fix(quickproxy): reject Exclude entries outside their rule's URLPrefix

An Exclude entry only ever matches requests that already fall under
its rule's URLPrefix, so one written without that prefix (e.g.
"/health" on a rule for "/api") silently never triggered. Validate
that each Exclude entry itself starts with the rule's URLPrefix,
failing NewService instead of accepting a no-op config.

pkg/server/quickproxy/quickproxy.go

@@ -0,0 +1,222 @@
+// Package quickproxy provides a small reverse-proxy layer that tries a set
+// of configured upstream targets first, and falls back to a caller-supplied
+// http.Handler (typically static file serving) when the upstream is
+// unreachable or returns 404.
+package quickproxy
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"sort"
+	"strings"
+	"time"
+)
+
+// Rule maps a URL path prefix to an upstream target.
+// A Rule with URLPrefix "/" acts as a catch-all passthrough.
+type Rule struct {
+	// URLPrefix is the URL path prefix this rule matches. Must start with "/".
+	URLPrefix string
+
+	// Target is the upstream base URL, e.g. "http://localhost:3000".
+	// The incoming request path and query are forwarded unchanged; only the
+	// scheme and host are rewritten to Target's.
+	Target string
+
+	// Exclude is a list of URL path prefixes that this rule should not
+	// proxy, even though they fall under URLPrefix. Each entry is a full
+	// path from root and must itself start with URLPrefix (e.g. rule
+	// URLPrefix "/api" excluding a subpath must use "/api/health", not
+	// "/health"). A request matching an Exclude prefix is treated as if
+	// this rule didn't match at all: matching continues against any other
+	// configured rule, falling back if none match. This is typically used
+	// to carve out paths (e.g. "/health") from a catch-all "/" rule so
+	// they're served by the fallback handler instead of being proxied.
+	Exclude []string
+}
+
+// DefaultTimeout is the dial and response-header timeout applied to
+// upstream requests when no WithTimeout option is given. It does not limit
+// response body streaming.
+const DefaultTimeout = 10 * time.Second
+
+// Option configures a Service.
+type Option func(*options)
+
+type options struct {
+	timeout time.Duration
+}
+
+// WithTimeout sets the dial and response-header timeout used when
+// connecting to upstream targets. It does not limit response body
+// streaming, so it won't interrupt long-lived downloads or SSE/WebSocket
+// connections once established.
+func WithTimeout(d time.Duration) Option {
+	return func(o *options) { o.timeout = d }
+}
+
+// compiledRule pairs a Rule with its ready-to-use reverse proxy.
+type compiledRule struct {
+	prefix   string
+	excludes []string
+	proxy    *httputil.ReverseProxy
+}
+
+// excluded reports whether path falls under one of the rule's Exclude prefixes.
+func (r *compiledRule) excluded(path string) bool {
+	for _, ex := range r.excludes {
+		if strings.HasPrefix(path, ex) {
+			return true
+		}
+	}
+	return false
+}
+
+// Service holds a compiled set of proxy rules and performs longest-prefix
+// matching against them. A Service is safe for concurrent use once
+// returned from NewService; Handler must be called once per Service to
+// wire up the fallback handler before the returned http.Handler is served.
+type Service struct {
+	rules []compiledRule // sorted by descending prefix length
+}
+
+// errUpstreamNotFound is a sentinel error returned from ModifyResponse to
+// make ReverseProxy invoke ErrorHandler (our fallback path) instead of
+// writing the upstream's 404 to the client. Nothing has been written to
+// the ResponseWriter yet when this happens.
+var errUpstreamNotFound = errors.New("quickproxy: upstream returned 404")
+
+// NewService compiles the given rules into a Service. Rules are matched by
+// longest URLPrefix, so a catch-all "/" rule can coexist with more specific
+// rules such as "/api".
