fix writing via XRootD (called from uproot)

[maxgalli]

This PR fixes scikit-hep/uproot5#1252.
Touching (and truncating) a file on EOS (implemented here) needs the File interface in XRootD (as explained here).
After this, other changes were needed, especially to allow for the r+b mode used in uproot when opening a file. To do this, some functions such as seek, write, read where re-implemented instead of using the default ones in AbstractBufferedFile.

Note: I believe some of these changes do not follow the logic that was originally implemented, so indications in this sense are much appreciated.

[nsmith-]

Looks ok on first pass. Can you add tests for the new methods?

[maxgalli]

Looks ok on first pass. Can you add tests for the new methods?

How about adding a writing via XRootD test in uproot? That way I don't have to re-write those functions in a test here, but just add the minimal reproducer that we saw causing the crash as a test there. Also, I see that in the CI we run the uproot tests anyways.

[maxgalli]

can we maybe run the tests? I tried them locally and the only one failing seems to be tests/test_basicio.py::test_touch_modified. However, I tried with the main branch and it fails as well. Since in the last build in main it seems to pass in the CI, it makes me think there might be something different with my local setup.

[nsmith-]

For the test I was thinking just to make a variant of

fsspec-xrootd/tests/test_basicio.py

Lines 177 to 183 in d33c203

	def test_write_fsspec(localserver, clear_server):
	remoteurl, localpath = localserver
	with fsspec.open(remoteurl + "/testfile.txt", "wt") as f:
	f.write(TESTDATA1)
	f.flush()
	with open(localpath + "/testfile.txt") as f:
	assert f.read() == TESTDATA1

that tests the newly enabled r+ mode.

[nsmith-]

Even better if it seeks and overwrites a bit as well