Patchwork D7796: rust-nodemap: input/output primitives

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Submitter phabricator
Date Jan. 6, 2020, 7:26 p.m.
Message ID <differential-rev-PHID-DREV-3mwbbura2ccoppyytg3h-req@mercurial-scm.org>
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Permalink /patch/44156/
State New
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phabricator - Jan. 6, 2020, 7:26 p.m.
gracinet created this revision.
Herald added subscribers: mercurial-devel, kevincox, durin42.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  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.
  
  In `load_bytes`, we anticipate a bit on the file format for
  the final version, allowing an offset for fixed data at the
  beginning of the file.
  
  This is enough to build examples running on real data and
  start gathering performance hints.

REPOSITORY
  rHG Mercurial

BRANCH
  default

REVISION DETAIL
  https://phab.mercurial-scm.org/D7796

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

CHANGE DETAILS




To: gracinet, #hg-reviewers
Cc: durin42, kevincox, mercurial-devel
phabricator - Jan. 16, 2020, 11:24 a.m.
This revision now requires changes to proceed.
kevincox added inline comments.
kevincox requested changes to this revision.

INLINE COMMENTS

> nodemap.rs:268
> +            Vec::from_raw_parts(
> +                vec.as_ptr() as *mut u8,
> +                vec.len() * BLOCK_SIZE,

This makes me uncomfortable. There is really no guarantee that there is no padding around `Block` or `RawElement`. If there is any padding this is UB. I would much prefer one of:

1. Store growable as a set of bytes and convert on get/set.
2. Add a method that outputs the bytes to any `Write`. In theory this will be slower but it is probably immaterial (especially if there is no padding).

> nodemap.rs:272
> +            )
> +        };
> +        mem::forget(vec);

If someone panics between the `from_raw_parts` and `mem::forget` this is a double free. Right now I think this can't happen but it is completely possible that the code changes between then and now. I would prefer using `Vec::from_raw_parts` to make sure that there is only one `Vec` alive with that pointer at a time. This risks a leak in the face of panic but I think that is much better.

> nodemap.rs:433
> +        amount: usize,
> +    ) -> Self {
> +        assert!(buffer.len() >= offset + amount);

This is a weird API. Why does new take a buffer and an offset? Can it take either a slice or just remove the offset parameter and always have the buffer start at 0? It is very strange to be passing in data that we own but isn't ever used.

REPOSITORY
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CHANGES SINCE LAST ACTION
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REVISION DETAIL
  https://phab.mercurial-scm.org/D7796

To: gracinet, #hg-reviewers, kevincox
Cc: durin42, kevincox, mercurial-devel
phabricator - Jan. 16, 2020, 12:10 p.m.
kevincox added inline comments.

INLINE COMMENTS

> kevincox wrote in nodemap.rs:268
> This makes me uncomfortable. There is really no guarantee that there is no padding around `Block` or `RawElement`. If there is any padding this is UB. I would much prefer one of:
> 
> 1. Store growable as a set of bytes and convert on get/set.
> 2. Add a method that outputs the bytes to any `Write`. In theory this will be slower but it is probably immaterial (especially if there is no padding).

I thought about this more and I think I am okay doing it this way. It seems like this should be well defined as long as there is no padding. However on that note I would want to add a check that there is no padding as expected. I would also like to ensure that this fails to compile if there is ever padding, even in release mode. I think this can be accomplished by something like:

  let _: [u8; 4 * BLOCK_SIZE] = std::mem::transmute([Block::new(); 4]);

I would probably want to repeat this check near any code that is relying on this invariant.

REPOSITORY
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CHANGES SINCE LAST ACTION
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REVISION DETAIL
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To: gracinet, #hg-reviewers, kevincox
Cc: durin42, kevincox, mercurial-devel
phabricator - Jan. 16, 2020, 12:54 p.m.
gracinet added a comment.


  @kevincox (replying hastily because I'm also finishing something unrelated today).
  Doesn't `mem::size_of` guarantee to take any padding into account? At least that's what the doc seems to say: https://doc.rust-lang.org/std/mem/fn.size_of.html

REPOSITORY
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CHANGES SINCE LAST ACTION
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REVISION DETAIL
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To: gracinet, #hg-reviewers, kevincox
Cc: durin42, kevincox, mercurial-devel
phabricator - Jan. 16, 2020, 3:34 p.m.
kevincox added a comment.


