Reproducible "builds"

[hudvin]

I am trying to get some sort of reproducible builds. I have pipeline to process pdf docs (split on separate pages, process them one by one, merge everything back in single document).
But if I send same file several times through pipeline I get output files with different hashes.

Some observations:

1. ID in trailer section is different.
2. Probably timestamp in metadata.
3. Random suffixes in FormXob sections

Is there any way to get exactly the same files after each run?

[JorjMcKie]

First of all, it's not a bug, but a feature.
I am going to convert your post to a Dicsussions item.
By PDF specification, the second array entry of the trailer /ID key must be updated on every save. MuPDF uses a random number generator to compute both /ID items when saving to a new file. For incremental saves, just the second item is updated.
Observations 2. & 3. are probably caused by your own app.

Is there any way to get exactly the same files after each run?

Shor answer is "no".
But you can approximate an equality check along these lines:

    # old_output = previous document version
    # new_output = current document
    assert old_output.xref_length() == new_output.xref_length()  # same number of PDF objects
    for xref in range(1, old_output.xref_length()):  # same object definition for each xref
        assert old_output.xref_object(xref, compressed=True) == new_output.xref_object(
            xref, compressed=True
        )
    assert old_output.xref_get_keys(-1) == new_output.xref_get_keys(-1)  # same PDF keys in trailer

If all of the above assertions work, there is an overwhelming probability that it is the same file.

[JorjMcKie]

Another option might be to locate the start of the PDF trailer in old and new versions (via Python .find()) and compute hashes up to these positions in the files.

[hudvin]

I managed to fix issue with ID in trailer. But in some documents I also have structures like
/FormXob.541847dbcfabf322fd53ec1ad48a68ca Do
and looks like numerical part is also random. I mean after two runs I have two documents with different /FormXob.<some_id>
I will check everything step by step to provide additional details.

[JorjMcKie]

/FormXob.541847dbcfabf322fd53ec1ad48a68ca Do

This is not done in PyMuPDF code. I remember I did that sort of thing years ago, but not in any recent version.