feat: Don't ignore receive_imf_inner() errors, try adding partially downloaded message instead (#7196)

Ignoring `receive_imf_inner()` errors, i.e. silently skipping messages on failures, leads to bugs
never fixed. As for temporary I/O errors, ignoring them leads to lost messages, in this case it's
better to bubble up the error and get the IMAP loop stuck. However if there's some logic error, it's
better to show it to the user so that it's more likely reported, and continue receiving messages. To
distinguish these cases, on error, try adding the message as partially downloaded with the error set
to `msgs.error`, this way the user also can retry downloading the message to finally see it if the
problem is fixed.

src/config.rs

@@ -450,6 +450,9 @@ pub enum Config {
     /// to avoid encrypting it differently and
     /// storing the same token multiple times on the server.
     EncryptedDeviceToken,
+
+    /// Return an error from `receive_imf_inner()` for a fully downloaded message. For tests.
+    FailOnReceivingFullMsg,
 }
 
 impl Config {

src/context.rs

@@ -1079,6 +1079,13 @@ impl Context {
                 .await?
                 .unwrap_or_default(),
         );
+        res.insert(
+            "fail_on_receiving_full_msg",
+            self.sql
+                .get_raw_config("fail_on_receiving_full_msg")
+                .await?
+                .unwrap_or_default(),
+        );
 
         let elapsed = time_elapsed(&self.creation_time);
         res.insert("uptime", duration_to_str(elapsed));

src/download.rs

@@ -238,14 +238,20 @@ impl MimeMessage {
     /// the mime-structure itself is not available.
     ///
     /// The placeholder part currently contains a text with size and availability of the message;
+    /// `error` is set as the part error;
     /// in the future, we may do more advanced things as previews here.
     pub(crate) async fn create_stub_from_partial_download(
         &mut self,
         context: &Context,
         org_bytes: u32,
+        error: Option<String>,
     ) -> Result<()> {
+        let prefix = match error {
+            None => "",
+            Some(_) => "[❗] ",
+        };
         let mut text = format!(
-            "[{}]",
+            "{prefix}[{}]",
             stock_str::partial_download_msg_body(context, org_bytes).await
         );
         if let Some(delete_server_after) = context.get_config_delete_server_after().await? {
@@ -259,9 +265,10 @@ impl MimeMessage {
 
         info!(context, "Partial download: {}", text);
 
-        self.parts.push(Part {
+        self.do_add_single_part(Part {
             typ: Viewtype::Text,
             msg: text,
+            error,
             ..Default::default()
         });
 

src/imap.rs

@@ -1480,7 +1480,7 @@ impl Session {
                     context,
                     "Passing message UID {} to receive_imf().", request_uid
                 );
-                match receive_imf_inner(
+                let res = receive_imf_inner(
                     context,
                     folder,
                     uidvalidity,
@@ -1488,20 +1488,31 @@ impl Session {
                     rfc724_mid,
                     body,
                     is_seen,
-                    partial,
+                    partial.map(|msg_size| (msg_size, None)),
                 )
-                .await
-                {
-                    Ok(received_msg) => {
-                        received_msgs_channel
-                            .send((request_uid, received_msg))
-                            .await?;
-                    }
-                    Err(err) => {
-                        warn!(context, "receive_imf error: {:#}.", err);
-                        received_msgs_channel.send((request_uid, None)).await?;
+                .await;
+                let received_msg = if let Err(err) = res {
+                    warn!(context, "receive_imf error: {:#}.", err);
+                    if partial.is_some() {
+                        return Err(err);
                     }
+                    receive_imf_inner(
+                        context,
+                        folder,
+                        uidvalidity,
+                        request_uid,
+                        rfc724_mid,
+                        body,
+                        is_seen,
+                        Some((body.len().try_into()?, Some(format!("{err:#}")))),
+                    )
+                    .await?
+                } else {
+                    res?
                 };
+                received_msgs_channel
+                    .send((request_uid, received_msg))
+                    .await?;
             }
 
             // If we don't process the whole response, IMAP client is left in a broken state where

src/mimeparser.rs

@@ -240,12 +240,12 @@ const MIME_AC_SETUP_FILE: &str = "application/autocrypt-setup";
 impl MimeMessage {
     /// Parse a mime message.
     ///
-    /// If `partial` is set, it contains the full message size in bytes
-    /// and `body` contains the header only.
+    /// If `partial` is set, it contains the full message size in bytes and an optional error text
+    /// for the partially downloaded message, and `body` contains the HEADER only.
     pub(crate) async fn from_bytes(
         context: &Context,
         body: &[u8],
-        partial: Option<u32>,
+        partial: Option<(u32, Option<String>)>,
     ) -> Result<Self> {
         let mail = mailparse::parse_mail(body)?;
 
