[PATCH 3/4] t5539: make timestamp requirements more explicit

The test for "no shallow lines after receiving ACK ready" is very sensitive to the timestamps of the commits we create. It's looking for the fetch negotiation to send a "ready", which in turn depends on the order in which we traverse commits during the negotiation.

It works reliably now because the base commit "7" is created without test_commit, and thus gets a commit time matching the current system clock. Whereas the new commits created in this test do use test_commit, and get the usual test_tick time from 2005. So the fetch into the "clone" repository results in a commit graph like this (I omitted some of the "unrelated" commits for clarity; they're all just a sequence of test_ticks):

  $ git log --graph --format='%ct %s %d'
  * 1112912953 new  (origin/master, origin/HEAD)
  * 1594322236 7  (grafted, master)
  * 1112912893 unrelated15  (origin/unrelated15, unrelated15)
  [...]
  * 1112912053 unrelated1  (origin/unrelated1, unrelated1)
  * 1112911993 new-too  (HEAD -> newnew, tag: new-too)

The important things to see are:

  - "7" is way in the future compared to the other commits

  - "new-too" in the fetching repo is older than "new" (and its
    "unrelated" ancestors) in the shallow repo

If we change our "setup shallow clone" step to use test_tick, too (and get rid of the dependency on the system clock), then the test will fail. The resulting graph looks like this:

  $ git log --graph --format='%ct %s %d'
  * 1112913373 new  (origin/master, origin/HEAD)
  * 1112912353 7  (grafted, master)
  * 1112913313 unrelated15  (origin/unrelated15, unrelated15)
  [...]
  * 1112912473 unrelated1  (origin/unrelated1, unrelated1)
  * 1112912413 new-too  (HEAD -> newnew, tag: new-too)

Our "new-too" is still older than "new" and "unrelated", but now "7" is older than all of them (because it advanced test_tick, which the other tests built on top of). In the original, we advertised "7" as the first "have" before anything else, but now "new-too" is more recent. You'd see the same thing in the unlikely event that the system clock was set before our test_tick default in 2005.

Let's make the timing requirements more explicit. The important thing is that the client advertise all of its shared commits first, before presenting its unique "new-too" commit. We can do that and get rid of the system clock dependency at the same time by creating all of the shared commits around time X (using test_tick), and then creating "new-too" with some time long before X. The resulting graph looks like this:

  $ git log --graph --format='%ct %s %d'
  * 1500001380 new  (origin/master, origin/HEAD)
  * 1500000420 7  (grafted, master)
  * 1500001320 unrelated15  (origin/unrelated15, unrelated15)
  [...]
  * 1500000480 unrelated1  (origin/unrelated1, unrelated1)
  * 1400000060 new-too  (HEAD -> newnew, tag: new-too)

That also lets us get rid of the hacky test_tick added by f0e802ca20 (t5539: update a flaky test, 2014-07-14). That was clearly dancing around the same problem, but only addressed the relationship between commits created in the two subshells (which did use test_tick, but overlapped because increments of test_tick in subshells are lost). Now that we're using consistent and well-placed times for both lines of history, we don't have to care about a one-tick difference between the two sides.

Signed-off-by: Jeff King <peff@peff.net>
---
Curiously, the test still passes if "7" has the same timestamp as
"new-too"! That's why I didn't notice the failure in the original
thread, where I looked at setting the deterministic default time to the
default test_tick time. I didn't dig in, but I believe it only worked
because we currently use insertion order to break timestamp ties. And
since "master" sorts before "newnew", we'd queue "7" before "new-too".

 t/t5539-fetch-http-shallow.sh | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)