  Ah yes. So my test would be ineffective.
  
  The problem that IIUC padding is not guaranteed to be initialized, and in rust all values need to be initialized. So if you cast a type with padding to a `u8` it is undefined behaviour.
  
  So we would need a test that shows that the type is the same size as the sum of the contained `i32`s.

REPOSITORY
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CHANGES SINCE LAST ACTION
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REVISION DETAIL
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To: gracinet, #hg-reviewers, kevincox
Cc: durin42, kevincox, mercurial-devel
phabricator - Jan. 16, 2020, 3:35 p.m.
kevincox added a comment.


  Maybe what we should do is keep the test I proposed, but change the definition of `BLOCK_SIZE` to be based upon the number of contained `i32` instead of `size_of`.

REPOSITORY
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CHANGES SINCE LAST ACTION
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REVISION DETAIL
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To: gracinet, #hg-reviewers, kevincox
Cc: durin42, kevincox, mercurial-devel

Patch

diff --git a/rust/hg-core/src/revlog/nodemap.rs b/rust/hg-core/src/revlog/nodemap.rs
--- a/rust/hg-core/src/revlog/nodemap.rs
+++ b/rust/hg-core/src/revlog/nodemap.rs
@@ -17,8 +17,10 @@ 
     RevlogIndex,
 };
 use std::fmt;
+use std::mem;
 use std::ops::Deref;
 use std::ops::Index;
+use std::slice;
 
 #[derive(Debug, PartialEq)]
 pub enum NodeMapError {
@@ -132,6 +134,8 @@ 
 #[derive(Clone, PartialEq)]
 pub struct Block([RawElement; 16]);
 
+pub const BLOCK_SIZE: usize = mem::size_of::<Block>();
+
 impl Block {
     fn new() -> Self {
         Block([-1; 16])
@@ -221,6 +225,57 @@ 
         }
     }
 
+    /// Create from an opaque bunch of bytes
+    ///
+    /// The created `NodeTreeBytes` is taken after the fixed `offset` 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 `offset + amount`
+    pub fn load_bytes(
+        bytes: Box<dyn Deref<Target = [u8]> + Send>,
+        offset: usize,
+        amount: usize,
+    ) -> Self {
+        NodeTree::new(Box::new(NodeTreeBytes::new(bytes, offset, 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();
+        let bytes = unsafe {
+            Vec::from_raw_parts(
+                vec.as_ptr() as *mut u8,
+                vec.len() * BLOCK_SIZE,
+                vec.capacity() * BLOCK_SIZE,
+            )
+        };
+        mem::forget(vec);
+        (readonly, bytes)
+    }
+
     /// Total number of blocks
     fn len(&self) -> usize {
         self.readonly.len() + self.growable.len() + 1
@@ -366,6 +421,42 @@ 
     }
 }
 
+pub struct NodeTreeBytes {
+    buffer: Box<dyn Deref<Target = [u8]> + Send>,
+    offset: usize,
+    len_in_blocks: usize,
+}
+
+impl NodeTreeBytes {
+    fn new(
+        buffer: Box<dyn Deref<Target = [u8]> + Send>,
+        offset: usize,
+        amount: usize,
+    ) -> Self {
+        assert!(buffer.len() >= offset + amount);
+        let len_in_blocks = amount / BLOCK_SIZE;
+        NodeTreeBytes {
+            buffer,
+            offset,
+            len_in_blocks,
+        }
+    }
+}
+
+impl Deref for NodeTreeBytes {
+    type Target = [Block];
+
+    fn deref(&self) -> &[Block] {
+        unsafe {
+            slice::from_raw_parts(
+                (&self.buffer).as_ptr().offset(self.offset as isize)
+                    as *const Block,
+                self.len_in_blocks,
+            )
+        }
+    }
+}
+
 struct NodeTreeVisitor<'n, 'p> {
     nt: &'n NodeTree,
     prefix: NodePrefixRef<'p>,
@@ -710,4 +801,31 @@ 
 
         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(())
+    }
+
 }