rust-nodemap: input/output primitives

These allow to initiate a `NodeTree` from an immutable opaque
sequence of bytes, which could be passed over from Python
(extracted from a `PyBuffer`) or directly mmapped from a file.

Conversely, we can consume
a `NodeTree`, extracting the bytes that express what
has been added to the immutable part, together with the
original immutable part.
This gives callers the choice to start a new Nodetree.
After writing to disk, some would prefer to reread for
best guarantees (very cheap if mmapping), some others will
find it more convenient to grow the memory that was considered
immutable in the `NodeTree` and continue from there.

This is enough to build examples running on real data and
start gathering performance hints.

Differential Revision: https://phab.mercurial-scm.org/D7796

rust/hg-core/src/revlog/nodemap.rs

@@ -17,8 +17,10 b' use super::{'
 };
 
 use std::fmt;
+use std::mem;
 use std::ops::Deref;
 use std::ops::Index;
+use std::slice;
 
 #[derive(Debug, PartialEq)]
 pub enum NodeMapError {
@@ -172,20 +174,42 b' impl From<Element> for RawElement {'
 /// represented at all, because we want an immutable empty nodetree
 /// to be valid.
 
-#[derive(Clone, PartialEq)]
-pub struct Block([RawElement; 16]);
+#[derive(Copy, Clone)]
+pub struct Block([u8; BLOCK_SIZE]);
+
+/// Not derivable for arrays of length >32 until const generics are stable
+impl PartialEq for Block {
+    fn eq(&self, other: &Self) -> bool {
+        &self.0[..] == &other.0[..]
+    }
+}
+
+pub const BLOCK_SIZE: usize = 64;
 
 impl Block {
     fn new() -> Self {
-        Block([-1; 16])
+        // -1 in 2's complement to create an absent node
+        let byte: u8 = 255;
+        Block([byte; BLOCK_SIZE])
     }
 
     fn get(&self, nybble: u8) -> Element {
-        Element::from(RawElement::from_be(self.0[nybble as usize]))
+        let index = nybble as usize * mem::size_of::<RawElement>();
+        Element::from(RawElement::from_be_bytes([
+            self.0[index],
+            self.0[index + 1],
+            self.0[index + 2],
+            self.0[index + 3],
+        ]))
     }
 
     fn set(&mut self, nybble: u8, element: Element) {
-        self.0[nybble as usize] = RawElement::to_be(element.into())
+        let values = RawElement::to_be_bytes(element.into());
+        let index = nybble as usize * mem::size_of::<RawElement>();
+        self.0[index] = values[0];
+        self.0[index + 1] = values[1];
+        self.0[index + 2] = values[2];
+        self.0[index + 3] = values[3];
     }
 }
 
@@ -230,9 +254,9 b' impl Index<usize> for NodeTree {'
 }
 
 /// Return `None` unless the `Node` for `rev` has given prefix in `index`.
-fn has_prefix_or_none<'p>(
+fn has_prefix_or_none(
     idx: &impl RevlogIndex,
-    prefix: NodePrefixRef<'p>,
+    prefix: NodePrefixRef,
     rev: Revision,
 ) -> Result<Option<Revision>, NodeMapError> {
     idx.node(rev)
@@ -262,6 +286,67 b' impl NodeTree {'
         }
     }
 
