A second download source, for when the first one refuses

Cloudflare rate-limits by volume, and the block it handed out while this was
being built lasted over twenty minutes — too fragile to be the only thing behind
a button. So the download phase tries its sources in order: the anycast endpoint
first, then a plain 100 MB file on Linode's Seattle mirror, which has no opinion
about how much has been pulled today.

Only a refusal moves to the next one; anything else would fail the same way
twice. Every result names the source it used, and the page says "from
linode-seattle" rather than "via SEA" when it was the fallback — that mirror is
in one fixed place, so somewhere else it reads low, and a figure that quietly
came from a different continent would be a lie.

Upload stays on Cloudflare, which has never been the one to refuse.

Also: the too-short-window guard was refusing to divide 512 MiB by ten
milliseconds on a link fast enough to hit the runaway budget before the first
tick. It now guards against dividing by zero and nothing more.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PLAN.md

@@ -611,10 +611,17 @@ Decisions worth keeping:
   in, it reported 31 ms for a 7 ms line. Jitter is the mean change between consecutive
   probes, not a standard deviation, so one slow probe doesn't run away with it.
 - **A refused phase is not a number.** The far end rate-limits by volume, and a test moves a
-  few hundred megabytes; several in a row earn 429s. So: a 20-second cooldown between tests,
-  a stream that retries twice before giving up, the rate computed over the window in which
-  bytes were actually still arriving, and a plain sentence when it was cut short. Coasting to
-  the end of the clock with nothing arriving reported a third of the truth.
+  few hundred megabytes; several in a row earn 429s, and the block observed while building this
+  lasted over twenty minutes. So: a 20-second cooldown counted from the last *attempt* (a
+  refused test still used the line), a stream that retries twice before giving up, the rate
+  computed over the window in which bytes were actually still arriving, and a plain sentence
+  when it was cut short. Coasting to the end of the clock with nothing arriving reported a
+  third of the truth.
+- **A second download source, for when the first refuses.** A plain 100 MB file on Linode's
+  Seattle mirror, which has no opinion about volume. It is only reached after a refusal, and
+  every result names the source it used — that mirror is in one place, so on a machine far from
+  Seattle it would read low, and a number that quietly came from somewhere else would be a lie.
+  Upload stays on Cloudflare, which has never been the one to refuse.
 - **The last result is published as sensors** (`net.internet.*`), so the line is a widget, a
   graph, an alert or a Prometheus series like anything else. Each carries the time the test
   ran, not the time it was asked for.

internal/network/network_test.go

@@ -8,7 +8,6 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
-	"strings"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -180,16 +179,16 @@ func TestPhaseMeasuresTheWindow(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	restore := func(u, up string, w, d, mw time.Duration, m int64) {
-		downURL, upURL, warmUp, downFor, minWindow, measureBytes = u, up, w, d, mw, m
-	}
-	defer restore(downURL, upURL, warmUp, downFor, minWindow, measureBytes)
-	downURL, upURL = srv.URL+"/down?bytes=%d", srv.URL+"/up"
+	defer func(up string, w, d, mw time.Duration, m int64) {
+		upURL, warmUp, downFor, minWindow, measureBytes = up, w, d, mw, m
+	}(upURL, warmUp, downFor, minWindow, measureBytes)
+	upURL = srv.URL + "/up"
 	warmUp, downFor, minWindow, measureBytes = 100*time.Millisecond, 3*time.Second, 200*time.Millisecond, 8<<20
+	here := Source{"local", func(int) string { return srv.URL + "/down" }}
 
 	tr := NewTester(nil)
 	began := time.Now()
-	mbps, moved, note, err := tr.phase(context.Background(), srv.Client(), "download", 2, downFor, 0, 1)
+	mbps, moved, note, err := tr.phase(context.Background(), srv.Client(), "download", 2, downFor, 0, 1, here)
 	took := time.Since(began)
 	if err != nil {
 		t.Fatalf("phase: %v", err)
@@ -216,11 +215,12 @@ func TestPhaseMeasuresTheWindow(t *testing.T) {
 	// A phase whose server refuses outright fails, and says what it was told.
 	bad := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(429) }))
 	defer bad.Close()
-	downURL = bad.URL + "/down?bytes=%d"
-	if _, _, _, err := tr.phase(context.Background(), bad.Client(), "download", 2, 300*time.Millisecond, 0, 1); err == nil {
+	refuser := Source{"refuser", func(int) string { return bad.URL + "/down" }}
+	_, _, _, err = tr.phase(context.Background(), bad.Client(), "download", 2, 300*time.Millisecond, 0, 1, refuser)
+	if err == nil {
 		t.Error("a phase against a server that refuses everything should fail")
-	} else if !strings.Contains(err.Error(), "rate-limiting") {
-		t.Errorf("error = %q, want it to name the rate limit", err)
+	} else if !refused(err) {
+		t.Errorf("error = %q, want the refusal that makes the fallback kick in", err)
 	}
 }
 
