Rev pruning algorithm deletes entire revtree if only branch is a tombstoned branch

[tleyden]

Spun out of #2687, regression caused by #2669

Steps to repro:

1.Run test_sg_sdk_interop_shared_docs[sync_gateway_default_functional_tests-1-1000] against commit 85f79d0:

$ pytest -s --xattrs --mode=cc --server-version=5.0.0-3193 \
--sync-gateway-version=85f79d0f05ab63b20a5af62f703f128ce317ade5 \
-k "test_sg_sdk_interop_shared_docs[sync_gateway_default_functional_tests-1-1000]"  \ 
testsuites/syncgateway/functional/tests/

Pause python interpreter after updates, before deletes.

Using curl, try to delete latest rev:

curl -X DELETE http://192.168.33.11:4985/db/doc_set_one_0?rev=1930-b847c22efcc197ec966475261fb518a5

Get error:

{"error":"Internal Server Error","reason":"Internal error: WriteUpdateWithXattr() not able to find revision (1931-c46c29586616e553b3fdd640c06b0604) in history of doc: \u0026{syncData:{CurrentRev:1931-c46c29586616e553b3fdd640c06b0604 NewestRev: Flags:1 Sequence:4965 UnusedSequences:[] RecentSequences:[4849 4850 4853 4855 4856 4857 4859 4860 4862 4866 4965] History:map[] Channels:map[shared:0xc42c50bbe0] Access:map[] RoleAccess:map[] Expiry:\u003cnil\u003e Cas:0x00009d3650f1cb14 TombstonedAt:1498557550 UpstreamCAS:\u003cnil\u003e UpstreamRev: TimeSaved:2017-06-27 05:59:10.667782544 -0400 EDT Deleted_OLD:false} body:map[_deleted:true] ID:doc_set_one_0 Cas:1498556627846758400}.  Cannot update doc."}tleyden_macbook:~ tleyden$

[tleyden]

Note: cannot repro with higher revs_limit, seems related to rev pruning

[tleyden]

Reverting commit the rev pruning change (commit 4d4e4f2), the test passes. So it's confirmed to be caused by that commit.

[tleyden]

Debugging into this sync gateway error message:

CRUD+: Did not update document "doc_set_one_0" w/ xattr: WriteUpdateWithXattr() not able to find revision (201-814e75b628c1932548b252a628c131b2) in history of doc: &{syncData:{CurrentRev:201-814e75b628c1932548b252a628c131b2 NewestRev: Flags:1 Sequence:458 UnusedSequences:[] RecentSequences:[433 437 440 441 443 444 445 449 450 451 454 455 456 457 458] History:map[] Channels:map[shared:0xc420ae1c60] Access:map[] RoleAccess:map[] Expiry:<nil> Cas:0x0000a34dd9fccb14 TombstonedAt:1498569449 UpstreamCAS:<nil> UpstreamRev: TimeSaved:2017-06-27 09:17:29.162515565 -0400 EDT Deleted_OLD:false} body:map[_deleted:true] ID:doc_set_one_0 Cas:1498569311271452672}.  Cannot update doc.

The doc.History is completely empty, which is why it can't find the revision.

In the log line directly above that error, I'm seeing:

2017-06-27T09:17:29.162-04:00 CRUD+: updateDoc("doc_set_one_0"): Pruned 61 old revisions

and digging into that I can see that the entire revtree is getting pruned away when it consists of only a single tombstoned/deleted branch:

Before revtree pruning

After revtree pruning

After pruning there is no rev history whatsoever.

I would expect it to keep 60 revisions after the rev tree pruning, since revs_limit is currently set to 60 in the config.

[tleyden]

I just pushed the following changes up to the feature/issue_2695_sg_returns_500_err_delete branch:

• failing unit test that reproduces the issue
• fix that passes both unit test and functional test

Going to clean up the changes and push up a PR.