Fix handling of terminal sparse output block

[thomas]

Summary

This change reorganizes the handling of sparse output, in the
case where the last block in a file is a block of zeroes.

The intent of the previous code in that case was to force
an explicit write, but the implementation was incorrect, and
as a result the write was never performed. This new implementation
instead uses ftruncate(2) to extend the file with a trailing hole.

PR: 189284
(original PR whose fix introduced this bug)

PR: 207092

Test Plan

New test cases added to make test, providing regression tests for
both PRs.

[sobomax]

Looks good to me, thanks!

bin/dd/dd.c

471 ↗

(On Diff #13192)

I am not a style(9) expert, but I think you might need to put blank "/*" and start comment on the next line, i.e.

/*
 * Seek past hole.  Note that we need to record the

[thomas]

style(9) fix as noted

[kib]

Is sparse copy to the devices supposed to work ? I expect that ftruncate(2) on special nodes results in EINVAL.

[thomas]

Do not call ftruncate unless output is a regular file

(In the case where output is a device, ftruncate would fail,
but is useless in the first place as it won't affect its size.)