--- a/mercurial/dirstatemap.py	Thu Jul 08 12:18:21 2021 +0200
+++ b/mercurial/dirstatemap.py	Mon Jul 12 22:46:52 2021 +0200
@@ -637,7 +637,9 @@
                     data = self._opener.read(self.docket.data_filename())
                 else:
                     data = b''
-                self._rustmap = rustmod.DirstateMap.new_v2(data)
+                self._rustmap = rustmod.DirstateMap.new_v2(
+                    data, self.docket.data_size
+                )
                 parents = self.docket.parents
             else:
                 self._rustmap, parents = rustmod.DirstateMap.new_v1(

--- a/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Thu Jul 08 12:18:21 2021 +0200
+++ b/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Mon Jul 12 22:46:52 2021 +0200
@@ -410,8 +410,15 @@
     }
 
     #[timed]
-    pub fn new_v2(on_disk: &'on_disk [u8]) -> Result<Self, DirstateError> {
-        Ok(on_disk::read(on_disk)?)
+    pub fn new_v2(
+        on_disk: &'on_disk [u8],
+        data_size: usize,
+    ) -> Result<Self, DirstateError> {
+        if let Some(data) = on_disk.get(..data_size) {
+            Ok(on_disk::read(data)?)
+        } else {
+            Err(DirstateV2ParseError.into())
+        }
     }
 
     #[timed]

--- a/rust/hg-core/src/dirstate_tree/on_disk.rs	Thu Jul 08 12:18:21 2021 +0200
+++ b/rust/hg-core/src/dirstate_tree/on_disk.rs	Mon Jul 12 22:46:52 2021 +0200
@@ -21,7 +21,7 @@
 use bytes_cast::BytesCast;
 use format_bytes::format_bytes;
 use std::borrow::Cow;
-use std::convert::TryFrom;
+use std::convert::{TryFrom, TryInto};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 /// Added at the start of `.hg/dirstate` when the "v2" format is used.
@@ -224,6 +224,11 @@
         DirstateParents { p1, p2 }
     }
 
+    pub fn data_size(&self) -> usize {
+        // This `unwrap` could only panic on a 16-bit CPU
+        self.header.data_size.get().try_into().unwrap()
+    }
+
     pub fn data_filename(&self) -> String {
         String::from_utf8(format_bytes!(b"dirstate.{}.d", self.uuid)).unwrap()
     }

--- a/rust/hg-cpython/src/dirstate/dirstate_map.rs	Thu Jul 08 12:18:21 2021 +0200
+++ b/rust/hg-cpython/src/dirstate/dirstate_map.rs	Mon Jul 12 22:46:52 2021 +0200
@@ -83,11 +83,12 @@
     @staticmethod
     def new_v2(
         on_disk: PyBytes,
+        data_size: usize,
     ) -> PyResult<PyObject> {
         let dirstate_error = |e: DirstateError| {
             PyErr::new::<exc::OSError, _>(py, format!("Dirstate error: {:?}", e))
         };
-        let inner = OwningDirstateMap::new_v2(py, on_disk)
+        let inner = OwningDirstateMap::new_v2(py, on_disk, data_size)
                 .map_err(dirstate_error)?;
         let map = Self::create_instance(py, Box::new(inner))?;
         Ok(map.into_object())

--- a/rust/hg-cpython/src/dirstate/owning.rs	Thu Jul 08 12:18:21 2021 +0200
+++ b/rust/hg-cpython/src/dirstate/owning.rs	Mon Jul 12 22:46:52 2021 +0200
@@ -48,9 +48,10 @@
     pub fn new_v2(
         py: Python,
         on_disk: PyBytes,
+        data_size: usize,
     ) -> Result<Self, DirstateError> {
         let bytes: &'_ [u8] = on_disk.data(py);
-        let map = DirstateMap::new_v2(bytes)?;
+        let map = DirstateMap::new_v2(bytes, data_size)?;
 
         // Like in `bytes` above, this `'_` lifetime parameter borrows from
         // the bytes buffer owned by `on_disk`.

--- a/rust/rhg/src/commands/status.rs	Thu Jul 08 12:18:21 2021 +0200
+++ b/rust/rhg/src/commands/status.rs	Mon Jul 12 22:46:52 2021 +0200
@@ -170,11 +170,13 @@
     let (mut dmap, parents) = if repo.has_dirstate_v2() {
         let parents;
         let dirstate_data;
+        let data_size;
         if let Some(docket_data) =
             repo.hg_vfs().read("dirstate").io_not_found_as_none()?
         {
             let docket = on_disk::read_docket(&docket_data)?;
             parents = Some(docket.parents());
+            data_size = docket.data_size();
             dirstate_data_mmap = repo
                 .hg_vfs()
                 .mmap_open(docket.data_filename())
@@ -182,9 +184,10 @@
             dirstate_data = dirstate_data_mmap.as_deref().unwrap_or(b"");
         } else {
             parents = None;
+            data_size = 0;
             dirstate_data = b"";
         }
-        let dmap = DirstateMap::new_v2(dirstate_data)?;
+        let dmap = DirstateMap::new_v2(dirstate_data, data_size)?;
         (dmap, parents)
     } else {
         dirstate_data_mmap =