Fix the compressed BLFWriter

[j-c-cook]

closes #1378

[codecov]

Codecov Report

Merging #1379 (876dacb) into develop (2da28c1) will increase coverage by 0.01%.
The diff coverage is 90.90%.

❗ Current head 876dacb differs from pull request most recent head f64b6e7. Consider uploading reports for the commit f64b6e7 to get more accurate results

@@             Coverage Diff             @@
##           develop    #1379      +/-   ##
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+ Coverage    66.21%   66.22%   +0.01%     
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  Files           86       86              
  Lines         9021     9027       +6     
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+ Hits          5973     5978       +5     
- Misses        3048     3049       +1     

[j-c-cook]

@zariiii9003 The call self.file.seek(0) results in an OSError. It appears that the code block is not required if write mode wb is used (rather than ab). However, if the append option is requested, then some of this idea may be required.

[christiansandberg]

The header contains things that are only known after the log has been written, so that's why we need to go back and write it later. Previously we closed and reopened the file to update it, but it seemed unnecessary if it was possible to seek instead.

[j-c-cook]

Remaining tasks:

• If the append option is requested, then open the compressed Writer with append mode
• Get the compressed BLFWriter functional with the append option

[j-c-cook]

If the type of self.file is a gzip.GzipFile, then the self.file.tell() call appears to enter gzip.seek(0, 1). Does it need to be called twice?

[j-c-cook]

The current changes make it so that blf.gz files are written with a complete header. The solution is not long term, because it simply ensures that nothing is written until the rollover occurs.

import can
from datetime import datetime


def u_to_dt(ts):
    return datetime.utcfromtimestamp(ts).strftime("%Y-%m-%d %H:%M:%S")


def main():
    file_name = "private_2022-08-24T084948_#000.blf.gz"

    with can.LogReader(file_name) as reader:
        for m in reader:
            print(m.timestamp)

    reader = can.LogReader(file_name)
    print(
        f"start: {u_to_dt(reader.start_timestamp)} "
        f"stop: {u_to_dt(reader.stop_timestamp)}"
    )


if __name__ == "__main__":
    main()

[j-c-cook]

I have modified the description of issue #1378 and this PR now contains changes that are out of scope. A combination of #1383, #1384 and #1385 will cover the material previously intended to be addressed in this PR.