openpgp: Use larger default buffer size in DetachedVerifier.

  - The DetachedVerifier uses the Transformer to construct a signed
    OpenPGP message on the fly from the given detached signature and
    data.  The transformer synthesizes a literal data packet with
    partial body encoding, and the size of the chunks depends on the
    amount of data requested by the reader.

  - The transformer is wrapped in a buffered_reader::Generic to
    implement BufferedReader on top of io::Read.

  - Previously, we used the default preferred chunk size.  This makes
    the transformer construct partial body chunks of size 8k,
    resulting in very poor performance.

  - This changes the preferred chunk size to 4MB.

  - The proper fix is to implement BufferedReader for Transformer to
    avoid unnecessary buffering.

  - Fixes #457.

1 files changed, 4 insertions, 1 deletions

diff --git a/openpgp/src/parse/stream.rs b/openpgp/src/parse/stream.rs
index 3e695420..c81b5d32 100644
--- a/openpgp/src/parse/stream.rs
+++ b/openpgp/src/parse/stream.rs
@@ -994,9 +994,12 @@ impl DetachedVerifier {
         let t = t.into();
         Verifier::from_buffered_reader(
             policy,
+            // XXX: impl BufferedReader for Transformer to reduce
+            // buffering.
             Box::new(buffered_reader::Generic::with_cookie(
                 Transformer::new(signature_bio, reader)?,
-                None, Default::default())),
+                Some(1 << 22), // 4MB.
+                Default::default())),
             helper, t)
     }
 }