tests: remove the DNS stand-ins from the watcher and resolver tests (closes #159)

The watcher tests used a stand-in resolver and the resolver timeout test
a stand-in DNS client, against the rule that DNS is never mocked. The
watcher tests now run the real resolver against live DNS. A change is
tested by saving values live DNS never returns (names under .invalid,
192.0.2.1) in the state a check starts from, or by marking a real
nameserver failed. The timeout test queries 192.0.2.1, where nothing
answers. The live-DNS retry and concurrency limit moved from the
resolver tests to internal/livedns, so both packages share them.
NewFromLoggerWithClient had no other use and is gone. TESTING.md and
the DNSClient comment now state the README's rule.

Model: opus-5-5

internal/resolver/livedns_test.go

@@ -8,8 +8,8 @@ import (
 	"sort"
 	"strings"
 	"testing"
-	"time"
 
+	"sneak.berlin/go/dnswatcher/internal/livedns"
 	"sneak.berlin/go/dnswatcher/internal/resolver"
 )
 
@@ -18,143 +18,32 @@ import (
 // ----------------------------------------------------------------
 //
 // Every test in this package resolves against the real, live DNS —
-// see TESTING.md. Nothing here mocks, fakes, stubs, records or
-// replays DNS, and nothing here skips or gates a test: the helpers
-// below only change *how* the live queries are issued, so that a
-// single dropped UDP packet or one slow authoritative server does
-// not turn a correct resolver into a red build.
+// see TESTING.md. Each live operation below goes through
+// livedns.Retry, which bounds how many resolutions are in flight at
+// once and retries an operation that got no answer (see package
+// livedns).
 //
-// Three mechanisms, all test-side:
+// Where an assertion spans several independent nameservers, a quorum
+// is enough: a strict majority answering as expected. A server that
+// fails to answer is tolerated, while a server that answers *wrongly*
+// still fails the test.
 //
-//  1. Bounded concurrency. The package's tests are parallel and the
-//     build hosts have many cores, so without a limit every test
-//     starts its own iterative resolution at the same instant and
-//     they all hit the first root server in rootServerList() within
-//     a few milliseconds of each other. Root servers rate-limit
-//     that, which shows up as a different arbitrary subset of tests
-//     failing on each run. liveGate caps how many resolutions are
-//     in flight at once.
-//
-//  2. Retry with exponential backoff. Each live operation gets
-//     several attempts with its own timeout. The retry predicate is
-//     strictly transport-level — "did a nameserver answer at all" —
-//     never the assertion the test is making. A resolver that
-//     answers incorrectly still fails on the first attempt.
-//
-//  3. Quorum. Where an assertion spans several independent
-//     nameservers, a strict majority answering as expected is
-//     enough; a server that fails to answer is tolerated, while a
-//     server that answers *wrongly* still fails the test.
-//
-// The tolerance in (3) is expressed as an ALLOWLIST of sanctioned
-// statuses, never as a blocklist of known-bad ones. A blocklist bans
-// the one wrong answer its author thought of and silently admits
-// every other status, including any added to the resolver later; an
-// allowlist fails on anything nobody explicitly sanctioned. Silence
-// (timeout, error) is the only thing quorum exists to tolerate. A
-// *wrong answer* — nxdomain for a name that exists, ok for one that
-// does not, nodata for either — is never tolerated at any count.
+// That tolerance is expressed as an ALLOWLIST of sanctioned statuses,
+// never as a blocklist of known-bad ones. A blocklist bans the one
+// wrong answer its author thought of and silently admits every other
+// status, including any added to the resolver later; an allowlist
+// fails on anything nobody explicitly sanctioned. Silence (timeout,
+// error) is the only thing quorum exists to tolerate. A *wrong
+// answer* — nxdomain for a name that exists, ok for one that does
+// not, nodata for either — is never tolerated at any count.
 
-const (
-	// liveAttempts is how many times a live DNS operation is
-	// attempted before the test fails.
-	liveAttempts = 3
+// minNameservers is the smallest nameserver count a well-run zone is
+// expected to publish.
+const minNameservers = 2
 
