openpgp: Fix crash in the packet parser.

  - The packet parser hashes packet bodies to provide a robust
    equality relation even when packet bodies are streamed.  To hash
    all bytes on the fly everywhere, we do that when it is consumed in
    PacketParser::consume.

  - This function assumes that if BufferedReader::data and friends
    returned n bytes, future calls to these interfaces will succeed if
    up to n bytes are requested, and no data was consumed in the
    meantime.

  - However, armor::Reader::data_helper did not provide that
    guarantee, making PacketParser::consume panic with the message "It
    is an error to consume more than data returns", which doesn't
    quite correctly name the problem at hand.

  - Fix this crash by fixing armor::Reader::data_helper in the same
    way the previous commit fixes
    buffered_reader::Generic::data_helper.

  - Fixes #957.

2 files changed, 17 insertions, 6 deletions

diff --git a/openpgp/src/armor.rs b/openpgp/src/armor.rs
index fb6ac87b..ed19c63a 100644
--- a/openpgp/src/armor.rs
+++ b/openpgp/src/armor.rs
@@ -1534,12 +1534,6 @@ impl<'a> Reader<'a> {
            self.cursor,
            self.buffer.as_ref().map(|buffer| buffer.len()));
 
-        // See if there is an error from the last invocation.
-        if let Some(e) = self.error.take() {
-            t!("Returning stashed error: {}", e);
-            return Err(e);
-        }
-
         if let Some(ref buffer) = self.buffer {
             // We have a buffer.  Make sure `cursor` is sane.
             assert!(self.cursor <= buffer.len());
@@ -1575,6 +1569,12 @@ impl<'a> Reader<'a> {
                     break;
                 }
 
+                // See if there is an error from the last invocation.
+                if let Some(e) = &self.error {
+                    t!("We have a stashed error, don't poll again: {}", e);
+                    break;
+                }
+
                 match self.do_read(&mut buffer_new
                                    [amount_buffered + amount_read..]) {
                     Ok(read) => {
diff --git a/openpgp/src/parse.rs b/openpgp/src/parse.rs
index d414cbe4..2dc9fad5 100644
--- a/openpgp/src/parse.rs
+++ b/openpgp/src/parse.rs
@@ -6491,4 +6491,15 @@ zXvj
 -----END PGP SIGNATURE-----
 ");
     }
+
+    /// Tests issue 957.
+    #[test]
+    fn panic_on_malformed_armor() {
+        parse_message("-----BEGIN PGP MESSAGE-----
+
+heLBX8Pq0kUBwQz2iFAzRwOdgTBvH5KsDU9lmE
+
+-----END PGP MESSAGE-----
+");
+    }
 }