Refactor tag assertion to FatalError check & Fix unexpected increase of tag

src/client.rs

@@ -8,7 +8,7 @@ use std::str;
 use std::sync::mpsc;
 
 use super::authenticator::Authenticator;
-use super::error::{Bad, Bye, Error, No, ParseError, Result, ValidateError};
+use super::error::{Bad, Bye, Error, No, ParseError, Result, TagCorrupted, ValidateError};
 use super::extensions;
 use super::parse::*;
 use super::types::*;
@@ -1345,7 +1345,11 @@ impl<T: Read + Write> Connection<T> {
 
     fn run_command(&mut self, untagged_command: &str) -> Result<()> {
         let command = self.create_command(untagged_command);
-        self.write_line(command.into_bytes().as_slice())
+        let result = self.write_line(command.into_bytes().as_slice());
+        if result.is_err() {
+            self.tag -= 1; // rollback tag increase in create_command()
+        }
+        result
     }
 
     fn run_command_and_read_response(&mut self, untagged_command: &str) -> Result<Vec<u8>> {
@@ -1397,7 +1401,17 @@ impl<T: Read + Write> Connection<T> {
                             ..
                         },
                     )) => {
-                        assert_eq!(tag.as_bytes(), match_tag.as_bytes());
+                        // check if tag matches
+                        if tag.as_bytes() != match_tag.as_bytes() {
+                            let expect = self.tag;
+                            let actual = tag
+                                .0
+                                .trim_start_matches(TAG_PREFIX)
+                                .parse::<u32>()
+                                .unwrap_or_default();
+                            break Err(Error::TagCorrupted(TagCorrupted { expect, actual }));
+                        }
+
                         Some(match status {
                             Status::Bad | Status::No | Status::Bye => Err((
                                 status,

src/error.rs

@@ -104,6 +104,8 @@ pub enum Error {
     /// In response to a STATUS command, the server sent OK without actually sending any STATUS
     /// responses first.
     MissingStatusResponse,
+    /// Tag mismatch between client and server. New session must be created.
+    TagCorrupted(TagCorrupted),
 }
 
 impl From<IoError> for Error {
@@ -170,6 +172,7 @@ impl fmt::Display for Error {
             Error::Append => f.write_str("Could not append mail to mailbox"),
             Error::Unexpected(ref r) => write!(f, "Unexpected Response: {:?}", r),
             Error::MissingStatusResponse => write!(f, "Missing STATUS Response"),
+            Error::TagCorrupted(ref data) => write!(f, "Mismatched Tag: {:?}", data),
         }
     }
 }
@@ -194,6 +197,7 @@ impl StdError for Error {
             Error::Append => "Could not append mail to mailbox",
             Error::Unexpected(_) => "Unexpected Response",
             Error::MissingStatusResponse => "Missing STATUS Response",
+            Error::TagCorrupted(ref e) => e.description(),
         }
     }
 
@@ -207,6 +211,7 @@ impl StdError for Error {
             #[cfg(feature = "native-tls")]
             Error::TlsHandshake(ref e) => Some(e),
             Error::Parse(ParseError::DataNotUtf8(_, ref e)) => Some(e),
+            Error::TagCorrupted(ref e) => Some(e),
             _ => None,
         }
     }
@@ -285,6 +290,36 @@ impl StdError for ValidateError {
     }
 }
 
+/// Tag was corrupted during session.
+#[derive(Debug)]
+#[non_exhaustive]
+pub struct TagCorrupted {
+    /// Expected tag number
+    pub(crate) expect: u32,
+    /// Actual tag number, 0 if parse failed
+    pub(crate) actual: u32,
+}
+
+impl fmt::Display for TagCorrupted {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Expected tag number is {}, actual {}",
+            self.expect, self.actual
+        )
+    }
+}
+
+impl StdError for TagCorrupted {
+    fn description(&self) -> &str {
+        "Tag is corrupted, session is in an inconsistent state"
+    }
+
+    fn cause(&self) -> Option<&dyn StdError> {
+        None
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;