feat(quickproxy): support per-rule Exclude path prefixes

Rule.Exclude lists path prefixes that should never be proxied by that
rule, even though they fall under its URLPrefix. A request matching an
Exclude prefix is treated as a non-match for that rule: matching
continues against other configured rules, falling back to the
caller-supplied handler if none apply. Lets a catch-all "/" rule proxy
everything except carved-out paths like "/health".

pkg/server/quickproxy/quickproxy.go

@@ -26,6 +26,15 @@ type Rule struct {
 	// 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 must start
+	// with "/". 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
@@ -51,9 +60,20 @@ func WithTimeout(d time.Duration) Option {
 // 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
@@ -98,8 +118,15 @@ func NewService(rules []Rule, opts ...Option) (*Service, error) {
 			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)
+			}
+		}
+
 		compiled = append(compiled, compiledRule{
 			prefix:   r.URLPrefix,
+			excludes: r.Exclude,
 			proxy:    newReverseProxy(target, cfg.timeout),
 		})
 	}
@@ -174,11 +201,17 @@ func (s *Service) Handler(fallback http.Handler) http.Handler {
 }
 
 // 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) {
+		if !strings.HasPrefix(path, s.rules[i].prefix) {
-			return &s.rules[i]
+			continue
 		}
+		if s.rules[i].excluded(path) {
+			continue
+		}
+		return &s.rules[i]
 	}
 	return nil
 }

pkg/server/quickproxy/quickproxy_test.go

@@ -23,7 +23,13 @@ func TestNewService_Validation(t *testing.T) {
 			{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},
 		{"valid", []Rule{{URLPrefix: "/api", Target: "http://localhost:1"}}, false},
+		{"valid with exclude", []Rule{
+			{URLPrefix: "/", Target: "http://localhost:1", Exclude: []string{"/health"}},
+		}, false},
 	}
 
 	for _, tt := range tests {
@@ -186,6 +192,81 @@ func TestHandler_LongestPrefixMatch(t *testing.T) {
 	}
 }
 
+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 {