[Proposal] Reduce unnecessary DB queries for Actions tasks

[wolfogre]

This looks familiar with #24543, but it's quite different.

Background

Gitea will start a transaction to find and assign a task to the runner when it requests a new one. However, we know that there may not be a task available most of the time, so Gitea has to roll back the transaction and respond with "no task yet, try again later."

Starting a transaction is an expensive operation. However, we don't have to do it every time.

Solution

Record a version number for table of Actions jobs

PS: You might wonder why it's not "table of Actions tasks". Never mind, just a few implementation details.

Increase the version number once the data table changes. The number can be stored in DB and cached in memory.

When Gitea receives a request for a task, it compares the version number in the request with the current version number. If they are equal, it responds with "no task available yet".

Otherwise, start a transaction to query and assign a task.

Gitea responds with the current version number regardless of whether there is a task available.

Runners fetch tasks with the version number

1. Fetch tasks with 0 version number when it’s the first time.
2. Fetch tasks with the responded version number when it didn't get a task last time.
3. Fetch tasks with 0 version number when it got a task last time.

Here's a simulation:

Gitea		Runner
	← 0
13 != 0, query db	→ 13, no task
	← 13
13 == 13, skip query	→ 13, no task
	← 13
13 != 14, query db	→ 14, a task
	← 0
14 != 0, query db	→ 14, no task
	← 14
14 == 14, skip query	→ 14, no task

Advantages

• Easy to implement. No need to modify existing mechanisms.
• Compatible with other improvement measures.