add check_trailing_bits: false

While testing message parsing code, there's a chance that the sender
isn't fully compliant with base64.  This leads to failure such as below

base64 decode: non-zero trailing bits at 20211 b='i' in HRtbD4NCi==

As long as it's not destructive, its more convenient to be able to
support non-RFC messages so we can extract message artifacts for its
decision making.

closes: https://github.com/KumoCorp/kumomta/pull/558

crates/mailparsing/src/mimepart.rs

@@ -16,13 +16,17 @@ use std::str::FromStr;
 use std::sync::Arc;
 
 /// Define our own because data_encoding::BASE64_MIME, despite its name,
-/// is not RFC2045 compliant, and will not ignore spaces
+/// is not RFC2045 compliant, and will not ignore spaces.
+/// check_trailing_bits is disabled because real-world MIME producers emit
+/// final quantums whose discarded padding bits are non-zero; a strict decoder
+/// would reject those otherwise-decodable bodies.
 const BASE64_RFC2045: data_encoding::Encoding = data_encoding_macro::new_encoding! {
     symbols: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
     padding: '=',
     ignore: " \r\n\t",
     wrap_width: 76,
     wrap_separator: "\r\n",
+    check_trailing_bits: false,
 };
 
 #[derive(Debug, Clone, PartialEq)]
@@ -2282,4 +2286,16 @@ Body\r
             }
         );
     }
+
+    /// Test the use case of check_trailing_bits being false.
+    /// This accepts a final quantum whose discarded padding bits are non-zero,
+    /// which a strict base64 decoder would reject.
+    /// For reference, valid base64 is aHRtbD4NCg==
+    #[test]
+    fn check_trailing_bits_test() {
+        // The low 4 trailing bits before `==` are non-zero in this sample.
+        // With check_trailing_bits=false we should still accept and decode it.
+        let decoded = BASE64_RFC2045.decode(b"aHRtbD4NCi==").unwrap();
+        assert_eq!(decoded, b"html>\r\n");
+    }
 }

docs/changelog/main.md

@@ -202,6 +202,11 @@
    startup, that would otherwise lead to rocksdb corrupting itself on
    the restart *after* the permissions were broken.
 
+ * Base64-encoded MIME bodies whose final quantum has non-zero discarded
+   padding bits are now decoded rather than rejected, matching the
+   leniency other mail software applies to such non-canonical encodings.
+   Thanks to @kayozaki! #558
+
 ## Fixes
 
  * An SMTP command line containing bytes that are not valid UTF-8 is now
@@ -234,3 +239,4 @@
    after it was first persisted are now written back to the meta spool,
    rather than being silently dropped on the next reload from spool.
    Thanks to @alexanderwburch! #570
+