backup: use shared flock for base snapshot

To allow other reading operations on the base snapshot as well. No
semantic changes with this patch alone, as all other locks on snapshots
are exclusive.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/tools/fs.rs

@@ -265,11 +265,31 @@ impl Default for FSXAttr {
     }
 }
 
+/// Attempt to acquire a shared flock on the given path, 'what' and
+/// 'would_block_message' are used for error formatting.
+pub fn lock_dir_noblock_shared(
+    path: &std::path::Path,
+    what: &str,
+    would_block_msg: &str,
+) -> Result<DirLockGuard, Error> {
+    do_lock_dir_noblock(path, what, would_block_msg, false)
+}
 
+/// Attempt to acquire an exclusive flock on the given path, 'what' and
+/// 'would_block_message' are used for error formatting.
 pub fn lock_dir_noblock(
     path: &std::path::Path,
     what: &str,
     would_block_msg: &str,
+) -> Result<DirLockGuard, Error> {
+    do_lock_dir_noblock(path, what, would_block_msg, true)
+}
+
+fn do_lock_dir_noblock(
+    path: &std::path::Path,
+    what: &str,
+    would_block_msg: &str,
+    exclusive: bool,
 ) -> Result<DirLockGuard, Error> {
     let mut handle = Dir::open(path, OFlag::O_RDONLY, Mode::empty())
         .map_err(|err| {
@@ -278,7 +298,7 @@ pub fn lock_dir_noblock(
 
     // acquire in non-blocking mode, no point in waiting here since other
     // backups could still take a very long time
-    proxmox::tools::fs::lock_file(&mut handle, true, Some(std::time::Duration::from_nanos(0)))
+    proxmox::tools::fs::lock_file(&mut handle, exclusive, Some(std::time::Duration::from_nanos(0)))
         .map_err(|err| {
             format_err!(
                 "unable to acquire lock on {} directory {:?} - {}", what, path,