+func NewService(rules []Rule, opts ...Option) (*Service, error) {
+	if len(rules) == 0 {
+		return nil, fmt.Errorf("quickproxy: no rules configured")
+	}
+
+	cfg := options{timeout: DefaultTimeout}
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+
+	seen := make(map[string]bool, len(rules))
+	compiled := make([]compiledRule, 0, len(rules))
+
+	for _, r := range rules {
+		if !strings.HasPrefix(r.URLPrefix, "/") {
+			return nil, fmt.Errorf("quickproxy: rule prefix %q must start with /", r.URLPrefix)
+		}
+		if seen[r.URLPrefix] {
+			return nil, fmt.Errorf("quickproxy: duplicate rule prefix %q", r.URLPrefix)
+		}
+		seen[r.URLPrefix] = true
+
+		target, err := url.Parse(r.Target)
+		if err != nil || target.Scheme == "" || target.Host == "" {
+			return nil, fmt.Errorf("quickproxy: invalid target %q for prefix %q", r.Target, r.URLPrefix)
+		}
+
+		for _, ex := range r.Exclude {
+			if !strings.HasPrefix(ex, "/") {
+				return nil, fmt.Errorf("quickproxy: exclude prefix %q for rule %q must start with /", ex, r.URLPrefix)
+			}
+			if !strings.HasPrefix(ex, r.URLPrefix) {
+				return nil, fmt.Errorf("quickproxy: exclude prefix %q for rule %q must itself start with the rule's URLPrefix", ex, r.URLPrefix)
+			}
+		}
+
+		compiled = append(compiled, compiledRule{
+			prefix:   r.URLPrefix,
+			excludes: r.Exclude,
+			proxy:    newReverseProxy(target, cfg.timeout),
+		})
+	}
+
+	// Longest prefix first, so the first match in Handler is always the
+	// most specific one.
+	sort.Slice(compiled, func(i, j int) bool {
+		return len(compiled[i].prefix) > len(compiled[j].prefix)
+	})
+
+	return &Service{rules: compiled}, nil
+}
+
+func newReverseProxy(target *url.URL, timeout time.Duration) *httputil.ReverseProxy {
+	transport := &http.Transport{
+		DialContext: (&net.Dialer{
+			Timeout: timeout,
+		}).DialContext,
+		ResponseHeaderTimeout: timeout,
+	}
+
+	return &httputil.ReverseProxy{
+		Transport: transport,
+		Director: func(req *http.Request) {
+			originalHost := req.Host
+
+			req.URL.Scheme = target.Scheme
+			req.URL.Host = target.Host
+			req.Host = target.Host
+
+			if originalHost != "" {
+				req.Header.Set("X-Forwarded-Host", originalHost)
+			}
+		},
+		ModifyResponse: func(resp *http.Response) error {
+			if resp.StatusCode == http.StatusNotFound {
+				return errUpstreamNotFound
+			}
+			return nil
+		},
+	}
+}
+
+// Handler returns an http.Handler that tries the configured proxy rules
+// first (longest-prefix match), and calls fallback when no rule matches,
+// the upstream is unreachable, or the upstream returns 404. Any other
+// upstream response (2xx, other 4xx, 5xx) is streamed through to the
+// client unchanged.
+//
+// Handler wires up ErrorHandler on the Service's compiled rules, so it
+// should be called once per Service, before the returned http.Handler
+// starts serving requests.
+func (s *Service) Handler(fallback http.Handler) http.Handler {
+	if fallback == nil {
+		fallback = http.NotFoundHandler()
+	}
+
+	for i := range s.rules {
+		s.rules[i].proxy.ErrorHandler = func(w http.ResponseWriter, r *http.Request, _ error) {
+			fallback.ServeHTTP(w, r)
+		}
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rule := s.match(r.URL.Path)
+		if rule == nil {
+			fallback.ServeHTTP(w, r)
+			return
+		}
+		rule.proxy.ServeHTTP(w, r)
+	})
+}
+
+// match returns the longest-prefix rule matching path, or nil if none match.
+// A rule whose Exclude covers path is skipped, and matching continues
+// against the next-longest-prefix rule.
