Encode and decode errors

Author: Byron

README.md

@@ -94,11 +94,11 @@ Please see _'Development Status'_ for a listing of all crates and their capabili
     some sort of reactor which feeds the client/server respectively with deserialized lines. This enables us to
     start out with a sync implementation, and later add an async one that reuses all the protocol code.
   * [ ] [PKT-Line](https://github.com/git/git/blob/master/Documentation/technical/protocol-common.txt#L52:L52)
-    * [ ] encode
-    * [ ] decode (zero-copy)
-    * [ ] [error line](https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt#L28:L28)
+    * [x] encode
+    * [x] decode (zero-copy)
+    * [x] [error line](https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt#L28:L28)
     * [ ] [V2 additions](https://github.com/git/git/blob/master/Documentation/technical/protocol-v2.txt#L35:L36)
-    * [ ]
+    * [ ] [side-band mode](https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt#L467:L467)
   * [ ] `Iterator` for multi-plexed pack lines from `Read`
   * [ ] parse and serialize [capabilities](https://github.com/git/git/blob/master/Documentation/technical/protocol-capabilities.txt#L1:L1)
   * [ ] **Version 1**
@@ -111,7 +111,6 @@ Please see _'Development Status'_ for a listing of all crates and their capabili
         * [ ] multi-ack 
         * [ ] multi-ack detailed
       * [ ] [server-response (pack)](https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt#L404:L404)
-        * [ ] [side-band mode](https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt#L467:L467)
     * [ ] push
   * [ ] [Version 2](https://github.com/git/git/blob/master/Documentation/technical/protocol-v2.txt)
 

git-protocol/src/packet_line/encode.rs

@@ -1,4 +1,4 @@
-use crate::packet_line::{FLUSH_LINE, MAX_DATA_LEN};
+use crate::packet_line::{ERR_PREFIX, FLUSH_LINE, MAX_DATA_LEN};
 use quick_error::quick_error;
 use std::io;
 
@@ -18,10 +18,23 @@ quick_error! {
         }
     }
 }
+
 pub fn flush_to_write(mut out: impl io::Write) -> io::Result<usize> {
     out.write_all(FLUSH_LINE).map(|_| 4)
 }
 
+pub fn error_to_write(data: &[u8], out: impl io::Write) -> Result<usize, Error> {
+    let data_with_prefix_end = data.len() + ERR_PREFIX.len();
+    if data_with_prefix_end > MAX_DATA_LEN {
+        return Err(Error::DataLengthLimitExceeded(data.len() - ERR_PREFIX.len()));
+    }
+    // This is a big buffer, but it's only used on error, so the program is on the way out
+    let mut buf = [0u8; MAX_DATA_LEN];
+    buf[..ERR_PREFIX.len()].copy_from_slice(ERR_PREFIX);
+    buf[ERR_PREFIX.len()..data_with_prefix_end].copy_from_slice(data);
+    data_to_write(&buf[..data_with_prefix_end], out)
+}
+
 pub fn data_to_write(data: &[u8], mut out: impl io::Write) -> Result<usize, Error> {
     if data.len() > MAX_DATA_LEN {
         return Err(Error::DataLengthLimitExceeded(data.len()));

git-protocol/src/packet_line/mod.rs

@@ -31,6 +31,17 @@ impl<'a> Borrowed<'a> {
     pub fn as_bstr(&self) -> &BStr {
         self.as_slice().into()
     }
+    pub fn to_error(&self) -> Error {
+        Error(self.as_slice())
+    }
+}
+
+pub struct Error<'a>(&'a [u8]);
+
+impl<'a> Error<'a> {
+    pub fn to_write(&self, out: impl io::Write) -> Result<usize, encode::Error> {
+        encode::error_to_write(self.0, out)
+    }
 }
 
 pub mod decode;

git-protocol/tests/packet_line/decode.rs

@@ -74,6 +74,14 @@ mod streaming {
         Ok(())
     }
 
+    #[test]
+    fn roundtrip_error_line() -> crate::Result {
+        let mut out = Vec::new();
+        PacketLine::Data(b"the error").to_error().to_write(&mut out)?;
+        assert_err_display(streaming(&out), "the error");
+        Ok(())
+    }
+
     mod incomplete {
         use git_protocol::packet_line::decode::{self, streaming, Stream};
 

git-protocol/tests/packet_line/encode.rs

@@ -1,7 +1,7 @@
 mod data_to_write {
     use crate::packet_line::assert_err_display;
     use bstr::ByteSlice;
-    use git_protocol::packet_line::encode::{data_to_write, flush_to_write};
+    use git_protocol::packet_line::encode::{data_to_write, error_to_write, flush_to_write};
     use std::io;
 
     fn vec_sized(size: usize) -> Vec<u8> {
@@ -30,6 +30,14 @@ mod data_to_write {
         Ok(())
     }
 
+    #[test]
+    fn successful_write_of_error_line() -> crate::Result {
+        let mut out = Vec::new();
+        assert_eq!(error_to_write(b"hello error", &mut out)?, 19);
+        assert_eq!(out.as_bstr(), b"0013ERR hello error".as_bstr());
+        Ok(())
+    }
+
     #[test]
     fn error_if_data_exceeds_limit() {
         assert_err_display(

tasks.md

@@ -6,8 +6,6 @@
     * to be used to obtain more real-world samples of typical git interactions for use in the test-suite
   * **git-protocol**
     * [ ] pkt-lines support
-      * [x] encoding including flush lines
-      * [ ] decoding
     * [ ] basic V1 parsing to understand data frames to allow placing them into individual files
 * **a way to intercept git-http communication**
   * Maybe with a custom proxy as well, can't hurt to try APIs in real-world programs