From 88e35f3c15f8b9f1d44dc6783d8428771e9d4064 Mon Sep 17 00:00:00 2001
From: soruh <mail@soruh.de>
Date: Sun, 23 Jul 2023 23:34:14 +0200
Subject: [PATCH] initial commit

---
 .gitignore        |   2 +
 Cargo.toml        |  13 ++
 src/atomic_arc.rs |  76 ++++++++++++
 src/lib.rs        | 301 ++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 392 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Cargo.toml
 create mode 100644 src/atomic_arc.rs
 create mode 100644 src/lib.rs

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..4fffb2f
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/target
+/Cargo.lock
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..4e63b6e
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "cow_file"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+memmap = "0.7.0"
+zerocopy = "0.6.1"
+
+[dev-dependencies]
+tempfile = "3.7.0"
diff --git a/src/atomic_arc.rs b/src/atomic_arc.rs
new file mode 100644
index 0000000..44d98c7
--- /dev/null
+++ b/src/atomic_arc.rs
@@ -0,0 +1,76 @@
+use std::{
+    marker::PhantomData,
+    sync::{
+        atomic::{AtomicPtr, Ordering},
+        Arc,
+    },
+};
+
+pub struct AtomicArc<T> {
+    inner: AtomicPtr<T>,
+    _phantom: PhantomData<Arc<T>>,
+}
+
+impl<T> Drop for AtomicArc<T> {
+    fn drop(&mut self) {
+        unsafe { Arc::decrement_strong_count(self.load()) }
+    }
+}
+
+impl<T> AtomicArc<T> {
+    fn load(&self) -> *const T {
+        self.inner.load(Ordering::Relaxed)
+    }
+
+    pub fn new(inner: Arc<T>) -> Self {
+        Self {
+            inner: AtomicPtr::new(Arc::into_raw(inner) as *mut T),
+            _phantom: PhantomData,
+        }
+    }
+
+    pub fn get(&self) -> Arc<T> {
+        unsafe {
+            let ptr = self.load();
+            Arc::increment_strong_count(ptr);
+            Arc::from_raw(ptr)
+        }
+    }
+
+    pub fn swap(&self, new: Arc<T>) -> Arc<T> {
+        unsafe {
+            let old = self
+                .inner
+                .swap(Arc::into_raw(new) as *mut T, Ordering::Relaxed);
+
+            Arc::from_raw(old)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use super::AtomicArc;
+
+    #[test]
+    fn test_basic() {
+        let atomic_arc = AtomicArc::new(Arc::new(1));
+
+        let first = atomic_arc.get();
+        assert_eq!(*first, 1);
+
+        atomic_arc.swap(Arc::new(2));
+
+        assert_eq!(*first, 1);
+
+        Arc::into_inner(first).unwrap();
+
+        let second = atomic_arc.get();
+
+        std::mem::drop(atomic_arc);
+
+        Arc::into_inner(second).unwrap();
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..e716bcb
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,301 @@
+use std::{collections::HashMap, fs::File, mem::size_of, ops::Range, sync::Arc};
+
+mod atomic_arc;
+
+use atomic_arc::AtomicArc;
+use memmap::{Mmap, MmapMut};
+use zerocopy::{AsBytes, FromBytes, LayoutVerified, Unaligned, LE};
+
+type U64 = zerocopy::byteorder::U64<LE>;
+
+#[derive(Clone, Copy, FromBytes, AsBytes, Unaligned, Debug, Hash, PartialEq, Eq)]
+#[repr(transparent)]
+struct FilePointer(U64);
+
+#[derive(Clone, Copy, FromBytes, AsBytes, Unaligned, Debug)]
+#[repr(C)]
+struct FileRange {
+    start: FilePointer,
+    len: U64,
+}
+
+impl FilePointer {
+    pub fn range(&self, len: u64) -> FileRange {
+        FileRange {
+            start: *self,
+            len: U64::new(len),
+        }
+    }
+}
+
+impl FileRange {
+    fn len(&self) -> u64 {
+        self.len.get()
+    }
+    fn as_range(&self) -> Range<usize> {
+        let start = self.start.0.get().try_into().unwrap();
+        start..start + usize::try_from(self.len()).unwrap()
+    }
+}
+
+#[derive(Clone, Copy, FromBytes, AsBytes, Unaligned)]
+#[repr(C)]
+struct Header {
+    magic: [u8; 16],
+    root: FilePointer,
+}
+
+impl Default for Header {
+    fn default() -> Self {
+        Self {
+            magic: [0; 16],
+            root: FilePointer(0.into()),
+        }
+    }
+}
+
+struct Snapshot {
+    root: FilePointer,
+    map: Mmap,
+}
+
+pub struct Reader {
+    state: Arc<AtomicArc<Snapshot>>,
+}
+
+struct Db {
+    file: File,
+    map: MmapMut,
+    header: Header,
+    state: Arc<AtomicArc<Snapshot>>,
+}
+
+#[derive(Clone, Copy)]
+struct Modification {
+    old_range: Option<FileRange>,
+    new_range: FileRange,
+}
+
+struct TransactionHandle<'t> {
+    db: &'t mut Db,
+    modifications: HashMap<FilePointer, Modification>,
+    allocator: Allocator,
+}
+
+struct Allocator {
+    head: FilePointer,
+}
+
+impl Allocator {
+    unsafe fn allocate_range(&mut self, size: u64) -> FileRange {
+        todo!()
+    }
+
+    unsafe fn free_range(&mut self, range: FileRange) {
+        todo!()
+    }
+
+    unsafe fn allocate_modification(
+        &mut self,
+        old_range: Option<FileRange>,
+        size: u64,
+    ) -> Modification {
+        let new_range = self.allocate_range(size);
+
+        Modification {
+            old_range,
+            new_range,
+        }
+    }
+}
+
+impl<'t> TransactionHandle<'t> {
+    fn get_data<'a>(&'a mut self, modification: Modification) -> (FileRange, &'a mut [u8]) {
+        (
+            modification.new_range,
+            &mut self.db.map[modification.new_range.as_range()],
+        )
+    }
+
+    pub unsafe fn free(&mut self, range: FileRange) {
+        self.allocator.free_range(range)
+    }
+
+    pub unsafe fn allocate(&mut self, new_size: u64) -> (FileRange, &mut [u8]) {
+        let modification = self.allocator.allocate_modification(None, new_size);
+
+        assert!(self
+            .modifications
+            .insert(modification.new_range.start, modification)
+            .is_none());
+
+        let modification = *self
+            .modifications
+            .get(&modification.new_range.start)
+            .unwrap();
+
+        self.get_data(modification)
+    }
+
+    pub unsafe fn modify(&mut self, range: FileRange) -> (FileRange, &mut [u8]) {
+        self.resize(range, range.len())
+    }
+
+    fn resize(&mut self, range: FileRange, new_size: u64) -> (FileRange, &mut [u8]) {
+        let modification = *self
+            .modifications
+            .entry(range.start)
+            .or_insert_with(|| unsafe {
+                self.allocator.allocate_modification(Some(range), new_size)
+            });
+
+        assert_eq!(
+            modification.new_range.len(),
+            new_size,
+            "tried to resize a region twice."
