Improve emitted change feed sequence after a split

[nickva]

When we get sequence before the split, we'll fill in the missing (now split) ranges with a special {split, OldNodeUUid} marker. However, when sequences are emitted in the changes API, that will make the N- prefix (SeqSum) bounce around from higher to lower numbers, while users expect those to be mostly incrementing. So take a conservative approach and assume it will be a full rewind for that range, and use 0 instead. This is a purely cosmetic thing, when we decode the sequence that prefix gets thrown away anyway.

To emphasize that the sequence prefix is mostly a visual aid, rename the seq/1 function to fake_packed_seq/1 with a comment about what it does and a note about the special split case.

Fixes a part of issue: #4640

[nickva]

Using the script from #4640 to test it as well:

#!/bin/bash

CDB_AUTH='adm:pass'
CDB_HOST='127.0.0.1'
CDB_PORT=15984

cdb () {
  curl -gs -H 'Content-Type: application/json' "$(cdb-host)$1" "${@:2}"
}

cdb-host () {
  echo "http:https://${CDB_AUTH}@${CDB_HOST}:${CDB_PORT}"
}

stop-jobs () {
  cdb '/_reshard/jobs' | jq -r '.jobs[] | .id' | while read -r id ; do
    cdb "/_reshard/jobs/$id" -X DELETE
  done
}

split-shards () {
  local db="$1"

  cdb "/$db/_shards" | jq -r '.shards | keys[]' | while read -r range ; do
    cdb '/_reshard/jobs' -X POST \
        -d '{ "type": "split", "db": "'$db'", "range": "'$range'" }'
  done
}

wait-for-q () {
  local db="$1"
  local q="$2"

  while [[ $(cdb "/$db" | jq -r '.cluster.q') != $q ]] ; do
    sleep 1
  done
}

db='test-db'

cdb "/$db" -X DELETE
cdb "/$db?q=2" -X PUT

for n in {1..10} ; do
  cdb "/$db/doc-$n" -X PUT -d '{ "n": '$n' }'
done

last_seq_q2="$(cdb "/$db/_changes" | jq -r '.last_seq')"

stop-jobs
split-shards "$db"
wait-for-q "$db" 4

last_seq_q4="$(cdb "/$db/_changes" | jq -r '.last_seq')"

cdb "/$db/_changes?since=$last_seq_q2" | jq

stop-jobs
split-shards "$db"
wait-for-q "$db" 8

last_seq_q8="$(cdb "/$db/_changes" | jq -r '.last_seq')"

cdb "/$db/_changes?since=$last_seq_q4" | jq
cdb "/$db/_changes?since=$last_seq_q2" | jq

