Support configuring session variable in lightning logical backend

[guoshouyan]

Feature Request

Can lightning support specifying session variables to use in lightning config?

In our case, we want to disable the async_commit in lightning, that is because during our test on stale read on TiDB cluster, we find out that async_commit could block stale_ts, which will block the whole tikv node. Since this issue will not be solved in a short time, we want to disable the async_commit in lightning for now and here is my pr: #46961

But I think we can generalize it by enabling lightning to let users specify a lit of session variable in the config that they want to use.

What is your thinking on this?

Is your feature request related to a problem? Please describe:

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[lance6716]

Hi, I found in the last we already proposed to set lightning session variables #28471 , but only limited to the scenario that lightning is used as a library. Maybe now we can change

	Vars                       map[string]string `toml:"-" json:"vars"`

to

	Vars                       map[string]string `toml:"session-vars" json:"vars"`

? Do you think it's acceptable?

[guoshouyan]

Hi, I found in the last we already proposed to set lightning session variables #28471 , but only limited to the scenario that lightning is used as a library. Maybe now we can change

	Vars                       map[string]string `toml:"-" json:"vars"`

to

	Vars                       map[string]string `toml:"session-vars" json:"vars"`

? Do you think it's acceptable?

Yeah, I think it should work in our case.

[guoshouyan]

Hi @lance6716 are you making the changes or you need me to make the changes?

[lance6716]

@guoshouyan I'm on holiday this week and haven't start to make the change. I also don't have a strict plan to assign this issue to you or me, we can use our spare time and check the linked PR of this issue to know if you or me have started, and I guess I will not come to work before Oct 08

[lance6716]

/assign @lance6716