-	// liveAttemptTimeout bounds one attempt. Worst case for an
-	// operation is liveAttempts * liveAttemptTimeout plus the
-	// backoff — about 26 seconds, well inside the 90-second
-	// `go test -timeout` backstop even when several operations
-	// exhaust their attempts.
-	liveAttemptTimeout = 8 * time.Second
-
-	// liveBackoffBase is the delay after the first failed
-	// attempt; it is multiplied by liveBackoffFactor each time.
-	liveBackoffBase = 500 * time.Millisecond
-
-	// liveBackoffFactor is the exponential backoff multiplier.
-	liveBackoffFactor = 2
-
-	// liveConcurrency caps how many live resolutions may be in
-	// flight across the whole package at once.
-	liveConcurrency = 6
-
-	// minNameservers is the smallest nameserver count a
-	// well-run zone is expected to publish.
-	minNameservers = 2
-)
-
-// liveGate bounds concurrent live resolutions package-wide. It has
-// to be package scoped: the whole point is that it is shared by
-// every parallel test in the package.
-//
-//nolint:gochecknoglobals // package-wide live query rate limit
-var liveGate = make(chan struct{}, liveConcurrency)
-
-var (
-	// errLiveNoAnswer reports that a live operation produced no
-	// usable answer, which is retried rather than asserted on.
-	errLiveNoAnswer = errors.New("no answer from live DNS")
-
-	// errLiveNoQuorum reports that too few of a domain's
-	// nameservers answered for a quorum assertion to be made.
-	errLiveNoQuorum = errors.New("no nameserver quorum")
-)
-
-// runLive executes one attempt of a live operation, holding a slot
-// in liveGate for its duration and bounding it with its own
-// timeout.
-func runLive(op func(ctx context.Context) error) error {
-	liveGate <- struct{}{}
-	defer func() { <-liveGate }()
-
-	ctx, cancel := context.WithTimeout(
-		context.Background(), liveAttemptTimeout,
-	)
-	defer cancel()
-
-	return op(ctx)
-}
-
-// retryLive runs op until it reports success, retrying transport
-// failures with exponential backoff, and fails the test if every
-// attempt fails. op returns an error only for a failure to obtain
-// an answer — never for an answer the test disagrees with, which
-// belongs in an assertion so that it fails immediately. op stores
-// whatever it obtained where its caller can find it.
-func retryLive(
-	t *testing.T,
-	what string,
-	op func(ctx context.Context) error,
-) {
-	t.Helper()
-
-	var last error
-
-	backoff := liveBackoffBase
-
-	for attempt := range liveAttempts {
-		if attempt > 0 {
-			t.Logf(
-				"%s: attempt %d of %d failed (%v), "+
-					"retrying in %s",
-				what, attempt, liveAttempts, last, backoff,
-			)
-			time.Sleep(backoff)
-
-			backoff *= liveBackoffFactor
-		}
-
-		last = runLive(op)
-		if last == nil {
-			return
-		}
-	}
-
-	t.Fatalf(
-		"%s: no answer after %d live attempts: %v",
-		what, liveAttempts, last,
-	)
-}
+// errLiveNoQuorum reports that too few of a domain's nameservers
+// answered for a quorum assertion to be made.
+var errLiveNoQuorum = errors.New("no nameserver quorum")
 
 // liveQuorum is how many of total nameservers must agree for a
 // multi-nameserver assertion to hold: a strict majority.
@@ -272,7 +161,7 @@ func liveFindAuthoritative(
 
 	var out []string
 
-	retryLive(
+	livedns.Retry(
 		t,
 		"FindAuthoritativeNameservers("+domain+")",
 		func(ctx context.Context) error {
@@ -284,7 +173,7 @@ func liveFindAuthoritative(
 			if len(ns) == 0 {
 				return fmt.Errorf(
 					"%w: %s has no nameservers",
-					errLiveNoAnswer, domain,
+					livedns.ErrNoAnswer, domain,
 				)
 			}
 
@@ -308,7 +197,7 @@ func liveLookupNS(
 
 	var out []string
 
-	retryLive(
+	livedns.Retry(
 		t,
 		"LookupNS("+domain+")",
 		func(ctx context.Context) error {
@@ -320,7 +209,7 @@ func liveLookupNS(
 			if len(ns) == 0 {
 				return fmt.Errorf(
 					"%w: %s has no nameservers",
-					errLiveNoAnswer, domain,
+					livedns.ErrNoAnswer, domain,
 				)
 			}
 
@@ -350,7 +239,7 @@ func liveQueryNameserver(
 
 	var out *resolver.NameserverResponse
 
-	retryLive(
+	livedns.Retry(
 		t,
 		what,
 		func(ctx context.Context) error {
@@ -365,7 +254,7 @@ func liveQueryNameserver(
 				resp.Status == resolver.StatusError {
 				return fmt.Errorf(
 					"%w: %s returned %s: %s",
-					errLiveNoAnswer, nameserver,
+					livedns.ErrNoAnswer, nameserver,
 					resp.Status, resp.Error,
 				)
 			}
@@ -392,7 +281,7 @@ func liveQueryAllNameservers(
 
 	var out map[string]*resolver.NameserverResponse
 
-	retryLive(
+	livedns.Retry(
 		t,
 		"QueryAllNameservers("+hostname+")",
 		func(ctx context.Context) error {
@@ -404,7 +293,7 @@ func liveQueryAllNameservers(
 			if len(results) == 0 {
 				return fmt.Errorf(
 					"%w: no nameservers queried for %s",
-					errLiveNoAnswer, hostname,
+					livedns.ErrNoAnswer, hostname,
 				)
 			}
 
@@ -437,7 +326,7 @@ func liveResolveIPs(
 
 	var out []string
 
-	retryLive(
+	livedns.Retry(
 		t,
 		"ResolveIPAddresses("+hostname+")",
 		func(ctx context.Context) error {
@@ -449,7 +338,7 @@ func liveResolveIPs(
 			if len(ips) == 0 {
 				return fmt.Errorf(
 					"%w: no addresses for %s",
-					errLiveNoAnswer, hostname,
+					livedns.ErrNoAnswer, hostname,
 				)
 			}
 
@@ -476,7 +365,7 @@ func liveResolveIPsAllowingEmpty(
 
 	var out []string
 
-	retryLive(
+	livedns.Retry(
 		t,
 		"ResolveIPAddresses("+hostname+")",
 		func(ctx context.Context) error {