cleanup: doh.powerdns.org is not working anymore (#924)

While there, `.../internal/sessionresolver` => `.../sessionresolver`

See https://github.com/ooni/probe/issues/2255

internal/engine/sessionresolver/resolvermaker.go

@@ -0,0 +1,120 @@
+package sessionresolver
+
+//
+// Code for creating a new child resolver
+//
+
+import (
+	"math/rand"
+	"strings"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// resolvemaker contains rules for making a resolver.
+type resolvermaker struct {
+	url   string
+	score float64
+}
+
+// systemResolverURL is the URL of the system resolver.
+const systemResolverURL = "system:///"
+
+// allmakers contains all the makers in a list. We use the http3
+// prefix to indicate we wanna use http3. The code will translate
+// this to https and set the proper netx options.
+var allmakers = []*resolvermaker{{
+	url: "https://cloudflare-dns.com/dns-query",
+}, {
+	url: "http3://cloudflare-dns.com/dns-query",
+}, {
+	url: "https://dns.google/dns-query",
+}, {
+	url: "http3://dns.google/dns-query",
+}, {
+	url: "https://dns.quad9.net/dns-query",
+}, {
+	url: systemResolverURL,
+}, {
+	url: "https://mozilla.cloudflare-dns.com/dns-query",
+}, {
+	url: "http3://mozilla.cloudflare-dns.com/dns-query",
+}}
+
+// allbyurl contains all the resolvermakers by URL
+var allbyurl map[string]*resolvermaker
+
+// init fills allbyname and gives a nonzero initial score
+// to all resolvers except for the system resolver. We set
+// the system resolver score to zero, so that it's less
+// likely than other resolvers in this list.
+func init() {
+	allbyurl = make(map[string]*resolvermaker)
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+	for _, e := range allmakers {
+		allbyurl[e.url] = e
+		if e.url != systemResolverURL {
+			e.score = rng.Float64()
+		}
+	}
+}
+
+// logger returns the configured logger or a default
+func (r *Resolver) logger() model.Logger {
+	return model.ValidLoggerOrDefault(r.Logger)
+}
+
+// newChildResolver creates a new child model.Resolver.
+func (r *Resolver) newChildResolver(h3 bool, URL string) (model.Resolver, error) {
+	if r.newChildResolverFn != nil {
+		return r.newChildResolverFn(h3, URL)
+	}
+	return netx.NewDNSClient(netx.Config{
+		BogonIsError: true,
+		ByteCounter:  r.ByteCounter, // nil is handled by netx
+		HTTP3Enabled: h3,
+		Logger:       r.logger(),
+		ProxyURL:     r.ProxyURL,
+	}, URL)
+}
+
+// newresolver creates a new resolver with the given config and URL. This is
+// where we expand http3 to https and set the h3 options.
+func (r *Resolver) newresolver(URL string) (model.Resolver, error) {
+	h3 := strings.HasPrefix(URL, "http3://")
+	if h3 {
+		URL = strings.Replace(URL, "http3://", "https://", 1)
+	}
+	return r.newChildResolver(h3, URL)
+}
+
+// getresolver returns a resolver with the given URL. This function caches
+// already allocated resolvers so we only allocate them once.
+func (r *Resolver) getresolver(URL string) (model.Resolver, error) {
+	defer r.mu.Unlock()
+	r.mu.Lock()
+	if re, found := r.res[URL]; found {
+		return re, nil // already created
+	}
+	re, err := r.newresolver(URL)
+	if err != nil {
+		return nil, err // config err?
+	}
+	if r.res == nil {
+		r.res = make(map[string]model.Resolver)
+	}
+	r.res[URL] = re
+	return re, nil
+}
+
+// closeall closes the cached resolvers.
+func (r *Resolver) closeall() {
+	defer r.mu.Unlock()
+	r.mu.Lock()
+	for _, re := range r.res {
+		re.CloseIdleConnections()
+	}
+	r.res = nil
+}