Marshal the value into the null value if the slot is not migrating

[hll1213181368]

we read from an old cluster that had -1 denoting !isMigrating, but new cluster migrating_slot value is string type. so we should return "-1" is not -1.

[git-hulk]

@hll1213181368 Can you share which version(or commit id) you're using? I cannot find which version was using the string type for the migration slot.

[hll1213181368]

@hll1213181368 Can you share which version(or commit id) you're using? I cannot find which version was using the string type for the migration slot.

@git-hulk

in the slot.go file. The function of MigrateSlot's String() returns string type to caller. The cases are following:
1）return null if new version is not migrating.
2）return string if new version is migrating.
3）return "-1" if old version is not migrating.

So I think, we should return string to caller.

[hll1213181368]

@git-hulk Please review my pull. I modify the cluster is not migrating to return nil. Compatibly with old version return -1.

[greatsharp]

int -1 is used before @bseto chaning migrate single slot to SlotRange,
suggest to return "migrating_slot":null when fetching old cluster info with -1 from storage,
and suggest to update old cluster info with -1 back into storage with "migrating_slot":null too.

[git-hulk]

@greatsharp @hll1213181368 I'm sorry that I cannot get what's going wrong in your case.

What your change is from -1 to null when storing, but the PR title is fixing the compatibility issue.

[hll1213181368]

@greatsharp @hll1213181368 I'm sorry that I cannot get what's going wrong in your case.

What your change is from -1 to null when storing, but the PR title is fixing the compatibility issue.

@git-hulk 我直接中文交流吧。
之前旧版本单slot迁移的时候，migrating_slot返回字段值为整型，不处于迁移中，migrating_slot返回-1，这些信息保存在etcd中。

现在新版本支持slot范围迁移，migrating_slot返回的是字符串，例如：正在迁移1024-2048区间段，返回的是“1024-2048”；如果不处于迁移中，返回的是null。

现在为了兼容之前旧集群保存在etcd中的migrating_slot为-1信息。针对新旧集群，如果不处于迁移中，都返回nil。旧集群在新版本控制器做切换更新拓扑后，会把之前-1的更改为nil。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.30%. Comparing base (6c56470) to head (2fc56e6).
⚠️ Report is 105 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #346      +/-   ##
============================================
+ Coverage     43.38%   47.30%   +3.91%     
============================================
  Files            37       45       +8     
  Lines          2971     4446    +1475     
============================================
+ Hits           1289     2103     +814     
- Misses         1544     2135     +591     
- Partials        138      208      +70     

Flag	Coverage Δ
unittests	47.30% <100.00%> (+3.91%)	⬆️

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[hll1213181368]

@git-hulk Please review my PR.

After we will commit new PR to fix tryUpdateMigrationStatus function code：
if !sourceNodeClusterInfo.MigratingSlot.Equal(shard.MigratingSlot.SlotRange){}

---

sourceNodeClusterInfo.MigratingSlot maybe return nil