feat(tvix/tools/weave): init

Scalable tracing GC for the cache.nixos.org dataset.

Change-Id: I6c7852796f28e1a1c7607384ffb55f44407e1185
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10765
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/tools/weave/src/lib.rs

@@ -0,0 +1,102 @@
+use anyhow::Result;
+use rayon::prelude::*;
+use std::{fs::File, slice};
+
+use polars::{
+    datatypes::BinaryChunked,
+    export::arrow::array::BinaryArray,
+    prelude::{ParquetReader, SerReader},
+};
+
+pub use crate::bytes::*;
+mod bytes;
+
+pub const INDEX_NULL: u32 = !0;
+pub const DONE: &str = "\u{2714}";
+
+/// A terrific hash function, turning 20 bytes of cryptographic hash
+/// into 8 bytes of cryptographic hash.
+pub fn hash64(h: &[u8; 20]) -> u64 {
+    let mut buf = [0; 8];
+    buf.copy_from_slice(&h[..8]);
+    u64::from_ne_bytes(buf)
+}
+
+/// Read a dense `store_path_hash` array from `narinfo.parquet`,
+/// returning it as an owned [FixedBytes].
+pub fn load_ph_array() -> Result<FixedBytes<20>> {
+    eprint!("… load store_path_hash\r");
+    // TODO(edef): this could use a further pushdown, since polars is more hindrance than help here
+    // We know this has to fit in memory (we can't mmap it without further encoding constraints),
+    // and we want a single `Vec<[u8; 20]>` of the data.
+    let ph_array = into_fixed_binary_rechunk::<20>(
+        ParquetReader::new(File::open("narinfo.parquet").unwrap())
+            .with_columns(Some(vec!["store_path_hash".into()]))
+            .set_rechunk(true)
+            .finish()?
+            .column("store_path_hash")?
+            .binary()?,
+    );
+
+    u32::try_from(ph_array.len()).expect("dataset exceeds 2^32");
+    eprintln!("{DONE}");
+
+    Ok(ph_array)
+}
+
+/// Iterator over `&[[u8; N]]` from a dense [BinaryChunked].
+pub fn as_fixed_binary<const N: usize>(
+    chunked: &BinaryChunked,
+) -> impl Iterator<Item = &[[u8; N]]> + DoubleEndedIterator {
+    chunked.downcast_iter().map(|array| {
+        assert_fixed_dense::<N>(array);
+        exact_chunks(array.values()).unwrap()
+    })
+}
+
+/// Convert a dense [BinaryChunked] into a single chunk as [FixedBytes],
+/// without taking a reference to the offsets array and validity bitmap.
+fn into_fixed_binary_rechunk<const N: usize>(chunked: &BinaryChunked) -> FixedBytes<N> {
+    let chunked = chunked.rechunk();
+    let mut iter = chunked.downcast_iter();
+    let array = iter.next().unwrap();
+
+    assert_fixed_dense::<N>(array);
+    Bytes(array.values().clone()).map(|buf| exact_chunks(buf).unwrap())
+}
+
+/// Ensures that the supplied Arrow array consists of densely packed bytestrings of length `N`.
+/// In other words, ensure that it is free of nulls, and that the offsets have a fixed stride of `N`.
+fn assert_fixed_dense<const N: usize>(array: &BinaryArray<i64>) {
+    let null_count = array.validity().map_or(0, |bits| bits.unset_bits());
+    if null_count > 0 {
+        panic!("null values present");
+    }
+
+    let length_check = array
+        .offsets()
+        .as_slice()
+        .par_windows(2)
+        .all(|w| (w[1] - w[0]) == N as i64);
+
+    if !length_check {
+        panic!("lengths are inconsistent");
+    }
+}
+
+fn exact_chunks<const K: usize>(buf: &[u8]) -> Option<&[[u8; K]]> {
+    // SAFETY: We ensure that `buf.len()` is a multiple of K, and there are no alignment requirements.
+    unsafe {
+        let ptr = buf.as_ptr();
+        let len = buf.len();
+
+        if len % K != 0 {
+            return None;
+        }
+
+        let ptr = ptr as *mut [u8; K];
+        let len = len / K;
+
+        Some(slice::from_raw_parts(ptr, len))
+    }
+}