Flaky test brotli/basic on focal amd64?

The brotli/basic test case periodically fails. It appears that a few retriggers can result in a pass:

    https://autopkgtest.ubuntu.com/packages/libs/libsoup2.4/focal/amd64

The failures look like this in autopkgtest logs:

    ERROR:../tests/brotli-decompressor-test.c:59:test_brotli: assertion failed ((char*)out_bytes->data == contents): ("***\nU" == "***\n")

I've replaced most of the text with '***' to highlight the difference is a 'U' character appended.

The test case is essentially doing a decompression of 'brotli-data/compressed.br' and comparing that it matches 'brotli-data/uncompressed.txt'. I've done this manually using the brotli cli, and the files do indeed have the same content. I've also built the package and run the test case locally in a focal lxc container on amd64:

    $ cd libsoup2.4-gu/debian/libsoup2.4-tests/usr/libexec/installed-tests/libsoup-2.4
    $ while : ; do ./brotli-decompressor-test | grep 'brotli/basic'; sleep 0.1; done

I left that to run several hundred cycles but no failures reported at all.

There are a bunch of instances of this failure, here's a few for reference:

    https://autopkgtest.ubuntu.com/results/autopkgtest-focal/focal/amd64/libs/libsoup2.4/20210602_152107_d1d56@/log.gz
    https://autopkgtest.ubuntu.com/results/autopkgtest-focal/focal/amd64/libs/libsoup2.4/20210531_201241_8b1be@/log.gz
    https://autopkgtest.ubuntu.com/results/autopkgtest-focal/focal/amd64/libs/libsoup2.4/20200508_211619_2aec8@/log.gz

That last link is the earliest instance I've been able to find.

You'll notice in the second two links they also have a failure "not ok 2 /ssl/tls-interaction", but the first link shows only the brotli failure.

Interestingly, I've not found instances of this failure against other architectures than amd64, nor have I seen it on releases other than focal. No dice from googling for bug reports or other reports of test failures with libsoup2.4 + brotli.

The test's code is doing this, basically:

        SoupBrotliDecompressor *dec = soup_brotli_decompressor_new ();
        do {
                result = g_converter_convert (G_CONVERTER (dec), in_buf, length, out_buf, sizeof out_buf, 0,
                                              &bytes_read, &bytes_written, &error);
                g_byte_array_append (out_bytes, out_buf, bytes_written);
  in_buf += bytes_read;
                length -= bytes_read;

 } while (result == G_CONVERTER_CONVERTED);

soup_brotli_decompressor's code is here:

    https://gitlab.gnome.org/GNOME/libsoup/-/blob/master/libsoup/content-decoder/soup-brotli-decompressor.c

But I'm not spotting anything that would obviously cause a 'U' to be randomly appended sometimes. And why 'U'? UINT? Unknown? User error?

There are some interesting bug reports in GNOME about brotli in libsoup's bug tracker (e.g. #106, #146, #119, and #193), but none that match this.