Maybe okay state

src/job.rs

@@ -1,7 +1,18 @@
 use super::*;
+use std::fs::{
+    File, Metadata,    
+};
 
 #[derive(Debug)]
 pub struct Job
 {
-    //todo
+    fd: File,
+    stat: Metadata,
+
+    /// We grab the slice of memory we write to from here
+    state: state::State,
+    /// From this offset
+    offset: usize,
 }
+
+//TODO: job's work :^)

src/map.rs

@@ -2,7 +2,7 @@ use std::ops::RangeBounds;
 use std::slice::SliceIndex;
 use std::{fs,io};
 use std::path::Path;
-
+use std::ops::Drop;
 use memmap::MmapMut;
 
 #[derive(Debug)]
@@ -12,6 +12,14 @@ pub struct MemoryMapMut
     map: MmapMut,
 }
 
+impl Drop for MemoryMapMut
+{
+    fn drop(&mut self)
+    {
+	let _ = self.map.flush();
+    }
+}
+
 impl AsRef<[u8]> for MemoryMapMut
 {
     fn as_ref(&self) -> &[u8]
@@ -28,7 +36,6 @@ impl AsMut<[u8]> for MemoryMapMut
     }
 }
 
-
 impl MemoryMapMut
 {
     #[inline] pub fn as_slice_mut(&mut self) -> &mut [u8]

src/state.rs

@@ -7,6 +7,50 @@ use std::sync::{
 };
 use std::fmt;
 use std::error;
+use std::cell::UnsafeCell;
+use std::{slice::SliceIndex, ops::RangeBounds};
+
+#[derive(Debug)]
+#[repr(transparent)]
+struct StateInner
+{
+    map: UnsafeCell<super::map::MemoryMapMut>,
+}
+
+// SAFETY: The whole point of this is internal mutablility across thread boundaries.
+unsafe impl Sync for StateInner{} 
+
+#[derive(Debug, Clone)]
+pub struct State(Arc<StateInner>);
+
+impl State
+{
+    /// Create a new state from this map
+    #[inline] pub fn new(map: super::map::MemoryMapMut) -> Self
+    {
+	Self(Arc::new(StateInner{map: UnsafeCell::new(map)}))
+    }
+
+    /// Try to consume this instance into its map. This will only succeed if there are no more references to the state than this one.
+    #[inline] pub fn try_into_inner(self) -> Result<super::map::MemoryMapMut, Self>
+    {
+	match Arc::try_unwrap(self.0) {
+	    Ok(v) => Ok(v.map.into_inner()),
+	    Err(e) => Err(Self(e)),
+	}
+    }
+    
+    /// Slice the map directly.
+    ///
+    /// # Safety
+    /// The caller must make sure *no* slices of this map overlap with eachother.
+    //  SAFETY: The map structure itself is never mutated, only its backing memory is accessed. This is fine, I think. If not, we can switch to using raw pointers and volatile writes. The backing memory itself is flushed to file when the map is dropped.
+    pub unsafe fn slice<R: RangeBounds<usize> + SliceIndex<[u8], Output = [u8]>>(&self, range: R) -> &mut [u8]
+    {
+	let slice = (*(self.0.map.get())).as_slice_mut();
+	&mut slice[range]
+    }
+}
 
 /// A multi-consumer message receiver
 #[derive(Debug)]