Implementation of a PAX header writer

This crate currently supports reading the PAX headers. This adds a way to write
some PAX headers in a Builder.

src/pax.rs

@@ -1,5 +1,6 @@
 #![allow(dead_code)]
 use std::io;
+use std::io::Write;
 use std::slice;
 use std::str;
 
@@ -145,3 +146,45 @@ impl<'entry> PaxExtension<'entry> {
         self.value
     }
 }
+
+/// Extension trait for `Builder` to append PAX extended headers.
+impl<T: Write> crate::Builder<T> {
+    /// Append PAX extended headers to the archive.
+    ///
+    /// Takes in an iterator over the list of headers to add to convert it into a header set formatted.
+    ///
+    /// Returns io::Error if an error occurs, else it returns ()
+    pub fn append_pax_extensions<'a>(&mut self, headers: impl IntoIterator<Item=(String, String)>) -> Result<(), io::Error> {
+        // Store the headers formatted before write
+        let mut data: Vec<u8> = Vec::new();
+
+        // For each key in headers, convert into a sized space and add it to data.
+        // This will then be written in the file
+        for (key, value) in headers {
+            let mut len_len = 1;
+            let mut max_len = 10;
+            let rest_len = 3 + key.len() + value.len();
+            while rest_len + len_len >= max_len {
+                len_len += 1;
+                max_len *= 10;
+            }
+            let len = rest_len + len_len;
+            write!(&mut data, "{} {}={}\n", len, key, value)?;
+        }
+
+        // Ignore the header append if it's empty.
+        if data.is_empty() {
+            return Ok(());
+        }
+
+        // Create a header of type XHeader, set the size to the length of the
+        // data, set the entry type to XHeader, and set the checksum
+        // then append the header and the data to the archive.
+        let mut header = crate::Header::new_ustar();
+        let data_as_bytes: &[u8] = &data;
+        header.set_size(data_as_bytes.len() as u64);
+        header.set_entry_type(crate::EntryType::XHeader);
+        header.set_cksum();
+        self.append(&header, data_as_bytes)
+    }
+}

tests/all.rs

@@ -6,12 +6,14 @@ extern crate xattr;
 
 use std::fs::{self, File};
 use std::io::prelude::*;
-use std::io::{self, Cursor};
+use std::io::{self, BufWriter, Cursor};
 use std::iter::repeat;
 use std::path::{Path, PathBuf};
 
 use filetime::FileTime;
-use tar::{Archive, Builder, Entries, EntryType, Header, HeaderMode};
+use tar::{
+    Archive, Builder, BuilderExt, Entries, Entry, EntryType, Header, HeaderMode, PaxBuilder,
+};
 use tempfile::{Builder as TempBuilder, TempDir};
 
 macro_rules! t {
@@ -925,6 +927,38 @@ fn pax_simple() {
     assert_eq!(third.value(), Ok("1453146164.953123768"));
 }
 
+#[test]
+fn pax_simple_write() {
+    let td = t!(TempBuilder::new().prefix("tar-rs").tempdir());
+    let pax_path = td.path().join("pax.tar");
+    let file: File = t!(File::create(&pax_path));
+    let mut ar: Builder<BufWriter<File>> = Builder::new(BufWriter::new(file));
+
+    let mut pax_builder: PaxBuilder = PaxBuilder::new();
+    let key: String = "arbitrary_pax_key".to_string();
+    let value: String = "arbitrary_pax_value".to_string();
+    pax_builder.add(&key, &value);
+
+    t!(ar.append_pax_extensions(&pax_builder));
+    t!(ar.append_file("test2", &mut t!(File::open(&pax_path))));
+    t!(ar.finish());
+    drop(ar);
+
+    let mut archive_opened = Archive::new(t!(File::open(pax_path)));
+    let mut entries = t!(archive_opened.entries());
+    let mut f: Entry<File> = t!(entries.next().unwrap());
+    let pax_headers = t!(f.pax_extensions());
+
+    assert!(pax_headers.is_some(), "pax_headers is None");
+    let mut pax_headers = pax_headers.unwrap();
+    let pax_arbitrary = t!(pax_headers.next().unwrap());
+
+    assert_eq!(pax_arbitrary.key(), Ok("arbitrary_pax_key"));
+    assert_eq!(pax_arbitrary.value(), Ok("arbitrary_pax_value"));
+
+    assert!(entries.next().is_none());
+}
+
 #[test]
 fn pax_path() {
     let mut ar = Archive::new(tar!("pax2.tar"));

tests/entry.rs

@@ -1,7 +1,8 @@
 extern crate tar;
 extern crate tempfile;
 
-use std::fs::{create_dir, File};
+use std::fs::create_dir;
+use std::fs::File;
 use std::io::Read;
 
 use tempfile::Builder;