Try to extract group ID from In-Reply-To and References as a last resort

This should help if parent message can't be found because messages were
reordered or deleted.

This does not prevent group IDs from being removed from Message-IDs in
the future, in which case it will become dead code.

src/dc_tools.rs

@@ -279,6 +279,31 @@ pub(crate) fn dc_create_outgoing_rfc724_mid(grpid: Option<&str>, from_addr: &str
     }
 }
 
+/// Extract the group id (grpid) from a message id (mid)
+///
+/// # Arguments
+///
+/// * `mid` - A string that holds the message id.  Leading/Trailing <>
+/// characters are automatically stripped.
+pub(crate) fn dc_extract_grpid_from_rfc724_mid(mid: &str) -> Option<&str> {
+    let mid = mid.trim_start_matches('<').trim_end_matches('>');
+
+    if mid.len() < 9 || !mid.starts_with("Gr.") {
+        return None;
+    }
+
+    if let Some(mid_without_offset) = mid.get(3..) {
+        if let Some(grpid_len) = mid_without_offset.find('.') {
+            /* strict length comparison, the 'Gr.' magic is weak enough */
+            if grpid_len == 11 || grpid_len == 16 {
+                return Some(mid_without_offset.get(0..grpid_len).unwrap());
+            }
+        }
+    }
+
+    None
+}
+
 // the returned suffix is lower-case
 pub fn dc_get_filesuffix_lc(path_filename: impl AsRef<str>) -> Option<String> {
     Path::new(path_filename.as_ref())
@@ -760,18 +785,56 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_dc_extract_grpid_from_rfc724_mid() {
+        // Should return None if we pass invalid mid
+        let mid = "foobar";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, None);
+
+        // Should return None if grpid has a length which is not 11 or 16
+        let mid = "Gr.12345678.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, None);
+
+        // Should return extracted grpid for grpid with length of 11
+        let mid = "Gr.12345678901.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, Some("12345678901"));
+
+        // Should return extracted grpid for grpid with length of 11
+        let mid = "Gr.1234567890123456.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, Some("1234567890123456"));
+
+        // Should return extracted grpid for grpid with length of 11
+        let mid = "<Gr.12345678901.morerandom@domain.de>";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, Some("12345678901"));
+
+        // Should return extracted grpid for grpid with length of 11
+        let mid = "<Gr.1234567890123456.morerandom@domain.de>";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
+        assert_eq!(grpid, Some("1234567890123456"));
+    }
+
     #[test]
     fn test_dc_create_outgoing_rfc724_mid() {
         // create a normal message-id
         let mid = dc_create_outgoing_rfc724_mid(None, "foo@bar.de");
         assert!(mid.starts_with("Mr."));
         assert!(mid.ends_with("bar.de"));
+        assert!(dc_extract_grpid_from_rfc724_mid(mid.as_str()).is_none());
 
         // create a message-id containing a group-id
         let grpid = dc_create_id();
         let mid = dc_create_outgoing_rfc724_mid(Some(&grpid), "foo@bar.de");
         assert!(mid.starts_with("Gr."));
         assert!(mid.ends_with("bar.de"));
+        assert_eq!(
+            dc_extract_grpid_from_rfc724_mid(mid.as_str()),
+            Some(grpid.as_str())
+        );
     }
 
     #[test]