip_buffer: Vec<u8>,
     callback: F,
 }
@@ -48,11 +49,16 @@ impl <'a, R: Read, F: Fn(&Path) -> Result<(), Error>> SequentialDecoder<'a, R, F
         Self {
             reader,
             feature_flags,
+            allow_existing_dirs: false,
             skip_buffer,
             callback,
         }
     }
 
+    pub fn set_allow_existing_dirs(&mut self, allow: bool) {
+        self.allow_existing_dirs = allow;
+    }
+
     pub (crate) fn get_reader_mut(&mut self) -> & mut R {
         self.reader
     }
@@ -632,7 +638,7 @@ impl <'a, R: Read, F: Fn(&Path) -> Result<(), Error>> SequentialDecoder<'a, R, F
             let dir = if filename.is_empty() {
                 nix::dir::Dir::openat(pfd, ".", OFlag::O_DIRECTORY,  Mode::empty())?
             } else {
-                dir_mkdirat(pfd, filename, true)
+                dir_mkdirat(pfd, filename, !self.allow_existing_dirs)
                     .map_err(|err| format_err!("unable to open directory {:?} - {}", full_path, err))?
             };
             (Some(dir.as_raw_fd()), Some(dir))