--- a/mercurial/dirstateutils/v2.py	Thu Oct 14 08:58:07 2021 -0700
+++ b/mercurial/dirstateutils/v2.py	Thu Oct 14 16:06:31 2021 +0200
@@ -18,7 +18,7 @@
 # Must match the constant of the same name in
 # `rust/hg-core/src/dirstate_tree/on_disk.rs`
 TREE_METADATA_SIZE = 44
-NODE_SIZE = 43
+NODE_SIZE = 44
 
 
 # Must match the `TreeMetadata` Rust struct in
@@ -50,7 +50,7 @@
 # * 4 bytes: expected size
 # * 4 bytes: mtime seconds
 # * 4 bytes: mtime nanoseconds
-NODE = struct.Struct('>LHHLHLLLLBlll')
+NODE = struct.Struct('>LHHLHLLLLHlll')
 
 
 assert TREE_METADATA_SIZE == TREE_METADATA.size

--- a/mercurial/helptext/internals/dirstate-v2.txt	Thu Oct 14 08:58:07 2021 -0700
+++ b/mercurial/helptext/internals/dirstate-v2.txt	Thu Oct 14 16:06:31 2021 +0200
@@ -372,7 +372,7 @@
   This counter is used to implement `has_tracked_dir`.
 
 * Offset 30:
-  A single `flags` byte that packs some boolean values as bits.
+  A `flags` fields  that packs some boolean values as bits of a 16-bit integer.
   Starting from least-significant, bit masks are::
 
     WDIR_TRACKED = 1 << 0
@@ -384,22 +384,29 @@
     MODE_IS_SYMLINK = 1 << 6
 
   The meaning of each bit is described below.
-  Other bits are unset.
 
-* Offset 31:
+  Other bits are unset.
+  They may be assigned meaning if the future,
+  with the limitation that Mercurial versions that pre-date such meaning
+  will always reset those bits to unset when writing nodes.
+  (A new node is written for any mutation in its subtree,
+  leaving the bytes of the old node unreachable
+  until the data file is rewritten entirely.)
+
+* Offset 32:
   A `size` field described below, as a 32-bit integer.
   Unlike in dirstate-v1, negative values are not used.
 
-* Offset 35:
+* Offset 36:
   The seconds component of an `mtime` field described below,
   as a 32-bit integer.
   Unlike in dirstate-v1, negative values are not used.
 
-* Offset 39:
+* Offset 40:
   The nanoseconds component of an `mtime` field described below,
   as a 32-bit integer.
 
-* (Offset 43: end of this node)
+* (Offset 44: end of this node)
 
 The meaning of the boolean values packed in `flags` is:
 

--- a/rust/hg-core/src/dirstate_tree/on_disk.rs	Thu Oct 14 08:58:07 2021 -0700
+++ b/rust/hg-core/src/dirstate_tree/on_disk.rs	Thu Oct 14 16:06:31 2021 +0200
@@ -33,7 +33,7 @@
 
 /// Must match constants of the same names in `mercurial/dirstateutils/v2.py`
 const TREE_METADATA_SIZE: usize = 44;
-const NODE_SIZE: usize = 43;
+const NODE_SIZE: usize = 44;
 
 /// Make sure that size-affecting changes are made knowingly
 #[allow(unused)]
@@ -94,15 +94,14 @@
     children: ChildNodes,
     pub(super) descendants_with_entry_count: Size,
     pub(super) tracked_descendants_count: Size,
-    flags: Flags,
+    flags: U16Be,
     size: U32Be,
     mtime: PackedTruncatedTimestamp,
 }
 
 bitflags! {
-    #[derive(BytesCast)]
     #[repr(C)]
-    struct Flags: u8 {
+    struct Flags: u16 {
         const WDIR_TRACKED = 1 << 0;
         const P1_TRACKED = 1 << 1;
         const P2_INFO = 1 << 2;
@@ -296,8 +295,12 @@
         })
     }
 
+    fn flags(&self) -> Flags {
+        Flags::from_bits_truncate(self.flags.get())
+    }
+
     fn has_entry(&self) -> bool {
-        self.flags.intersects(
+        self.flags().intersects(
             Flags::WDIR_TRACKED | Flags::P1_TRACKED | Flags::P2_INFO,
         )
     }
@@ -318,7 +321,7 @@
         &self,
     ) -> Result<Option<TruncatedTimestamp>, DirstateV2ParseError> {
         Ok(
-            if self.flags.contains(Flags::HAS_MTIME) && !self.has_entry() {
+            if self.flags().contains(Flags::HAS_MTIME) && !self.has_entry() {
                 Some(self.mtime.try_into()?)
             } else {
                 None
@@ -327,12 +330,12 @@
     }
 
     fn synthesize_unix_mode(&self) -> u32 {
-        let file_type = if self.flags.contains(Flags::MODE_IS_SYMLINK) {
+        let file_type = if self.flags().contains(Flags::MODE_IS_SYMLINK) {
             libc::S_IFLNK
         } else {
             libc::S_IFREG
         };
-        let permisions = if self.flags.contains(Flags::MODE_EXEC_PERM) {
+        let permisions = if self.flags().contains(Flags::MODE_EXEC_PERM) {
             0o755
         } else {
             0o644
@@ -342,15 +345,15 @@
 
     fn assume_entry(&self) -> DirstateEntry {
         // TODO: convert through raw bits instead?
-        let wdir_tracked = self.flags.contains(Flags::WDIR_TRACKED);
-        let p1_tracked = self.flags.contains(Flags::P1_TRACKED);
-        let p2_info = self.flags.contains(Flags::P2_INFO);
-        let mode_size = if self.flags.contains(Flags::HAS_MODE_AND_SIZE) {
+        let wdir_tracked = self.flags().contains(Flags::WDIR_TRACKED);
+        let p1_tracked = self.flags().contains(Flags::P1_TRACKED);
+        let p2_info = self.flags().contains(Flags::P2_INFO);
+        let mode_size = if self.flags().contains(Flags::HAS_MODE_AND_SIZE) {
             Some((self.synthesize_unix_mode(), self.size.into()))
         } else {
             None
         };
-        let mtime = if self.flags.contains(Flags::HAS_MTIME) {
+        let mtime = if self.flags().contains(Flags::HAS_MTIME) {
             Some(self.mtime.truncated_seconds.into())
         } else {
             None
@@ -600,7 +603,7 @@
                         tracked_descendants_count: node
                             .tracked_descendants_count
                             .into(),
-                        flags,
+                        flags: flags.bits().into(),
                         size,
                         mtime,
                     }