Limit each client's requests to the image routes (closes #236)

The new setting image_requests_per_minute (PIXA_IMAGE_REQUESTS_PER_MINUTE),
default 600, caps the GET and HEAD requests one client sends to /v1/image/
and /v1/e/ per minute; the next is answered 429 with Retry-After. It uses
the login limit's RateLimit middleware, so clients are told apart the same
way, and sits in the image routes' group behind the CORS middleware, so a
preflight is not counted. 0 turns it off; a negative or non-integer value
stops startup.

TestEnvironmentSetsEveryKey gains the new variable, under the owner's
standing approval in #64. To stay
within the linters' limits, the sentry_dsn check moves unchanged into
validateSentryDSN, and the cache_max_bytes explicit flag is set in one
line.

Model: opus-5-5

internal/config/config.go

@@ -34,6 +34,7 @@ const (
 	DefaultUpstreamFetchTimeout       = 30 * time.Second
 	DefaultUpstreamMaxResponseSize    = 50 << 20 // 50 MiB
 	DefaultDownstreamTimeout          = 60 * time.Second
+	DefaultImageRequestsPerMinute     = 600
 )
 
 // Configuration key names.
@@ -61,6 +62,7 @@ const (
 	keyUpstreamFetchTimeout       = "upstream_fetch_timeout"
 	keyUpstreamMaxResponseSize    = "upstream_max_response_size"
 	keyDownstreamTimeout          = "downstream_timeout"
+	keyImageRequestsPerMinute     = "image_requests_per_minute"
 )
 
 // placeholderSigningKey is the dummy signing_key shipped in
@@ -158,6 +160,11 @@ type Config struct {
 	// of the per-request timeout middleware.
 	DownstreamTimeout time.Duration
 
+	// ImageRequestsPerMinute is how many requests one client may make to
+	// the image routes per minute; the next is refused with 429 Too Many
+	// Requests. 0 turns the limit off.
+	ImageRequestsPerMinute int
+
 	// BlockedNetworks are operator-supplied CIDR ranges to refuse in
 	// addition to the built-in SSRF blocklist. Enforced by the upstream
 	// fetcher's dialer; the built-in ranges always apply.
@@ -302,6 +309,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 			keyAccessControlAllowOrigin, DefaultAccessControlAllowOrigin),
 		DownstreamTimeout: loader.durationVal(
 			keyDownstreamTimeout, DefaultDownstreamTimeout),
+		ImageRequestsPerMinute: loader.intVal(
+			keyImageRequestsPerMinute, DefaultImageRequestsPerMinute),
 		CacheMaxBytes:    loader.int64Val(keyCacheMaxBytes, 0),
 		BlockedNetworks:  blockedNetworks,
 		TrustedProxies:   trustedProxies,
@@ -311,9 +320,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 	// The default for an omitted cache_max_bytes is worked out when
 	// the cache opens; here we only record whether the operator set
 	// the key explicitly.
-	if _, present := lookupValue(sc, keyCacheMaxBytes); present {
-		c.CacheMaxBytesExplicit = true
-	}
+	_, c.CacheMaxBytesExplicit = lookupValue(sc, keyCacheMaxBytes)
 
 	// Build DBURL from StateDir if not explicitly set. The derived URL
 	// is a default: it applies only when db_url is omitted, never to an
@@ -426,7 +433,7 @@ func isKnownConfigKey(key string) bool {
 		keyMaxConcurrentProcessing, keyCacheMaxBytes, keyBlockedNetworks,
 		keyTrustedProxies, keyAccessControlAllowOrigin, keyUpstreamFetchTimeout,
 		keyUpstreamMaxResponseSize, keyDownstreamTimeout, keyRefererBlocklist,
-		"env":
+		keyImageRequestsPerMinute, "env":
 		return true
 	}
 
@@ -461,6 +468,7 @@ func envVarNames() map[string]string {
 		keyUpstreamFetchTimeout:       "PIXA_UPSTREAM_FETCH_TIMEOUT",
 		keyUpstreamMaxResponseSize:    "PIXA_UPSTREAM_MAX_RESPONSE_SIZE",
 		keyDownstreamTimeout:          "PIXA_DOWNSTREAM_TIMEOUT",
+		keyImageRequestsPerMinute:     "PIXA_IMAGE_REQUESTS_PER_MINUTE",
 	}
 }
 
@@ -613,6 +621,14 @@ func (c *Config) validate() error {
 			settingName(keyCacheMaxBytes), c.CacheMaxBytes, errMustNotBeNegative)
 	}
 
+	// Zero is valid (it turns the limit off); only negative values are
+	// rejected.
+	if c.ImageRequestsPerMinute < 0 {
+		return fmt.Errorf("%s: value %d %w",
+			settingName(keyImageRequestsPerMinute), c.ImageRequestsPerMinute,
+			errMustNotBeNegative)
+	}
+
 	err = c.validateUpstreamMaxResponseSize()
 	if err != nil {
 		return err
@@ -625,12 +641,9 @@ func (c *Config) validate() error {
 		}
 	}
 
-	if c.SentryDSN != "" {
-		parsed, err := url.Parse(c.SentryDSN)
-		if err != nil || parsed.Scheme == "" || parsed.Host == "" {
-			return fmt.Errorf("%s: value %q is %w",
-				settingName(keySentryDSN), c.SentryDSN, errNotAValidURL)
-		}
+	err = c.validateSentryDSN()
+	if err != nil {
+		return err
 	}
 
 	if (c.MetricsUsername == "") != (c.MetricsPassword == "") {
@@ -642,6 +655,22 @@ func (c *Config) validate() error {
 	return c.validateAccessControlAllowOrigin()
 }
 
+// validateSentryDSN checks that sentry_dsn, when set, is a URL with a scheme
+// and a host.
+func (c *Config) validateSentryDSN() error {
+	if c.SentryDSN == "" {
+		return nil
+	}
+
+	parsed, err := url.Parse(c.SentryDSN)
+	if err != nil || parsed.Scheme == "" || parsed.Host == "" {
+		return fmt.Errorf("%s: value %q is %w",
+			settingName(keySentryDSN), c.SentryDSN, errNotAValidURL)
+	}
+
+	return nil
+}
+
 // validateUpstreamMaxResponseSize checks that upstream_max_response_size
 // is from 1 byte to 1 GiB. An upstream response is read whole into
 // memory, and the image processor reads one byte past this limit, which