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@AlekSi I cannot write the data directly into the crc32 hasher, I need to parse the whole message, check if the OpMsgChecksumPresent flag is present, detach the original checksum and compare it to the calculated checksum. See here https://www.mongodb.com/docs/manual/reference/mongodb-wire-protocol/#std-label-wire-msg-checksum

For this implementation to work, the data can first be read into a TeeReader, then the checksum is calculated, then the data is then read again from the TeeReader into a bufio.Reader so that it can used for the rest of the implementation. However, this is duplicated work

Sorry, I don't follow. TeeReader, MultiWriter, and other IO helpers like those two do not keep read or written data in memory. So there is no duplication of work.

I also don't think we should parse the whole message to calculate the checksum. After all, we calculate the checksum of bytes, not parsed data structure.

I think the algorithm could be something like that:

1. Read the message header.
2. If it does not contain ChecksumPresent, do change anything.
3. If it does contain that flag, create a TeeReader that wraps the existing reader and writes to the CRC hasher. Write read header bytes to hasher.
4. Use that reader to read bytes for parsing while also hashing them.
5. In the end, check the checksum.

@AlekSi I found a way around using a TeeReader and made use of the read bytes instead. Please kindly review and let me know what you think.

Codecov Report

Merging #2540 (15f94a8) into main (87b0a6e) will decrease coverage by 0.02%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##             main    #2540      +/-   ##
==========================================
- Coverage   66.31%   66.30%   -0.02%     
==========================================
  Files         419      419              
  Lines       20444    20480      +36     
==========================================
+ Hits        13558    13579      +21     
- Misses       5972     5982      +10     
- Partials      914      919       +5     

@AlekSi some of the tests are failing and I'm not sure why. My assumption is that some messages in the records do not contain valid checksums.

I will check it shortly