+func (s *Service) match(path string) *compiledRule {
+	for i := range s.rules {
+		if !strings.HasPrefix(path, s.rules[i].prefix) {
+			continue
+		}
+		if s.rules[i].excluded(path) {
+			continue
+		}
+		return &s.rules[i]
+	}
+	return nil
+}

pkg/server/quickproxy/quickproxy_test.go

@@ -0,0 +1,322 @@
+package quickproxy
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestNewService_Validation(t *testing.T) {
+	tests := []struct {
+		name    string
+		rules   []Rule
+		wantErr bool
+	}{
+		{"no rules", nil, true},
+		{"empty rules", []Rule{}, true},
+		{"bad prefix", []Rule{{URLPrefix: "api", Target: "http://localhost:1"}}, true},
+		{"bad target", []Rule{{URLPrefix: "/api", Target: "not-a-url"}}, true},
+		{"missing host", []Rule{{URLPrefix: "/api", Target: "http://"}}, true},
+		{"duplicate prefix", []Rule{
+			{URLPrefix: "/api", Target: "http://localhost:1"},
+			{URLPrefix: "/api", Target: "http://localhost:2"},
+		}, true},
+		{"bad exclude prefix", []Rule{
+			{URLPrefix: "/", Target: "http://localhost:1", Exclude: []string{"health"}},
+		}, true},
+		{"exclude outside rule's URLPrefix", []Rule{
+			{URLPrefix: "/api", Target: "http://localhost:1", Exclude: []string{"/health"}},
+		}, true},
+		{"valid", []Rule{{URLPrefix: "/api", Target: "http://localhost:1"}}, false},
+		{"valid with exclude", []Rule{
+			{URLPrefix: "/", Target: "http://localhost:1", Exclude: []string{"/health"}},
+		}, false},
+		{"valid with nested exclude", []Rule{
+			{URLPrefix: "/api", Target: "http://localhost:1", Exclude: []string{"/api/health"}},
+		}, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := NewService(tt.rules)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("NewService() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func fallbackHandler(body string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(body))
+	})
+}
+
+func TestHandler_ProxiesSuccessResponse(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("upstream:" + r.URL.Path))
+	}))
+	defer upstream.Close()
+
+	svc, err := NewService([]Rule{{URLPrefix: "/api", Target: upstream.URL}})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback"))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/widgets", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "upstream:/api/widgets" {
+		t.Fatalf("body = %q", got)
+	}
+}
+
+func TestHandler_404FallsBack(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte("upstream not found"))
+	}))
+	defer upstream.Close()
+
+	svc, err := NewService([]Rule{{URLPrefix: "/", Target: upstream.URL}})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback-content"))
+
+	req := httptest.NewRequest(http.MethodGet, "/missing.html", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "fallback-content" {
+		t.Fatalf("body = %q, want fallback-content", got)
+	}
+}
+
+func TestHandler_UnreachableUpstreamFallsBack(t *testing.T) {
+	// A closed listener address: nothing is listening, so dialing fails.
