package grpctool

import (
	"context"
	"time"

	"gitlab.com/gitlab-org/cluster-integration/gitlab-agent/v18/internal/tool/mathz"
	"google.golang.org/grpc"
	"google.golang.org/grpc/keepalive"
	"google.golang.org/grpc/stats"
)

// Load balancer in front of kas typically has a timeout parameter, limiting the maximum connection age.
// After it elapses, the load balancer aborts the connection and the client sees a connection reset.
// This is not good. To mitigate the problem all long-running server-side poll loops should stop beforehand.
//
// After gRPC max connection age time elapses, gRPC will send a HTTP/2 GOAWAY frame to the client, to let it know that it
// should not use this TCP connection for any new RPCs and, after all in-flight requests had finished, close it.
// After gRPC max connection age time elapses, max connection grace period starts, allowing in-flight requests to finish cleanly.
// After this second time interval elapses, gRPC aborts the connection.
// https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/issues/138 is the example of the above
// behavior.
// We tag each TCP connection with a timeout context set to expire after gRPC max connection age duration to be able to
// return from the request handler just after gRPC max connection age duration had elapsed. Returning after and not
// before is important so that the client receives GOAWAY first and makes no further requests on this TCP connection.

// https://github.com/grpc/grpc/issues/26703 feature request to help get rid of the messy stuff below.

type maxConnAgeCtxKeyType int

const (
	maxConnAgeCtxKey maxConnAgeCtxKeyType = iota
	maxConnAgeConnStartKey
)

const (
	maxConnectionAgeJitterPercent = 5
	// gRPC applies +/- 10% jitter to MaxConnectionAge parameter.
	// https://github.com/grpc/grpc-go/blob/v1.39.0/internal/transport/http2_server.go#L1339-L1347
	maxConnectionAgeGRPCJitterPercent = 10
	// High fraction of connection time as grace time makes agentk reopen TCP connections more frequently i.e.
	// after 100-5-10-65=20% of max connection age time has elapsed. That leaves 65% of time for the long-running
	// connections to wrap up if such a connection was established close to the 20% time boundary.
	maxConnectionAgeGracePercent = 65

	// This is a compile-time test that ensures that the sum of all of the above does not exceed 100%.
	_ uint = 100 - maxConnectionAgeGracePercent - maxConnectionAgeGRPCJitterPercent - maxConnectionAgeJitterPercent
)

func MaxConnectionAge2GRPCKeepalive(auxCtx context.Context, maxConnectionAge time.Duration) (grpc.ServerOption, stats.Handler) {
	kp, sh := maxConnectionAge2GRPCKeepalive(auxCtx, maxConnectionAge)
	return grpc.KeepaliveParams(kp), sh
}

func maxConnectionAge2GRPCKeepalive(auxCtx context.Context, maxConnectionAge time.Duration) (keepalive.ServerParameters, stats.Handler) {
	// See https://github.com/grpc/grpc-go/blob/v1.33.1/internal/transport/http2_server.go#L949-L1047
	// to better understand how keepalive works.
	kp := keepalive.ServerParameters{
		MaxConnectionAge: maxConnectionAge * (100 - maxConnectionAgeGracePercent - maxConnectionAgeGRPCJitterPercent - maxConnectionAgeJitterPercent) / 100,
		// Give pending RPCs some time to complete.
		MaxConnectionAgeGrace: maxConnectionAge * maxConnectionAgeGracePercent / 100,
		// Trying to stay below 60 seconds (typical load-balancer timeout)
		Time: 50 * time.Second,
	}
	sh := NewServerMaxConnAgeStatsHandler(auxCtx, maxConnectionAge*(100-maxConnectionAgeGracePercent-maxConnectionAgeJitterPercent)/100)
	return kp, sh
}

func MaxConnectionAgeContextFromStreamContext(streamCtx context.Context) context.Context {
	return streamCtx.Value(maxConnAgeCtxKey).(context.Context)
}

func AddMaxConnectionAgeContext(ctx, ageCtx context.Context) context.Context {
	return context.WithValue(ctx, maxConnAgeCtxKey, ageCtx)
}

type serverMaxConnAgeStatsHandler struct {
	auxCtx           context.Context
	maxConnectionAge time.Duration
}

func NewServerMaxConnAgeStatsHandler(auxCtx context.Context, maxConnectionAge time.Duration) stats.Handler {
	return serverMaxConnAgeStatsHandler{
		auxCtx:           auxCtx,
		maxConnectionAge: maxConnectionAge,
	}
}

func (m serverMaxConnAgeStatsHandler) TagRPC(ctx context.Context, info *stats.RPCTagInfo) context.Context {
	var (
		ageCtx    context.Context
		ageCancel context.CancelFunc
	)
	if m.maxConnectionAge == 0 {
		ageCtx, ageCancel = context.WithCancel(m.auxCtx)
	} else {
		remainingConnAge := m.maxConnectionAge - time.Since(ctx.Value(maxConnAgeConnStartKey).(time.Time))
		ageCtx, ageCancel = context.WithTimeout(m.auxCtx, mathz.DurationWithPositiveJitter(remainingConnAge, maxConnectionAgeJitterPercent))
	}
	context.AfterFunc(ctx, ageCancel)
	return AddMaxConnectionAgeContext(ctx, ageCtx)
}

func (m serverMaxConnAgeStatsHandler) HandleRPC(ctx context.Context, rpcStats stats.RPCStats) {
}

func (m serverMaxConnAgeStatsHandler) TagConn(ctx context.Context, info *stats.ConnTagInfo) context.Context {
	return context.WithValue(ctx, maxConnAgeConnStartKey, time.Now())
}

func (m serverMaxConnAgeStatsHandler) HandleConn(ctx context.Context, connStats stats.ConnStats) {
}

type ServerNoopMaxConnAgeStatsHandler struct {
}

func (m ServerNoopMaxConnAgeStatsHandler) TagRPC(ctx context.Context, info *stats.RPCTagInfo) context.Context {
	return AddMaxConnectionAgeContext(ctx, context.Background())
}

func (m ServerNoopMaxConnAgeStatsHandler) HandleRPC(ctx context.Context, rpcStats stats.RPCStats) {
}

func (m ServerNoopMaxConnAgeStatsHandler) TagConn(ctx context.Context, info *stats.ConnTagInfo) context.Context {
	return ctx
}

func (m ServerNoopMaxConnAgeStatsHandler) HandleConn(ctx context.Context, connStats stats.ConnStats) {
}
