Use Auto-Submitted: auto-generated header to identify bots

New `dc_msg_is_bot()` C API and corresponding `Message.is_bot()`
Python API can be used to check if incoming message is sent by a bot,
e.g. to avoid two echo bots replying indefinitely to each other.

"Bot" flag is not set for outgoing messages, but may be set for
BCC-self messages. For now documentation says that `dc_msg_is_bot()`
return value is unspecified for outgoing messages. It can be better
specified later if needed for specific applications, e.g. sharing an
account with a helper bot.

src/message.rs

@@ -601,6 +601,11 @@ impl Message {
         self.param.get_int(Param::GuaranteeE2ee).unwrap_or_default() != 0
     }
 
+    /// Returns true if message is Auto-Submitted.
+    pub fn is_bot(&self) -> bool {
+        self.param.get_bool(Param::Bot).unwrap_or_default()
+    }
+
     pub fn get_ephemeral_timer(&self) -> EphemeralTimer {
         self.ephemeral_timer
     }
@@ -2868,4 +2873,50 @@ mod tests {
 
         Ok(())
     }
+
+    #[async_std::test]
+    async fn test_is_bot() -> Result<()> {
+        let alice = TestContext::new_alice().await;
+
+        // Alice receives a message from Bob the bot.
+        dc_receive_imf(
+            &alice,
+            b"From: Bob <bob@example.com>\n\
+                    To: alice@example.com\n\
+                    Chat-Version: 1.0\n\
+                    Message-ID: <123@example.com>\n\
+                    Auto-Submitted: auto-generated\n\
+                    Date: Fri, 29 Jan 2021 21:37:55 +0000\n\
+                    \n\
+                    hello\n",
+            "INBOX",
+            1,
+            false,
+        )
+        .await?;
+        let msg = alice.get_last_msg().await;
+        assert_eq!(msg.get_text().unwrap(), "hello".to_string());
+        assert!(msg.is_bot());
+
+        // Alice receives a message from Bob who is not the bot anymore.
+        dc_receive_imf(
+            &alice,
+            b"From: Bob <bob@example.com>\n\
+                    To: alice@example.com\n\
+                    Chat-Version: 1.0\n\
+                    Message-ID: <456@example.com>\n\
+                    Date: Fri, 29 Jan 2021 21:37:55 +0000\n\
+                    \n\
+                    hello again\n",
+            "INBOX",
+            2,
+            false,
+        )
+        .await?;
+        let msg = alice.get_last_msg().await;
+        assert_eq!(msg.get_text().unwrap(), "hello again".to_string());
+        assert!(!msg.is_bot());
+
+        Ok(())
+    }
 }

src/mimefactory.rs

@@ -491,6 +491,11 @@ impl<'a> MimeFactory<'a> {
                 "Auto-Submitted".to_string(),
                 "auto-replied".to_string(),
             ));
+        } else if context.get_config_bool(Config::Bot).await? {
+            headers.unprotected.push(Header::new(
+                "Auto-Submitted".to_string(),
+                "auto-generated".to_string(),
+            ));
         }
 
         if self.req_mdn {

src/mimeparser.rs

@@ -498,6 +498,12 @@ impl MimeMessage {
 
             self.parts.push(part);
         }
+
+        if self.header.contains_key("auto-submitted") {
+            for part in &mut self.parts {
+                part.param.set(Param::Bot, "1");
+            }
+        }
     }
 
     async fn avatar_action_from_header(

src/param.rs

@@ -60,6 +60,9 @@ pub enum Param {
     /// For Messages
     WantsMdn = b'r',
 
+    /// For Messages: a message with Auto-Submitted header ("bot").
+    Bot = b'b',
+
     /// For Messages: unset or 0=not forwarded,
     /// 1=forwarded from unknown msg_id, >9 forwarded from msg_id
     Forwarded = b'a',