discovery: use thanos resolver for endpoint groups

Signed-off-by: Michael Hoffmann <mhoffm@posteo.de>

cmd/thanos/query.go

@@ -492,6 +492,16 @@ func runQuery(
 		}
 	}
 
+	// Register resolver for the "thanos:///" scheme for endpoint-groups
+	dns.RegisterGRPCResolver(
+		dns.NewProvider(
+			logger,
+			extprom.WrapRegistererWithPrefix("thanos_query_endpoint_groups_", reg),
+			dns.ResolverType(dnsSDResolver),
+		),
+		dnsSDInterval,
+	)
+
 	dnsEndpointProvider := dns.NewProvider(
 		logger,
 		extprom.WrapRegistererWithPrefix("thanos_query_endpoints_", reg),
@@ -891,14 +901,12 @@ func prepareEndpointSet(
 			}
 
 			for _, eg := range endpointGroupAddrs {
-				addr := fmt.Sprintf("dns:///%s", eg)
-				spec := query.NewGRPCEndpointSpec(addr, false, extgrpc.EndpointGroupGRPCOpts()...)
+				spec := query.NewGRPCEndpointSpec(fmt.Sprintf("thanos:///%s", eg), false, extgrpc.EndpointGroupGRPCOpts()...)
 				specs = append(specs, spec)
 			}
 
 			for _, eg := range strictEndpointGroups {
-				addr := fmt.Sprintf("dns:///%s", eg)
-				spec := query.NewGRPCEndpointSpec(addr, true, extgrpc.EndpointGroupGRPCOpts()...)
+				spec := query.NewGRPCEndpointSpec(fmt.Sprintf("thanos:///%s", eg), true, extgrpc.EndpointGroupGRPCOpts()...)
 				specs = append(specs, spec)
 			}
 

pkg/discovery/dns/grpc.go

@@ -0,0 +1,88 @@
+// Copyright (c) The Thanos Authors.
+// Licensed under the Apache License 2.0.
+
+package dns
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	grpcresolver "google.golang.org/grpc/resolver"
+)
+
+var (
+	_ grpcresolver.Builder  = &builder{}
+	_ grpcresolver.Resolver = &resolver{}
+)
+
+type builder struct {
+	resolveInterval time.Duration
+	provider        *Provider
+}
+
+func RegisterGRPCResolver(provider *Provider, interval time.Duration) {
+	grpcresolver.Register(&builder{
+		resolveInterval: interval,
+		provider:        provider,
+	})
+}
+
+func (b *builder) Scheme() string { return "thanos" }
+
+func (b *builder) Build(t grpcresolver.Target, cc grpcresolver.ClientConn, _ grpcresolver.BuildOptions) (grpcresolver.Resolver, error) {
+	ctx, cancel := context.WithCancel(context.Background())
+	r := &resolver{
+		provider: b.provider,
+		target:   t.Endpoint(),
+		ctx:      ctx,
+		cancel:   cancel,
+		cc:       cc,
+		interval: b.resolveInterval,
+	}
+	r.wg.Add(1)
+	go r.run()
+
+	return r, nil
+}
+
+type resolver struct {
+	provider *Provider
+
+	target   string
+	ctx      context.Context
+	cancel   context.CancelFunc
+	cc       grpcresolver.ClientConn
+	interval time.Duration
+
+	wg sync.WaitGroup
+}
+
+func (r *resolver) Close() {
+	r.cancel()
+	r.wg.Wait()
+}
+
+func (r *resolver) ResolveNow(_ grpcresolver.ResolveNowOptions) {}
+
+func (r *resolver) run() {
+	defer r.wg.Done()
+	for {
+		err := r.provider.Resolve(r.ctx, []string{r.target})
+		if err != nil {
+			r.cc.ReportError(err)
+		} else {
+			state := grpcresolver.State{}
+			for _, addr := range r.provider.AddressesForHost(r.target) {
+				raddr := grpcresolver.Address{Addr: addr}
+				state.Addresses = append(state.Addresses, raddr)
+			}
+			_ = r.cc.UpdateState(state)
+		}
+		select {
+		case <-r.ctx.Done():
+			return
+		case <-time.After(r.interval):
+		}
+	}
+}

pkg/discovery/dns/provider.go

@@ -164,3 +164,16 @@ func (p *Provider) Addresses() []string {
 	}
 	return result
 }
+
+// AddressesForHost returns the latest addresses present for the host in the Provider.
+func (p *Provider) AddressesForHost(host string) []string {
+	p.RLock()
+	defer p.RUnlock()
+
+	addrs := p.resolved[host]
+
+	res := make([]string, len(addrs))
+	copy(res, addrs)
+
+	return res
+}