@@ -612,9 +612,9 @@ impl MimeMessage {
         };
 
         match partial {
-            Some(org_bytes) => {
+            Some((org_bytes, err)) => {
                 parser
-                    .create_stub_from_partial_download(context, org_bytes)
+                    .create_stub_from_partial_download(context, org_bytes, err)
                     .await?;
             }
             None => match mail {
@@ -634,7 +634,7 @@ impl MimeMessage {
                         error: Some(format!("Decrypting failed: {err:#}")),
                         ..Default::default()
                     };
-                    parser.parts.push(part);
+                    parser.do_add_single_part(part);
                 }
             },
         };
@@ -1543,7 +1543,7 @@ impl MimeMessage {
         Ok(true)
     }
 
-    fn do_add_single_part(&mut self, mut part: Part) {
+    pub(crate) fn do_add_single_part(&mut self, mut part: Part) {
         if self.was_encrypted() {
             part.param.set_int(Param::GuaranteeE2ee, 1);
         }

src/receive_imf.rs

@@ -196,7 +196,7 @@ pub(crate) async fn receive_imf_from_inbox(
         rfc724_mid,
         imf_raw,
         seen,
-        is_partial_download,
+        is_partial_download.map(|msg_size| (msg_size, None)),
     )
     .await
 }
@@ -494,9 +494,8 @@ async fn get_to_and_past_contact_ids(
 /// If the message is so wrong that we didn't even create a database entry,
 /// returns `Ok(None)`.
 ///
-/// If `is_partial_download` is set, it contains the full message size in bytes.
-/// Do not confuse that with `replace_msg_id` that will be set when the full message is loaded
-/// later.
+/// If `partial` is set, it contains the full message size in bytes and an optional error text for
+/// the partially downloaded message.
 #[expect(clippy::too_many_arguments)]
 pub(crate) async fn receive_imf_inner(
     context: &Context,
@@ -506,7 +505,7 @@ pub(crate) async fn receive_imf_inner(
     rfc724_mid: &str,
     imf_raw: &[u8],
     seen: bool,
-    is_partial_download: Option<u32>,
+    partial: Option<(u32, Option<String>)>,
 ) -> Result<Option<ReceivedMsg>> {
     if std::env::var(crate::DCC_MIME_DEBUG).is_ok() {
         info!(
@@ -515,9 +514,16 @@ pub(crate) async fn receive_imf_inner(
             String::from_utf8_lossy(imf_raw),
         );
     }
+    if partial.is_none() {
+        ensure!(
+            !context
+                .get_config_bool(Config::FailOnReceivingFullMsg)
+                .await?
+        );
+    }
 
-    let mut mime_parser = match MimeMessage::from_bytes(context, imf_raw, is_partial_download).await
-    {
+    let is_partial_download = partial.as_ref().map(|(msg_size, _err)| *msg_size);
+    let mut mime_parser = match MimeMessage::from_bytes(context, imf_raw, partial).await {
         Err(err) => {
             warn!(context, "receive_imf: can't parse MIME: {err:#}.");
             if rfc724_mid.starts_with(GENERATED_PREFIX) {
@@ -551,22 +557,11 @@ pub(crate) async fn receive_imf_inner(
     // make sure, this check is done eg. before securejoin-processing.
     let (replace_msg_id, replace_chat_id);
     if let Some((old_msg_id, _)) = message::rfc724_mid_exists(context, rfc724_mid).await? {
-        if is_partial_download.is_some() {
-            // Should never happen, see imap::prefetch_should_download(), but still.
-            info!(
-                context,
-                "Got a partial download and message is already in DB."
-            );
-            return Ok(None);
-        }
         let msg = Message::load_from_db(context, old_msg_id).await?;
         replace_msg_id = Some(old_msg_id);
         replace_chat_id = if msg.download_state() != DownloadState::Done {
             // the message was partially downloaded before and is fully downloaded now.
-            info!(
-                context,
-                "Message already partly in DB, replacing by full message."
-            );
+            info!(context, "Message already partly in DB, replacing.");
             Some(msg.chat_id)
         } else {
             None