refactor(sessionresolver): adapt to changing network conditions (#238)

* feat(sessionresolver): try many and use what works

* fix(sessionresolver): make sure we can use quic

* fix: the config struct is unnecessary

* fix: make kvstore optional

* feat: write simple integration test

* feat: start adding tests

* feat: continue writing tests

* fix(sessionresolver): add more unit tests

* fix(sessionresolver): finish adding tests

* refactor(sessionresolver): changes after code review

internal/engine/internal/sessionresolver/childresolver_test.go

@@ -0,0 +1,80 @@
+package sessionresolver
+
+import (
+	"context"
+	"errors"
+	"io"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+type FakeResolver struct {
+	Closed bool
+	Data   []string
+	Err    error
+	Sleep  time.Duration
+}
+
+func (r *FakeResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	select {
+	case <-time.After(r.Sleep):
+		return r.Data, r.Err
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
+func (r *FakeResolver) CloseIdleConnections() {
+	r.Closed = true
+}
+
+func TestTimeLimitedLookupSuccess(t *testing.T) {
+	reso := &Resolver{}
+	re := &FakeResolver{
+		Data: []string{"8.8.8.8", "8.8.4.4"},
+	}
+	ctx := context.Background()
+	out, err := reso.timeLimitedLookup(ctx, re, "dns.google")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(re.Data, out); diff != "" {
+		t.Fatal(diff)
+	}
+}
+
+func TestTimeLimitedLookupFailure(t *testing.T) {
+	reso := &Resolver{}
+	re := &FakeResolver{
+		Err: io.EOF,
+	}
+	ctx := context.Background()
+	out, err := reso.timeLimitedLookup(ctx, re, "dns.google")
+	if !errors.Is(err, re.Err) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != nil {
+		t.Fatal("expected nil here")
+	}
+}
+
+func TestTimeLimitedLookupWillTimeout(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+	reso := &Resolver{}
+	re := &FakeResolver{
+		Err:   io.EOF,
+		Sleep: 20 * time.Second,
+	}
+	ctx := context.Background()
+	out, err := reso.timeLimitedLookup(ctx, re, "dns.google")
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != nil {
+		t.Fatal("expected nil here")
+	}
+}