constant_memory: fix memory consumption when zipping file

Add fix in constant memory mode to properly buffer reading the
worksheet file when writing it to zip archive.

This will ensure memory consumption remain constant.

Fixes #120

src/packager.rs

@@ -44,7 +44,7 @@ use std::collections::HashSet;
 use std::io::{Seek, Write};
 
 #[cfg(feature = "constant_memory")]
-use std::io::Read;
+use std::io::{BufRead, BufReader};
 
 use std::sync::{Arc, Mutex};
 #[cfg(not(target_arch = "wasm32"))]
@@ -440,15 +440,17 @@ impl<W: Write + Seek + Send> Packager<W> {
                 // We also need to flush the last remaining row.
                 worksheet.flush_last_row();
 
-                let mut buffer = Vec::new();
                 worksheet.file_writer.rewind().unwrap();
-                worksheet
-                    .file_writer
-                    .get_ref()
-                    .read_to_end(&mut buffer)
-                    .unwrap();
-
-                self.zip.write_all(&buffer)?;
+                let mut reader = BufReader::new(worksheet.file_writer.get_ref());
+                loop {
+                    let buffer = reader.fill_buf().unwrap();
+                    let length = buffer.len();
+                    if length == 0 {
+                        break;
+                    }
+                    self.zip.write_all(&buffer).unwrap();
+                    reader.consume(length);
+                }
             }
 
             // Section 3. In memory metadata at end of the file.