While looking for slow tests in CI, t4205 stood out on macOS: its EXPENSIVE
tests take several times longer there than on Linux. A CI experiment showed
that much of the time was spent in tr(1), which is very slow on macOS for
the 2GB of data that t4205 generates.

Patch 1 renames test-genzeros.c so that the actual changes in patch 2 show
up as a regular diff. Patch 2 adds "genbytes" and uses it instead of
"genzeros | tr", which is where the speedup comes from.

Open question: is patch 3 wanted? It folds "genzeros" into "genbytes",
spelling the NUL byte as "NUL", so that there is a single command.
Alternatively, we could drop it and keep "genzeros" as a thin wrapper,
avoiding changes to its 10 call sites. I kept patch 3 separate so that it is
easy to drop, but it could also be squashed into patch 2.

Some other decisions that may be worth pointing out:

 * The buffer is no longer static: it used to be static so that it was
   zero-initialized for free, but now that it is filled with the requested
   byte on every call, a local buffer avoids shared state. It keeps its size
   of 256KB. Smaller buffers made no measurable difference in my tests, but
   there was no reason to change it.

 * Invalid counts are still silently treated as 0, as before. That could be
   tightened, but it is unrelated to this series.

Kristofer Karlsson (3):
  t/helper: rename test-genzeros.c to test-genbytes.c
  t: add "test-tool genbytes" to avoid slow tr(1) on macOS
  t: replace "test-tool genzeros" with "genbytes NUL"

 Makefile                          |  2 +-
 t/helper/meson.build              |  2 +-
 t/helper/test-genbytes.c          | 64 +++++++++++++++++++++++++++++++
 t/helper/test-genzeros.c          | 37 ------------------
 t/helper/test-tool.c              |  2 +-
 t/helper/test-tool.h              |  2 +-
 t/t0613-reftable-write-options.sh |  2 +-
 t/t1007-hash-object.sh            |  8 ++--
 t/t1051-large-conversion.sh       |  4 +-
 t/t1450-fsck.sh                   |  4 +-
 t/t4141-apply-too-large.sh        |  2 +-
 t/t4205-log-pretty-formats.sh     |  2 +-
 t/t5300-pack-object.sh            |  6 +--
 t/t5318-commit-graph.sh           |  2 +-
 t/t7006-pager.sh                  |  3 +-
 t/t9850-shell.sh                  |  2 +-
 16 files changed, 85 insertions(+), 59 deletions(-)
 create mode 100644 t/helper/test-genbytes.c
 delete mode 100644 t/helper/test-genzeros.c

base-commit: fb255012acbcbe81189cb8014da0280b09c66ecc

Submitted-As: https://lore.kernel.org/git/pull.2301.git.1794063565.gitgitgadget@gmail.com