Fix BigTable multi-entry row cached Tail.

Fix BigTable multi-entry row cached Tail.

When performing a Tail, LogDog has to scan through all of a log's
BigTable rows to identify the last contiguous entry. In order to
optimize this, the BigTable storage implementation caches the last
contiguous tail index in memcache.

BigTable rows are keyed on their LAST log entry index, not their first,
so the cached log index was the last index in the row. If the row
contained multiple log entries, this violated an earlier continuity check,
which was expecting the first index in the row, not the last, to match the
cached index.

We fix this by caching the first index in the row, matching continuity
expectaitons.

BUG=chromium:678831
TEST=unit,local
  - Added regression unit test, passes now.
  - Use multi-round Tail on `logdog_bigtable_test` before and after,
    verify that it works now.

R=vadimsh@chromium.org

Review-Url: https://codereview.chromium.org/2611253005
Committed: https://github.com/luci/luci-go/commit/77c4eac9a4bf4c8818dbddcea7b91cba90a20d75

[dnj, 2017-01-06 18:50:36]

dnj@chromium.org changed reviewers:
+ estaab@chromium.org

[dnj, 2017-01-06 18:50:36]

PTAL

[dnj, 2017-01-06 18:51:41]

https://codereview.chromium.org/2611253005/diff/1/logdog/common/storage/bigta...
File logdog/common/storage/bigtable/logdog_bigtable_test/main.go (right):

https://codereview.chromium.org/2611253005/diff/1/logdog/common/storage/bigta...
logdog/common/storage/bigtable/logdog_bigtable_test/main.go:1: // Copyright 2016
The LUCI Authors. All rights reserved.
You can probably not care about this. It's just a local BigTable testing utility
that is useful for reproducing storage/bigtable issues against a live BigTable
instance. I added a "rounds" field and enabled cache so I could reproduce this
issue locally.

[Vadim Sh., 2017-01-06 20:31:34]

lgtm

[dnj, 2017-01-06 20:54:49]

The CQ bit was checked by dnj@chromium.org

[commit-bot: I haz the power, 2017-01-06 20:54:54]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-06 21:01:11]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1483736089780110, "parent_rev":
"3b9b350e96778cdf0bd6381a2b3c23503faed52d", "commit_rev":
"77c4eac9a4bf4c8818dbddcea7b91cba90a20d75"}

[commit-bot: I haz the power, 2017-01-06 21:01:37]

Description was changed from

==========
Fix BigTable multi-entry row cached Tail.

When performing a Tail, LogDog has to scan through all of a log's
BigTable rows to identify the last contiguous entry. In order to
optimize this, the BigTable storage implementation caches the last
contiguous tail index in memcache.

BigTable rows are keyed on their LAST log entry index, not their first,
so the cached log index was the last index in the row. If the row
contained multiple log entries, this violated an earlier continuity check,
which was expecting the first index in the row, not the last, to match the
cached index.

We fix this by caching the first index in the row, matching continuity
expectaitons.

BUG=chromium:678831
TEST=unit,local
  - Added regression unit test, passes now.
  - Use multi-round Tail on `logdog_bigtable_test` before and after,
    verify that it works now.

R=vadimsh@chromium.org
==========

to

==========
Fix BigTable multi-entry row cached Tail.

When performing a Tail, LogDog has to scan through all of a log's
BigTable rows to identify the last contiguous entry. In order to
optimize this, the BigTable storage implementation caches the last
contiguous tail index in memcache.

BigTable rows are keyed on their LAST log entry index, not their first,
so the cached log index was the last index in the row. If the row
contained multiple log entries, this violated an earlier continuity check,
which was expecting the first index in the row, not the last, to match the
cached index.

We fix this by caching the first index in the row, matching continuity
expectaitons.

BUG=chromium:678831
TEST=unit,local
  - Added regression unit test, passes now.
  - Use multi-round Tail on `logdog_bigtable_test` before and after,
    verify that it works now.

R=vadimsh@chromium.org

Review-Url: https://codereview.chromium.org/2611253005
Committed:
https://github.com/luci/luci-go/commit/77c4eac9a4bf4c8818dbddcea7b91cba90a20d75
==========

[commit-bot: I haz the power, 2017-01-06 21:01:37]

Committed patchset #1 (id:1) as
https://github.com/luci/luci-go/commit/77c4eac9a4bf4c8818dbddcea7b91cba90a20d75