Reduce unnecessary DB queries for Actions tasks (#219)

implement: https://github.com/go-gitea/gitea/issues/24544

Changes:
- Add a global variable `tasksVersion` to store the lastest version number which returned by Gitea.
- Pass `tasksVersion` to Gitea when invoking `FetchTask`.
- If there is no task in the `FetchTask` response, store the version from the `FetchTask` response into `tasksVersion` variable.
- If there is a task in the `FetchTask` response, set `tasksVersion` to zero to focre query db in next `FetchTask` request.

Related:
- Protocol: https://gitea.com/gitea/actions-proto-def/pulls/10
- Gitea side: https://github.com/go-gitea/gitea/pull/25199

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/219
Co-authored-by: sillyguodong <gedong_1994@163.com>
Co-committed-by: sillyguodong <gedong_1994@163.com>

internal/app/poll/poller.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 
 	runnerv1 "code.gitea.io/actions-proto-go/runner/v1"
 	"github.com/bufbuild/connect-go"
@@ -20,9 +21,10 @@ import (
 )
 
 type Poller struct {
-	client client.Client
-	runner *run.Runner
-	cfg    *config.Config
+	client       client.Client
+	runner       *run.Runner
+	cfg          *config.Config
+	tasksVersion atomic.Int64 // tasksVersion used to store the version of the last task fetched from the Gitea.
 }
 
 func New(cfg *config.Config, client client.Client, runner *run.Runner) *Poller {
@@ -77,7 +79,11 @@ func (p *Poller) fetchTask(ctx context.Context) (*runnerv1.Task, bool) {
 	reqCtx, cancel := context.WithTimeout(ctx, p.cfg.Runner.FetchTimeout)
 	defer cancel()
 
-	resp, err := p.client.FetchTask(reqCtx, connect.NewRequest(&runnerv1.FetchTaskRequest{}))
+	// Load the version value that was in the cache when the request was sent.
+	v := p.tasksVersion.Load()
+	resp, err := p.client.FetchTask(reqCtx, connect.NewRequest(&runnerv1.FetchTaskRequest{
+		TasksVersion: v,
+	}))
 	if errors.Is(err, context.DeadlineExceeded) {
 		err = nil
 	}
@@ -86,8 +92,20 @@ func (p *Poller) fetchTask(ctx context.Context) (*runnerv1.Task, bool) {
 		return nil, false
 	}
 
-	if resp == nil || resp.Msg == nil || resp.Msg.Task == nil {
+	if resp == nil || resp.Msg == nil {
 		return nil, false
 	}
+
+	if resp.Msg.TasksVersion > v {
+		p.tasksVersion.CompareAndSwap(v, resp.Msg.TasksVersion)
+	}
+
+	if resp.Msg.Task == nil {
+		return nil, false
+	}
+
+	// got a task, set `tasksVersion` to zero to focre query db in next request.
+	p.tasksVersion.CompareAndSwap(resp.Msg.TasksVersion, 0)
+
 	return resp.Msg.Task, true
 }