Compatibility issue about multi Group TSO

[nolouch]

Compatibility issue about Group TSO

ref #5895

MaxTS

Problem

Async Commit use max_ts to commit. The problem is, if multi group tso is used with async commit/1pc enabled, the commit_ts calculation becomes complicated. We can't record only one max_tsany more. For example, in this case:

• Read k1 from TSO Group1, start_ts (allocated from local allocator 1) = 100, then the max_ts is updated to 100;
• Write k2 from TSO Group2, start_ts (allocated from local allocator 2) = 50, calculated commit_ts = 101 (max_ts + 1)
  Commit k2 from TSO Group2
• Read k2 from TSO Group1, start_ts (allocated from local allocator 2) = 51, then the previous transaction's result is invisible.

Resolve TS

the resolve ts formula:

min_ts = pd.tso()
new_resolved_ts = min(min_ts, all(lock.start_ts))
resolved_ts = min(new_resolved_ts, resolved_ts)

Problem

If there is no locks, it maybe wrong update the resolved ts, fix with:

min_ts = min(all group.tso())

[ystaticy]

In keyspace scenario, there is a globally unique GC Worker to calculate the globally unique GC safepoint.
The calculation of GC safepoint depends on three values:

1. min(all keyspace service safepoint)
2. now-10min
3. min(all keyspace. minstartts)
   now in the second item above, it's always get from a unique timeline.
   now, err := w.getOracleTime()

But if TSO in differrent group, multiple timelines will be created.
So there is a possible options:
If TSO is in differrent group, but we still use the globally unique gc safepoint.
now should take the minimum value of all TSO groups. It need PD provide a new interface to return min(all TSO group nowTS)

Subsequent plan: the calculation of GC safepoint will be separated by keyspace,but the design discussion and code review may take more time.

[ystaticy]

There is another problem:
In keyspace scenario, if we use the globally unique gc safepoint.
now, gc safepoint = ts1;
We need to constrain the new tso group timeline to be later than ts1.
Otherwise, the data in the new tso group will be directly gc.

[nolouch]

cc @rleungx @binshi-bing

[rleungx]

native_br also needs to get the global min ts.