./repro.sh
{"error":"not_found","reason":"Database does not exist."}
{"ok":true}
{"ok":true,"id":"doc-1","rev":"1-731bef401491606a3b246ed178e697c1"}
{"ok":true,"id":"doc-2","rev":"1-43a36d04d31d38efc5b9245c53e00627"}
{"ok":true,"id":"doc-3","rev":"1-2acff5ec499bf1cf4a478896765b824e"}
{"ok":true,"id":"doc-4","rev":"1-2371fbbf6336dcb0ff571e1f4f449ad0"}
{"ok":true,"id":"doc-5","rev":"1-16599e9b349642aef785eac10b9f94a3"}
{"ok":true,"id":"doc-6","rev":"1-cd1f7574f3318036b66bb76bd4f7ed91"}
{"ok":true,"id":"doc-7","rev":"1-f996a2834eb8c362514d171f99324abd"}
{"ok":true,"id":"doc-8","rev":"1-817027c9296c2d2d688c568237e89624"}
{"ok":true,"id":"doc-9","rev":"1-46d26f2360296f940c344ef881b52066"}
{"ok":true,"id":"doc-10","rev":"1-6829757f6498d33021e0a764f0a492af"}
[{"ok":true,"id":"001-486d0ee2a74434c94bec8f9a7034842bd5882af99ea9edee32c9f8222ff916e8","node":"[email protected]","shard":"shards/00000000-7fffffff/test-db.1686875897"}]
[{"ok":true,"id":"001-bafee5292431febf513fedab86c793bf770513576f812b3f032d55aca0f1a83d","node":"[email protected]","shard":"shards/80000000-ffffffff/test-db.1686875897"}]
{
  "results": [],
  "last_seq": "20-g1AAAAEjeJzLYWBgYMlgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_f___7MymBNZcoEC7IbG5gZmhmaYynEakeQAJJPqUUxJSzY0tUgxJsGUPBYgydAApIAG7QeZxAY2ydjCOMUkKY1kkw5ATPqPMMk8JdnEMjEVU1cWACVXR8c",
  "pending": 0
}
{"ok":true}
{"ok":true}
[{"ok":true,"id":"001-c48499ca13a5a9602e6bf1a23c9f38744f71fa0bf9c4d949de3414f90e69b82f","node":"[email protected]","shard":"shards/00000000-3fffffff/test-db.1686875897"}]
[{"ok":true,"id":"001-a6b3d236b0e1e865dcd90fed71de21871828ebfa080ca9e80b3891f96a302515","node":"[email protected]","shard":"shards/40000000-7fffffff/test-db.1686875897"}]
[{"ok":true,"id":"001-d0c624fb555c48d3aa45b99750d5093703b9ea31ac3d69701995ccf9b70c9945","node":"[email protected]","shard":"shards/80000000-bfffffff/test-db.1686875897"}]
[{"ok":true,"id":"001-6e67e89cb8171afd46d59b9208ee69d2c8f80085ae320d1ced00e30139bec65e","node":"[email protected]","shard":"shards/c0000000-ffffffff/test-db.1686875897"}]
{  "results": [],
  "last_seq": "34-g1AAAAJDeJyV0FEKwjAMANDihvrnEfQE0s5lXb_cTbRZK2PMeQS9id5Eb6I3qZkV5hh-lEACafNo2jDG5lVk2KI9GSsKkcg1pxANHUw0w6Vzrq4iHR-pMUuMzfgBxtf_EriijNuBYkACbLIQpeiU3VeJPkoq-SYXZYiy75TzQJHaKmVD3tLGlNmFCkHXfivgkKMMl25euveS1oh5kgZLDy89O2nq_yhVtgQVLL285H4kQK6sGE_Vb5jejjE",
  "pending": 0
}
{  "results": [
    {
      "seq": "2-g1AAAAKJeJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBOZcoEC7EYpqWYGaaaYynEakaQAJJPsoaawZDClMLAWF-RkloDNM0k0NjK0TCXFPAeQefHUMy8BZF491czLYwGSDA1ACmjkfJCZbGhmJpqbJVumWZJs5gKImfupaeYBiJn3qWnmA4iZ_0kyMwsA0VGkAA",
      "id": "doc-3",
      "changes": [
        {
          "rev": "1-2acff5ec499bf1cf4a478896765b824e"
        }
      ]
    },
    {
      "seq": "3-g1AAAAJ_eJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBOZcoEC7EYpqWYGaaaYynEakaQAJJPsoaawZDClMLAWF-RkloDNM0k0NjK0TCXFPAeQefHUMy8BZF491czLYwGSDA1ACmjkfJCZbGhmJpqbJVumWZJs5gKImfupaeYBiJn3qWnmA4iZ4PBkhISiaZKBZaohpq4sAF0RoL0",
      "id": "doc-1",
      "changes": [
        {
          "rev": "1-731bef401491606a3b246ed178e697c1"
        }
      ]
    },
    {
      "seq": "5-g1AAAAJreJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBNZcoEC7EYpqWYGaaaYynEakaQAJJPsoaYwgE1JMTU3NTU2I8UUB5Ap8SimmJgbGFsYJpNiSgLIlHqYjzKYUhhYiwtyMksg5iUaGxlappJgXh4LkGRoAFJAI-eDzGRDMzPR3CzZMs2SZDMXQMzcT00zD0DMvE9NMx9AzASHJyMkFE2TDCxTDTF1ZQEAnFiaZA",