+    /// Create from an opaque bunch of bytes
+    ///
+    /// The created `NodeTreeBytes` from `buffer`,
+    /// of which exactly `amount` bytes are used.
+    ///
+    /// - `buffer` could be derived from `PyBuffer` and `Mmap` objects.
+    /// - `offset` allows for the final file format to include fixed data
+    ///   (generation number, behavioural flags)
+    /// - `amount` is expressed in bytes, and is not automatically derived from
+    ///   `bytes`, so that a caller that manages them atomically can perform
+    ///   temporary disk serializations and still rollback easily if needed.
+    ///   First use-case for this would be to support Mercurial shell hooks.
+    ///
+    /// panics if `buffer` is smaller than `amount`
+    pub fn load_bytes(
+        bytes: Box<dyn Deref<Target = [u8]> + Send>,
+        amount: usize,
+    ) -> Self {
+        NodeTree::new(Box::new(NodeTreeBytes::new(bytes, amount)))
+    }
+
+    /// Retrieve added `Block` and the original immutable data
+    pub fn into_readonly_and_added(
+        self,
+    ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<Block>) {
+        let mut vec = self.growable;
+        let readonly = self.readonly;
+        if readonly.last() != Some(&self.root) {
+            vec.push(self.root);
+        }
+        (readonly, vec)
+    }
+
+    /// Retrieve added `Blocks` as bytes, ready to be written to persistent
+    /// storage
+    pub fn into_readonly_and_added_bytes(
+        self,
+    ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<u8>) {
+        let (readonly, vec) = self.into_readonly_and_added();
+        // Prevent running `v`'s destructor so we are in complete control
+        // of the allocation.
+        let vec = mem::ManuallyDrop::new(vec);
+
+        // Transmute the `Vec<Block>` to a `Vec<u8>`. Blocks are contiguous
+        // bytes, so this is perfectly safe.
+        let bytes = unsafe {
+            // Assert that `Block` hasn't been changed and has no padding
+            let _: [u8; 4 * BLOCK_SIZE] =
+                std::mem::transmute([Block::new(); 4]);
+
+            // /!\ Any use of `vec` after this is use-after-free.
+            // TODO: use `into_raw_parts` once stabilized
+            Vec::from_raw_parts(
+                vec.as_ptr() as *mut u8,
+                vec.len() * BLOCK_SIZE,
+                vec.capacity() * BLOCK_SIZE,
+            )
+        };
+        (readonly, bytes)
+    }
+
     /// Total number of blocks
     fn len(&self) -> usize {
         self.readonly.len() + self.growable.len() + 1
@@ -410,6 +495,38 b' impl NodeTree {'
     }
 }
 
+pub struct NodeTreeBytes {
+    buffer: Box<dyn Deref<Target = [u8]> + Send>,
+    len_in_blocks: usize,
+}
+
+impl NodeTreeBytes {
+    fn new(
+        buffer: Box<dyn Deref<Target = [u8]> + Send>,
+        amount: usize,
+    ) -> Self {
+        assert!(buffer.len() >= amount);
+        let len_in_blocks = amount / BLOCK_SIZE;
+        NodeTreeBytes {
+            buffer,
+            len_in_blocks,
+        }
+    }
+}
+
+impl Deref for NodeTreeBytes {
+    type Target = [Block];
+
+    fn deref(&self) -> &[Block] {
+        unsafe {
+            slice::from_raw_parts(
+                (&self.buffer).as_ptr() as *const Block,
+                self.len_in_blocks,
+            )
+        }
+    }
+}
+
 struct NodeTreeVisitor<'n, 'p> {
     nt: &'n NodeTree,
     prefix: NodePrefixRef<'p>,
@@ -539,12 +656,15 b' mod tests {'
 
     #[test]
     fn test_raw_block() {
-        let mut raw = [-1; 16];
-        raw[0] = 0;
-        raw[1] = RawElement::to_be(15);
-        raw[2] = RawElement::to_be(-2);
-        raw[3] = RawElement::to_be(-1);
-        raw[4] = RawElement::to_be(-3);
+        let mut raw = [255u8; 64];
+
+        let mut counter = 0;
+        for val in [0, 15, -2, -1, -3].iter() {
+            for byte in RawElement::to_be_bytes(*val).iter() {
+                raw[counter] = *byte;
+                counter += 1;
+            }
+        }
         let block = Block(raw);
         assert_eq!(block.get(0), Element::Block(0));
         assert_eq!(block.get(1), Element::Block(15));
@@ -787,4 +907,30 b' mod tests {'
 
         Ok(())
     }
+
+    #[test]
+    fn test_into_added_empty() {
+        assert!(sample_nodetree().into_readonly_and_added().1.is_empty());
+        assert!(sample_nodetree()
+            .into_readonly_and_added_bytes()
+            .1
+            .is_empty());
 }
+
+    #[test]
+    fn test_into_added_bytes() -> Result<(), NodeMapError> {
+        let mut idx = TestNtIndex::new();
+        idx.insert(0, "1234")?;
+        let mut idx = idx.commit();
+        idx.insert(4, "cafe")?;
+        let (_, bytes) = idx.nt.into_readonly_and_added_bytes();
+
+        // only the root block has been changed
+        assert_eq!(bytes.len(), BLOCK_SIZE);
+        // big endian for -2
+        assert_eq!(&bytes[4..2 * 4], [255, 255, 255, 254]);
+        // big endian for -6
+        assert_eq!(&bytes[12 * 4..13 * 4], [255, 255, 255, 250]);
+        Ok(())
+    }
+}