+	unreachable := "http://127.0.0.1:1"
+
+	svc, err := NewService([]Rule{{URLPrefix: "/", Target: unreachable}}, WithTimeout(500*time.Millisecond))
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback-content"))
+
+	req := httptest.NewRequest(http.MethodGet, "/anything", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "fallback-content" {
+		t.Fatalf("body = %q, want fallback-content", got)
+	}
+}
+
+func TestHandler_NonNotFoundErrorsPassThrough(t *testing.T) {
+	codes := []int{http.StatusOK, http.StatusForbidden, http.StatusBadRequest, http.StatusInternalServerError}
+
+	for _, code := range codes {
+		upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(code)
+			_, _ = w.Write([]byte("upstream response"))
+		}))
+
+		svc, err := NewService([]Rule{{URLPrefix: "/", Target: upstream.URL}})
+		if err != nil {
+			upstream.Close()
+			t.Fatalf("NewService: %v", err)
+		}
+
+		handler := svc.Handler(fallbackHandler("fallback-content"))
+
+		req := httptest.NewRequest(http.MethodGet, "/x", nil)
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != code {
+			t.Errorf("status for upstream code %d = %d, want %d", code, rr.Code, code)
+		}
+		if got := rr.Body.String(); got != "upstream response" {
+			t.Errorf("body for upstream code %d = %q, want passthrough", code, got)
+		}
+
+		upstream.Close()
+	}
+}
+
+func TestHandler_LongestPrefixMatch(t *testing.T) {
+	specific := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("specific"))
+	}))
+	defer specific.Close()
+
+	general := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("general"))
+	}))
+	defer general.Close()
+
+	svc, err := NewService([]Rule{
+		{URLPrefix: "/", Target: general.URL},
+		{URLPrefix: "/api/v1", Target: specific.URL},
+	})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback"))
+
+	for path, want := range map[string]string{
+		"/api/v1/thing": "specific",
+		"/api/other":    "general",
+		"/anything":     "general",
+	} {
+		req := httptest.NewRequest(http.MethodGet, path, nil)
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if got := rr.Body.String(); got != want {
+			t.Errorf("path %s: body = %q, want %q", path, got, want)
+		}
+	}
+}
+
+func TestHandler_ExcludeFallsBackToFallback(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("upstream:" + r.URL.Path))
+	}))
+	defer upstream.Close()
+
+	svc, err := NewService([]Rule{
+		{URLPrefix: "/", Target: upstream.URL, Exclude: []string{"/health"}},
+	})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback-content"))
+
+	req := httptest.NewRequest(http.MethodGet, "/health", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if got := rr.Body.String(); got != "fallback-content" {
+		t.Fatalf("body = %q, want fallback-content", got)
+	}
+
+	req = httptest.NewRequest(http.MethodGet, "/health/live", nil)
+	rr = httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if got := rr.Body.String(); got != "fallback-content" {
+		t.Fatalf("body = %q, want fallback-content", got)
+	}
+
+	req = httptest.NewRequest(http.MethodGet, "/other", nil)
+	rr = httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if got := rr.Body.String(); got != "upstream:/other" {
+		t.Fatalf("body = %q, want upstream:/other", got)
+	}
+}
+
+func TestHandler_ExcludeFallsThroughToNextRule(t *testing.T) {
+	specific := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("specific"))
+	}))
+	defer specific.Close()
+
+	general := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("general"))
+	}))
+	defer general.Close()
+
+	svc, err := NewService([]Rule{
+		{URLPrefix: "/api", Target: general.URL},
+		{URLPrefix: "/api/v1", Target: specific.URL, Exclude: []string{"/api/v1/health"}},
+	})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback"))
+
+	for path, want := range map[string]string{
+		"/api/v1/thing":  "specific",
+		"/api/v1/health": "general",
+	} {
+		req := httptest.NewRequest(http.MethodGet, path, nil)
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if got := rr.Body.String(); got != want {
+			t.Errorf("path %s: body = %q, want %q", path, got, want)
+		}
+	}
+}
+
+func TestHandler_NoMatchFallsBack(t *testing.T) {
+	svc, err := NewService([]Rule{{URLPrefix: "/api", Target: "http://127.0.0.1:1"}})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback-content"))
+
+	req := httptest.NewRequest(http.MethodGet, "/other", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "fallback-content" {
+		t.Fatalf("body = %q, want fallback-content", got)
+	}
+}
+
+func TestHandler_AllMethodsProxied(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(r.Method + ":" + string(body)))
+	}))
+	defer upstream.Close()
+
+	svc, err := NewService([]Rule{{URLPrefix: "/api", Target: upstream.URL}})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	handler := svc.Handler(fallbackHandler("fallback"))
+
+	methods := []string{http.MethodGet, http.MethodPost, http.MethodPut, http.MethodPatch, http.MethodDelete}
+	for _, method := range methods {
+		req := httptest.NewRequest(method, "/api/widgets", nil)
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		want := method + ":"
+		if got := rr.Body.String(); got != want {
+			t.Errorf("method %s: body = %q, want %q", method, got, want)
+		}
+	}
+}