+        );
+
+        let n = usize::try_from(range.len().min(new_size)).unwrap();
+
+        {
+            let old_range = range.as_range();
+            let new_range = modification.new_range.as_range();
+
+            assert!(!old_range.contains(&new_range.start));
+            assert!(!old_range.contains(&new_range.end));
+
+            assert!(!new_range.contains(&old_range.start));
+            assert!(!new_range.contains(&old_range.end));
+        }
+
+        // this is fine, because we just allocated the space we copy into, so it can't overlap with the source
+        let old_data: &'static [u8] = unsafe { &*(&self.db.map[range.as_range()] as *const _) };
+
+        let data = self.get_data(modification);
+
+        data.1[..n].copy_from_slice(&old_data[..n]);
+
+        data
+    }
+}
+
+impl Db {
+    fn transaction(f: fn(TransactionHandle)) {
+        // let handle = TransactionHandle {};
+    }
+
+    fn header_ptr() -> FilePointer {
+        FilePointer(0.into())
+    }
+
+    fn root_ptr() -> FilePointer {
+        FilePointer(16.into())
+    }
+
+    pub fn create_reader(&self) -> Reader {
+        Reader {
+            state: self.state.clone(),
+        }
+    }
+
+    fn update_root(&mut self, new_root: FilePointer) -> Arc<Snapshot> {
+        // flush all data in file
+        self.map.flush().unwrap();
+
+        // update root pointer and immediately flush
+        self.write(Self::root_ptr(), new_root);
+        self.map
+            .flush_range(Self::root_ptr().0.get() as usize, size_of::<FilePointer>())
+            .unwrap();
+
+        // TODO: we could do a second write + flush here for better consistency
+
+        // update data that readers see
+        self.state.swap(Arc::new(Snapshot {
+            root: new_root,
+            map: self.create_readonly_map(),
+        }))
+    }
+
+    fn read<T: FromBytes>(&self, at: FilePointer) -> T {
+        self.read_range(at.range(size_of::<T>() as u64))
+    }
+
+    fn read_range<T: FromBytes>(&self, range: FileRange) -> T {
+        LayoutVerified::<_, T>::new(&self.map[range.as_range()])
+            .unwrap()
+            .read()
+    }
+
+    fn write<T: AsBytes>(&mut self, at: FilePointer, data: T) {
+        self.write_range(at.range(size_of::<T>() as u64), data)
+    }
+
+    fn write_range<T: AsBytes>(&mut self, range: FileRange, data: T) {
+        LayoutVerified::<_, T>::new(&mut self.map[range.as_range()])
+            .unwrap()
+            .write(data)
+    }
+
+    fn remap(&mut self) {
+        let map = unsafe { MmapMut::map_mut(&self.file) }.unwrap();
+        self.map = map;
+    }
+
+    fn create_readonly_map(&self) -> Mmap {
+        unsafe { Mmap::map(&self.file) }.unwrap()
+    }
+
+    fn resize(&mut self, len: u64) {
+        self.file.set_len(len).unwrap();
+        self.remap();
+    }
+
+    pub fn new(file: File) -> Self {
+        let len = file.metadata().unwrap().len();
+        if len == 0 {
+            file.set_len(size_of::<Header>() as u64).unwrap();
+        }
+
+        dbg!(&len);
+
+        let mut map = unsafe { MmapMut::map_mut(&file) }.unwrap();
+
+        // TODO use the crate Db object and call Db::write(Db::header_ptr())
+        let mut header_bytes =
+            LayoutVerified::<_, Header>::new(&mut map[..size_of::<Header>()]).unwrap();
+
+        let header = if len == 0 {
+            let header = Header::default();
+            header_bytes.write(header);
+            header
+        } else {
+            header_bytes.read()
+        };
+
+        let state = Arc::new(AtomicArc::new(Arc::new(Snapshot {
+            root: header.root,
+            map: todo!(),
+        })));
+
+        Self {
+            file,
+            map,
+            header,
+            state,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        Db::root_ptr();
+
+        let db = Db::new(tempfile::tempfile().unwrap());
+    }
+}