@@ -241,15 +241,77 @@ func TestCancelIsReportedAsCancelled(t *testing.T) {
 		}
 	}))
 	defer srv.Close()
-	defer func(u string, w, d time.Duration) { downURL, warmUp, downFor = u, w, d }(downURL, warmUp, downFor)
-	downURL, warmUp, downFor = srv.URL+"/down?bytes=%d", 10*time.Millisecond, 10*time.Second
+	defer func(w, d time.Duration) { warmUp, downFor = w, d }(warmUp, downFor)
+	warmUp, downFor = 10*time.Millisecond, 10*time.Second
+	here := Source{"local", func(int) string { return srv.URL + "/down" }}
 
 	tr := NewTester(nil)
 	ctx, cancel := context.WithCancel(context.Background())
 	go func() { time.Sleep(200 * time.Millisecond); cancel() }()
-	_, _, _, err := tr.phase(ctx, srv.Client(), "download", 1, downFor, 0, 1)
+	_, _, _, err := tr.phase(ctx, srv.Client(), "download", 1, downFor, 0, 1, here)
 	// Cancelled after bytes moved: the figure stands, and the phase does not claim an error.
 	if err != nil && !errors.Is(err, context.Canceled) {
 		t.Errorf("err = %v, want nil or a cancellation", err)
 	}
 }
+
+// When the first source refuses, the download is taken from the next one and the result says
+// so. This is the whole reason there is more than one.
+func TestDownloadFallsBackWhenRefused(t *testing.T) {
+	serving := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/up" {
+			_, _ = io.Copy(io.Discard, r.Body)
+			w.WriteHeader(204)
+			return
+		}
+		// paced, so the phase is governed by its window and not by a loopback moving a
+		// gibibyte before the first tick
+		buf := make([]byte, 1<<20)
+		fl, _ := w.(http.Flusher)
+		for i := 0; i < 64; i++ {
+			if _, err := w.Write(buf); err != nil {
+				return
+			}
+			if fl != nil {
+				fl.Flush()
+			}
+			select {
+			case <-time.After(2 * time.Millisecond):
+			case <-r.Context().Done():
+				return
+			}
+		}
+	}))
+	defer serving.Close()
+	refusing := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(429) }))
+	defer refusing.Close()
+
+	defer func(srcs []Source, up string, w, d, mw time.Duration, m int64) {
+		downSources, upURL, warmUp, downFor, minWindow, measureBytes = srcs, up, w, d, mw, m
+	}(downSources, upURL, warmUp, downFor, minWindow, measureBytes)
+	downSources = []Source{
+		{"refuser", func(int) string { return refusing.URL + "/down" }},
+		{"the-other-one", func(int) string { return serving.URL + "/down" }},
+	}
+	upURL = serving.URL + "/up"
+	warmUp, downFor, upFor, minWindow, measureBytes = 50*time.Millisecond, time.Second, 300*time.Millisecond, 100*time.Millisecond, 4<<20
+
+	tr := NewTester(nil)
+	if _, err := tr.Start(); err != nil {
+		t.Fatal(err)
+	}
+	deadline := time.Now().Add(30 * time.Second)
+	for tr.Running() && time.Now().Before(deadline) {
+		time.Sleep(50 * time.Millisecond)
+	}
+	res := tr.State().Result
+	if res == nil {
+		t.Fatal("no result")
+	}
+	if res.Source != "the-other-one" {
+		t.Errorf("source = %q, want the second one — the first refused", res.Source)
+	}
+	if res.DownMbps <= 0 {
+		t.Errorf("download = %v, want a figure from the source that did serve: %+v", res.DownMbps, res)
+	}
+}

internal/network/speedtest.go

@@ -35,6 +35,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log"
 	"math"
 	"net"
 	"net/http"
@@ -47,8 +48,26 @@ import (
 
