open: optimise truncate

src/main.rs

@@ -47,6 +47,7 @@ fn work(arg::Operation{output, inputs}: arg::Operation) -> Result<(), Box<dyn st
     //  - Read the completion stream receiver until all file jobs have been signaled as completed
     //  - wait on all worker threads to complete.
     //  - ensure all data was written.
+    //  - truncate the output file to the correct size (sum of all the input sizes) if needed (we do not truncate before fallocate()ing it, so if a previous larger file existed there, it will have trailing garbage)
 
     Ok(())
 }

src/map.rs

@@ -63,6 +63,7 @@ impl MemoryMapMut
 	    .create(true)
 	    .truncate(true)
 	    .write(true)
+	    .read(true)
 	    .open(from)?;
 	
 	Ok(Self{

src/open.rs

@@ -34,7 +34,7 @@ impl Output
 	fallocate(&mut file, to)?;
 	map::MemoryMapMut::map(file)
 	    .map_err(OutputError::Map)
-	    .and_then(|map| if map.len() != to {
+	    .and_then(|map| if map.len() < to {
 		Err(OutputError::Size{expected: to, got: map.len()})
 	    } else {
 		Ok(map)
@@ -48,7 +48,10 @@ impl Output
     fs::OpenOptions::new()
 	.create(true)
 	.write(true)
+	.read(true)
+	//.truncate(true) //TODO: Remove this, and truncate any excess space that may exist from a clobbered previous file at the end
 	.open(path)
+	
 	.map(Output)
 	.map_err(OutputError::Open)
 }
@@ -93,7 +96,7 @@ impl fmt::Display for OutputError
 	    Self::Open(io) => write!(f, "Failed to open file: {}", io),
 	    Self::Allocate(errno) => write!(f, "fallocate() failed with error {}: {}", errno, errno_str(*errno)),
 	    Self::Map(io) => write!(f, "mmap() failed: {}", io),
-	    Self::Size{expected, got} => write!(f, "mapped file size mismatch: expected {}, got {}", expected, got),
+	    Self::Size{expected, got} => write!(f, "mapped file size mismatch: expected at least {}, got {}", expected, got),
 	}
     }
 }