INF-1671/fix: clamp and floor Retry-After in backoffDelay

Treat the Retry-After header as a lower bound clamped to maxBackoff rather
than an override. A hostile or misconfigured Retry-After: 999999 no longer
parks the CLI for days, and Retry-After: 0 after a rate-limit reset no longer
collapses into a zero-delay retry storm. Mirrors the twoadm-cli httpx
hardening so the two CLIs keep the same retry semantics (INF-1314).

Author: brtkwr

internal/httpx/backoff_test.go

@@ -34,3 +34,34 @@ func TestBackoffDelayBadRetryAfterFallsBack(t *testing.T) {
 		t.Errorf("garbage Retry-After should fall back to backoff, got %v", d)
 	}
 }
+
+func TestBackoffDelayClampsLargeRetryAfter(t *testing.T) {
+	// A hostile or misconfigured upstream must not be able to park the CLI for
+	// days: the hint is clamped to maxBackoff.
+	resp := &http.Response{Header: http.Header{"Retry-After": []string{"999999"}}}
+	if d := backoffDelay(0, resp); d != maxBackoff {
+		t.Errorf("Retry-After: 999999 → %v, want clamp to %v", d, maxBackoff)
+	}
+}
+
+func TestBackoffDelayFloorsZeroRetryAfter(t *testing.T) {
+	// Retry-After: 0 (e.g. a fleet-wide rate-limit reset) must not collapse
+	// into a zero-delay retry storm; it floors at the jittered backoff.
+	resp := &http.Response{Header: http.Header{"Retry-After": []string{"0"}}}
+	d := backoffDelay(0, resp)
+	if d < baseBackoff {
+		t.Errorf("Retry-After: 0 → %v, want at least baseBackoff %v", d, baseBackoff)
+	}
+	if d > maxBackoff+maxBackoff/4 {
+		t.Errorf("Retry-After: 0 → %v exceeds cap+jitter window", d)
+	}
+}
+
+func TestBackoffDelayRetryAfterIsLowerBoundNotCeiling(t *testing.T) {
+	// At a high attempt our own backoff already sits at the cap; a small
+	// Retry-After must not shrink it below that.
+	resp := &http.Response{Header: http.Header{"Retry-After": []string{"1"}}}
+	if d := backoffDelay(6, resp); d < maxBackoff {
+		t.Errorf("backoff at attempt 6 should be >= %v despite Retry-After: 1, got %v", maxBackoff, d)
+	}
+}

internal/httpx/client.go

@@ -183,16 +183,24 @@ func shouldRetry(resp *http.Response, err error) bool {
 	return false
 }
 
-// backoffDelay honours `Retry-After` when set, otherwise applies exponential
-// backoff with jitter so a thundering herd doesn't synchronise.
+// backoffDelay applies exponential backoff with jitter so a thundering herd
+// doesn't synchronise, and treats `Retry-After` as a lower bound rather than an
+// override: the server hint is honoured only when it exceeds our own backoff,
+// and is clamped to maxBackoff. This keeps `Retry-After: 0` (e.g. a fleet-wide
+// rate-limit reset) from collapsing into a zero-delay retry storm, and stops a
+// hostile or misconfigured `Retry-After: 999999` from parking the CLI for days.
+// Mirrors twoadm-cli internal/httpx (see INF-1314).
 func backoffDelay(attempt int, resp *http.Response) time.Duration {
-	if resp != nil {
-		if v := resp.Header.Get("Retry-After"); v != "" {
-			if secs, err := strconv.Atoi(strings.TrimSpace(v)); err == nil && secs >= 0 {
-				return time.Duration(secs) * time.Second
-			}
-		}
+	d := jitteredBackoff(attempt)
+	if hint := retryAfter(resp); hint > d {
+		d = hint
 	}
+	return d
+}
+
+// jitteredBackoff returns the exponential backoff for `attempt` (0-based) with
+// [0, 25%) additive jitter, capped at maxBackoff.
+func jitteredBackoff(attempt int) time.Duration {
 	// Clamp attempt so the shift below can't overflow time.Duration on
 	// pathological MaxRetries values. baseBackoff << 6 = 16s, already
 	// past the cap, so any attempt ≥ 6 lands on maxBackoff anyway.
@@ -209,3 +217,25 @@ func backoffDelay(attempt int, resp *http.Response) time.Duration {
 	}
 	return d + time.Duration(rand.Int64N(jitter))
 }
+
+// retryAfter parses the `Retry-After` header (seconds form), clamped to
+// maxBackoff. Returns 0 when the header is absent, negative, or unparseable so
+// the caller falls back to jittered exponential backoff.
+func retryAfter(resp *http.Response) time.Duration {
+	if resp == nil {
+		return 0
+	}
+	v := strings.TrimSpace(resp.Header.Get("Retry-After"))
+	if v == "" {
+		return 0
+	}
+	secs, err := strconv.Atoi(v)
+	if err != nil || secs < 0 {
+		return 0
+	}
+	d := time.Duration(secs) * time.Second
+	if d > maxBackoff {
+		return maxBackoff
+	}
+	return d
+}