 // The endpoints, and the shape of a phase. Variables rather than constants only so the tests
 // can point them at a local server and shrink the windows; nothing else rebinds them.
+// Source is somewhere to pull bytes from. Cloudflare's endpoint is the first choice because
+// it is anycast — near wherever Ham Lab is running — and because the same host answers for
+// upload and for the trace. The fallback exists because that endpoint rate-limits by volume
+// and will refuse a connection that has been tested hard: a plain 100 MB file on a public
+// mirror has no such opinion. It is in Seattle, so on a machine far from there it will read
+// low, which is exactly why every result names the source it used.
+type Source struct {
+	Name string
+	// URL takes the number of bytes wanted, where the source can be asked; a fixed file
+	// ignores it and is fetched again each time round.
+	URL func(bytes int) string
+}
+
+var downSources = []Source{
+	{"cloudflare", func(b int) string { return fmt.Sprintf(cfDownURL, b) }},
+	{"linode-seattle", func(int) string { return "https://speedtest.seattle.linode.com/100MB-seattle.bin" }},
+}
+
 var (
-	downURL = "https://speed.cloudflare.com/__down?bytes=%d"
+	cfDownURL = "https://speed.cloudflare.com/__down?bytes=%d"
 	upURL     = "https://speed.cloudflare.com/__up"
 	// traceURL is Cloudflare's standard trace: a few lines of key=value naming the address it
 	// sees, the edge that answered and the country. A few hundred bytes.
@@ -100,6 +119,9 @@ type Result struct {
 	// address as that edge saw it.
 	Colo string `json:"colo,omitempty"`
 	IP   string `json:"ip,omitempty"`
+	// Source is where the download came from, which matters because the fallback is not in the
+	// same place as the anycast one. Empty means the usual.
+	Source string `json:"source,omitempty"`
 	// Note is set when the figures are real but something went wrong along the way — a stream
 	// that was refused part-way, most often. The number is what was actually seen; the note is
 	// why it may be low.
@@ -230,12 +252,21 @@ func (t *Tester) measure(ctx context.Context, cancel context.CancelFunc) {
 	var note string
 	if err == nil {
 		t.set(func(r *Run) { r.Phase, r.Done, r.Mbps = "download", 0.1, 0 })
-		res.DownMbps, res.DownBytes, note, err = t.phase(ctx, client, "download", downStreams, downFor, 0.1, 0.6)
+		// Each source in turn: a refusal from the first is exactly what the second is for, and
+		// anything else (no route, no DNS) would fail the same way twice, so it stops there.
+		for i, src := range downSources {
+			res.Source = src.Name
+			res.DownMbps, res.DownBytes, note, err = t.phase(ctx, client, "download", downStreams, downFor, 0.1, 0.6, src)
+			if err == nil || !refused(err) || i == len(downSources)-1 {
+				break
+			}
+			log.Printf("speed test: %s refused the download, trying %s", src.Name, downSources[i+1].Name)
+		}
 		res.Note = join(res.Note, note)
 	}
 	if err == nil {
 		t.set(func(r *Run) { r.Phase, r.Done, r.Mbps = "upload", 0.7, 0 })
-		res.UpMbps, res.UpBytes, note, err = t.phase(ctx, client, "upload", upStreams, upFor, 0.7, 1.0)
+		res.UpMbps, res.UpBytes, note, err = t.phase(ctx, client, "upload", upStreams, upFor, 0.7, 1.0, Source{Name: "cloudflare"})
 		res.Note = join(res.Note, note)
 	}
 	res.TookSec = round(time.Since(started).Seconds(), 1)
@@ -307,7 +338,7 @@ func (t *Tester) latency(ctx context.Context, client *http.Client) ([]float64, m
 	var out []float64
 	for i := 0; i <= latencyProbes; i++ { // the first is the handshake, and is not counted
 		start := time.Now()
-		req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf(downURL, 0), nil)
+		req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf(cfDownURL, 0), nil)
 		if err != nil {
 			return nil, m, err
 		}
@@ -344,7 +375,7 @@ func (t *Tester) latency(ctx context.Context, client *http.Client) ([]float64, m
 // refusing part-way, the figure is what was actually moved while it was moving, and the note
 // says what happened. Coasting to the end of the clock with no bytes arriving would report a
 // number a third of the truth.
-func (t *Tester) phase(parent context.Context, client *http.Client, dir string, streams int, forHow time.Duration, from, to float64) (float64, int64, string, error) {
+func (t *Tester) phase(parent context.Context, client *http.Client, dir string, streams int, forHow time.Duration, from, to float64, src Source) (float64, int64, string, error) {
 	// parent is the test's own context, which is cancelled when somebody presses Stop. ctx is
 	// this phase's, which *this function* cancels to shut the streams down — so the two are
 	// kept apart: asking ctx whether the test was cancelled would always say yes by then, and
@@ -371,7 +402,7 @@ func (t *Tester) phase(parent context.Context, client *http.Client, dir string,
 			for ctx.Err() == nil && moved.Load() < phaseBudget {
 				var err error
 				if dir == "download" {
-					err = stream(ctx, client, &moved)
+					err = stream(ctx, client, src, &moved)
 				} else {
 					err = upstream(ctx, client, &moved)
 				}
@@ -446,7 +477,8 @@ func (t *Tester) phase(parent context.Context, client *http.Client, dir string,
 		}
 		return 0, moved.Load(), "", fmt.Errorf("nothing moved in the %s phase", dir)
 	}
-	if secs < 0.1 { // too short a window to divide by
+	if secs < 0.02 { // too short a window to divide by; the guard is against dividing by zero,
+		// not a judgement about how long a good measurement takes
 		return 0, moved.Load(), "", fmt.Errorf("the %s phase was over before it could be measured", dir)
 	}
 	note := ""
@@ -467,9 +499,9 @@ func join(a, b string) string {
 	return a + "; " + b
 }
 