      "id": "doc-7",
      "changes": [
        {
          "rev": "1-f996a2834eb8c362514d171f99324abd"
        }
      ]
    },
    {
      "seq": "7-g1AAAAJreJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBNZcoEC7EYpqWYGaaaYynEakaQAJJPsoaYwgE1JMTU3NTU2I8UUB5Ap8SimmJgbGFsYJpNiSgLIlHqYjzKYUhhYiwtyMksg5iUaGxlappJgXh4LkGRoAFJAI-eDzGRDMzPR3CzZMs2SZDMXQMzcT00zD0DMvE9NMx9AzASHJzMkFE2TDCxTDTF1ZQEAnJiaZg",
      "id": "doc-5",
      "changes": [
        {
          "rev": "1-16599e9b349642aef785eac10b9f94a3"
        }
      ]
    },
    {
      "seq": "8-g1AAAAJheJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBNZcoEC7EYpqWYGaaaYynEakaQAJJPsoaYwgE1JMTU3NTU2I8UUB5Ap8SimmJgbGFsYJpNiSgLIlHqoKYxgU8wTUy0tU0lxSx4LkGRoAFJAg-aDTGLLYEphYC0uyMksAZuZaG6WbJlmSbKZCyBm7qemmQcgZt6nppkPIGaCQ5EZEhemSQaWqYaYurIASVGXpQ",
      "id": "doc-2",
      "changes": [
        {
          "rev": "1-43a36d04d31d38efc5b9245c53e00627"
        }
      ]
    },
    {
      "seq": "10-g1AAAAJheJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBNZcoEC7EYpqWYGaaaYynEakaQAJJPsoaYwgE1JMTU3NTU2I8UUB5Ap8SimmJgbGFsYJpNiSgLIlHqoKYxgU8wTUy0tU0lxSx4LkGRoAFJAg-aDTGLLYEphYC0uyMksAZuZaG6WbJlmSbKZCyBm7qemmQcgZt6nppkPIGaCQ5EVEhemSQaWqYaYurIASZGXpw",
      "id": "doc-9",
      "changes": [
        {
          "rev": "1-46d26f2360296f940c344ef881b52066"
        }
      ]
    },
    {
      "seq": "12-g1AAAAJheJzLYWBg4MhgTmHgz8tPSTV0MDQy1zMAQsMcoARTIkOS_P___7MymBNZcoEC7EYpqWYGaaaYynEakaQAJJPsoaYwgE1JMTU3NTU2I8UUB5Ap8SimmJgbGFsYJpNiSgLIlHqoKcxgU8wTUy0tU0lxSx4LkGRoAFJAg-aDTGLLYEphYC0uyMksAZuZaG6WbJlmSbKZCyBm7qemmQcgZt6nppkPIGaCQ5EVEhemSQaWqYaYurIATF2XqQ",
      "id": "doc-6",
      "changes": [
        {
          "rev": "1-cd1f7574f3318036b66bb76bd4f7ed91"
        }
      ]
    },
    {
      "seq": "18-g1AAAAJNeJyV0EsOwiAQBuBJH-rOI-gJDLSllJW9iUKhqU2tJrrXm-hN9CZ6k0qhiTGNC0IyJAN8-ZkGAGaVL2HeHqTCOY7oCumFG33gcRCLruvqyufBXjemkVQpKsn4-l9CLHUV60EBo0hCCYlTFyXvlc2PklAUZ7hwUba9chkU3yiUK8aUS5Y20BWuetPQrZcmlSchPB2b3dmYnKYFK5mzebfm4_tHzoXIosRZelrpZdLZaSVMFcQ909tKZmKhlYhATOHxq_oDFP6Rjg",
      "id": "doc-10",
      "changes": [
        {
          "rev": "1-6829757f6498d33021e0a764f0a492af"
        }
      ]
    },
    {
      "seq": "20-g1AAAAJDeJyV0N0NgjAQAOAL4M-bI-gEpgWO0ifZRFtaQwjiCLqJbqKb6CbYHxJjiA_NJXfJtf3Suw4Alk2sYNWflaYVTdmWmKCdOYgEyPUwDG0Ti-RkGotU6YIccXr9LyE3JsvdqIBTFDLErAhRKqvsf5SckaykdYhysMplVGKnMKE51yF_6ROT4WqKgW5WipyEBEvJwqW7lx7fyYSQskzzYOnppZeV5n5HOdc18mDp7SW3p5mXUBKu6fRV-wGH-I4j",
      "id": "doc-4",
      "changes": [
        {
          "rev": "1-2371fbbf6336dcb0ff571e1f4f449ad0"
        }
      ]
    },
    {
      "seq": "22-g1AAAAJDeJyV0N0NgjAQB_CL4MebI-gEpgWO0ifZRHu0hhDEEXQT3UQ30U2wUBJiiA9Nk2vSa3-5_msAWJWBhnVz0YbnPBI7ZhevbWOmgDZt21ZloMKzPVhG2qTshNPrfwna2kr7QYFe0SgQ49RHyTvl8KMkgsUZL3yUY6dcByXoFaGMlMZnlia0FW52s9B9zAYZZiT8pYeTnuPPlCLKosRbejnp3UkLl1EiTYHSW_o4qc9p7iQkJg2fvqq-ifSOJQ",
      "id": "doc-8",
      "changes": [
        {
          "rev": "1-817027c9296c2d2d688c568237e89624"
        }
      ]
    }
  ],
  "last_seq": "22-g1AAAAJDeJyV0N0NgjAQB_CL4MebI-gEpgWO0ifZRHu0hhDEEXQT3UQ30U2wUBJiiA9Nk2vSa3-5_msAWJWBhnVz0YbnPBI7ZhevbWOmgDZt21ZloMKzPVhG2qTshNPrfwna2kr7QYFe0SgQ49RHyTvl8KMkgsUZL3yUY6dcByXoFaGMlMZnlia0FW52s9B9zAYZZiT8pYeTnuPPlCLKosRbejnp3UkLl1EiTYHSW_o4qc9p7iQkJg2fvqq-ifSOJQ",
  "pending": 0
}