diff -r 8ff2d8359d0f -r 9eb07ab3f2d4 rust/hg-core/src/revlog/nodemap_docket.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/revlog/nodemap_docket.rs	Fri Dec 04 17:27:10 2020 +0100
@@ -0,0 +1,119 @@
+use memmap::Mmap;
+use std::convert::TryInto;
+use std::path::{Path, PathBuf};
+
+use super::revlog::{mmap_open, RevlogError};
+use crate::utils::strip_suffix;
+
+const ONDISK_VERSION: u8 = 1;
+
+pub(super) struct NodeMapDocket {
+    pub data_length: usize,
+    // TODO: keep here more of the data from `parse()` when we need it
+}
+
+impl NodeMapDocket {
+    /// Return `Ok(None)` when the caller should proceed without a persistent
+    /// nodemap:
+    ///
+    /// * This revlog does not have a `.n` docket file (it is not generated for
+    ///   small revlogs), or
+    /// * The docket has an unsupported version number (repositories created by
+    ///   later hg, maybe that should be a requirement instead?), or
+    /// * The docket file points to a missing (likely deleted) data file (this
+    ///   can happen in a rare race condition).
+    pub fn read_from_file(
+        index_path: &Path,
+    ) -> Result<Option<(Self, Mmap)>, RevlogError> {
+        let docket_path = index_path.with_extension("n");
+        let docket_bytes = match std::fs::read(&docket_path) {
+            Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+                return Ok(None)
+            }
+            Err(e) => return Err(RevlogError::IoError(e)),
+            Ok(bytes) => bytes,
+        };
+
+        let mut input = if let Some((&ONDISK_VERSION, rest)) =
+            docket_bytes.split_first()
+        {
+            rest
+        } else {
+            return Ok(None);
+        };
+        let input = &mut input;
+
+        let uid_size = read_u8(input)? as usize;
+        let _tip_rev = read_be_u64(input)?;
+        // TODO: do we care about overflow for 4 GB+ nodemap files on 32-bit
+        // systems?
+        let data_length = read_be_u64(input)? as usize;
+        let _data_unused = read_be_u64(input)?;
+        let tip_node_size = read_be_u64(input)? as usize;
+        let uid = read_bytes(input, uid_size)?;
+        let _tip_node = read_bytes(input, tip_node_size)?;
+
+        let uid =
+            std::str::from_utf8(uid).map_err(|_| RevlogError::Corrupted)?;
+        let docket = NodeMapDocket { data_length };
+
+        let data_path = rawdata_path(&docket_path, uid);
+        // TODO: use `std::fs::read` here when the `persistent-nodemap.mmap`
+        // config is false?
+        match mmap_open(&data_path) {
+            Ok(mmap) => {
+                if mmap.len() >= data_length {
+                    Ok(Some((docket, mmap)))
+                } else {
+                    Err(RevlogError::Corrupted)
+                }
+            }
+            Err(error) => {
+                if error.kind() == std::io::ErrorKind::NotFound {
+                    Ok(None)
+                } else {
+                    Err(RevlogError::IoError(error))
+                }
+            }
+        }
+    }
+}
+
+fn read_bytes<'a>(
+    input: &mut &'a [u8],
+    count: usize,
+) -> Result<&'a [u8], RevlogError> {
+    if let Some(start) = input.get(..count) {
+        *input = &input[count..];
+        Ok(start)
+    } else {
+        Err(RevlogError::Corrupted)
+    }
+}
+
+fn read_u8<'a>(input: &mut &[u8]) -> Result<u8, RevlogError> {
+    Ok(read_bytes(input, 1)?[0])
+}
+
+fn read_be_u64<'a>(input: &mut &[u8]) -> Result<u64, RevlogError> {
+    let array = read_bytes(input, std::mem::size_of::<u64>())?
+        .try_into()
+        .unwrap();
+    Ok(u64::from_be_bytes(array))
+}
+
+fn rawdata_path(docket_path: &Path, uid: &str) -> PathBuf {
+    let docket_name = docket_path
+        .file_name()
+        .expect("expected a base name")
+        .to_str()
+        .expect("expected an ASCII file name in the store");
+    let prefix = strip_suffix(docket_name, ".n.a")
+        .or_else(|| strip_suffix(docket_name, ".n"))
+        .expect("expected docket path in .n or .n.a");
+    let name = format!("{}-{}.nd", prefix, uid);
+    docket_path
+        .parent()
+        .expect("expected a non-root path")
+        .join(name)
+}