-// stream fetches one chunk, counting bytes as they arrive.
-func stream(ctx context.Context, client *http.Client, n *atomic.Int64) error {
-	req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf(downURL, downChunk), nil)
+// stream fetches one chunk from src, counting bytes as they arrive.
+func stream(ctx context.Context, client *http.Client, src Source, n *atomic.Int64) error {
+	req, err := http.NewRequestWithContext(ctx, "GET", src.URL(downChunk), nil)
 	if err != nil {
 		return err
 	}
@@ -522,11 +554,18 @@ func upstream(ctx context.Context, client *http.Client, n *atomic.Int64) error {
 // the only failure of this test anyone here is likely to meet.
 func answered(code int, status string) error {
 	if code == 429 || code == 403 {
-		return errors.New("the speed endpoint is rate-limiting this connection — it does that after a lot of testing; leave it a few minutes")
+		return errRefused
 	}
 	return fmt.Errorf("the speed endpoint answered %s", status)
 }
 
+// errRefused is the far end rate-limiting this connection, which is the only failure of this
+// test anyone here is likely to meet: it limits by volume, and a test moves a few hundred
+// megabytes. It is recognisable so the download phase can try its other source.
+var errRefused = errors.New("the speed endpoint is rate-limiting this connection — it does that after a lot of testing; leave it a few minutes")
+
+func refused(err error) bool { return errors.Is(err, errRefused) }
+
 // zeros is a request body of a fixed size that costs nothing to produce, counting itself as
 // it is read out onto the wire.
 type zeros struct {

web/src/Network.tsx

@@ -18,6 +18,12 @@ const scenery = (f: Iface) =>
 
 const KIND: Record<string, string> = { ethernet: 'wired', wifi: 'Wi-Fi', bridge: 'bridge', bond: 'bond', loopback: 'loopback', virtual: 'virtual', vlan: 'VLAN', wlan: 'Wi-Fi' }
 
+// Where the bytes came from. The usual source is anycast and its edge is the interesting
+// part; a fallback source is somewhere fixed, and then *which* one is the interesting part,
+// because a mirror far away reads low and the figure should not pretend otherwise.
+const where = (r: SpeedResult) =>
+  r.source && r.source !== 'cloudflare' ? ` from ${r.source}` : r.colo ? ` via ${r.colo}` : ''
+
 // A round number of megabits, which is how a line is sold and how it should be read.
 const mbps = (v: number) => (v >= 100 ? v.toFixed(0) : v.toFixed(1))
 const ago = (iso: string) => {
@@ -121,7 +127,7 @@ export default function Network() {
           {running
             ? <><Btn className="btn danger" onClick={cancel}>Stop</Btn><span className="muted small">running on {d.report.host} — it carries on if you close this</span></>
             : <><Btn className="btn on" busy={busy} onClick={start}>Run speed test</Btn>
-              <span className="muted small">{last ? `${fmtBytes((last.down_bytes ?? 0) + (last.up_bytes ?? 0))} moved in ${last.took_sec}s${last.colo ? ` via ${last.colo}` : ''}` : 'about 15 seconds'}</span></>}
+              <span className="muted small">{last ? `${fmtBytes((last.down_bytes ?? 0) + (last.up_bytes ?? 0))} moved in ${last.took_sec}s${where(last)}` : 'about ten seconds'}</span></>}
         </div>
         {failed && <div className="error">{failed === 'cancelled' ? 'Stopped.' : failed}</div>}
         {!failed && last?.note && <div className="notice">{last.note}</div>}

web/src/api.ts

@@ -182,7 +182,7 @@ export interface NetReport { at: string; host?: string; ifaces: Iface[]; gateway
 export interface Outside { at: string; ip?: string; colo?: string; country?: string; error?: string }
 export interface SpeedResult {
   at: string; down_mbps: number; up_mbps: number; latency_ms: number; jitter_ms: number
-  down_bytes: number; up_bytes: number; took_sec: number; colo?: string; ip?: string; note?: string; error?: string
+  down_bytes: number; up_bytes: number; took_sec: number; colo?: string; ip?: string; source?: string; note?: string; error?: string
 }
 // A test in progress, or the last one to have finished. `done` is 0–1 across the whole test
 // and `mbps` is the live figure for the phase under way.