refactor(billing)!: 计费全部沉入插件,内核零 billing 代码;规则由插件自持

按维护者要求(issue #1 回复方向):内核不处理任何 billing 特有逻辑;插件有完整
OS 权限,自己保存配置文件。据此不再保留任何"内核 ↔ billing 插件"的专用对接。

- `internal/billing`(Go 编译器 + 测试):删除;
- `internal/core` 的 billing 接口/降级应用/规则加载:删除(含 `billingPath`);
- `internal/gateway` 的 `/api/plugins/billing/rules` 端点与测试:删除;
- `config` 的 `BillingDSL/Profile/Rule/Token/Peak/LongContext` 与校验、
  `billing:`/`billing_file:` 配置段:删除。系统配置里不再有 billing 段。
内核只保留所有插件通用的能力:加载 .lua、插件状态持久化、插件目录
(`__llmsproxy_plugin_dir`)、`plugin.http` 路由、插件可用的 io/os。

- 规则文件移到**插件自己的目录**:`<plugin_dir>/billing.json`,由插件用 io 自己
  读写(tmp + rename 原子替换);加载插件时自己读文件并编译;
- 规则以**源名** `source` 为键(`url:"*"` 仅表示全局模型价目表):URL→源名映射
  属于内核配置,插件不为它和内核对接;编辑页在浏览器里取 `/api/sources` 选源名;
- 规则编辑 API 改为插件自己的 `/api/plugins/http/billing/rules`
  (GET/PUT/POST/DELETE,roles=admin),写盘也在插件内完成;
- 计费账目仍在插件状态文件(内核通用持久化)`<plugin_dir>/.billing.state.json`;
- `priceFor` 落地 profile 级 `default` 策略与显式 `mode=unpriced` 的优先级。

生产已迁移:`config.yaml` 的 `billing:` 规则转为 `plugins/billing.json`(按源名
重写),并从系统配置中移除;规则文件与系统配置从此各自独立保存。

- `internal/lua/billing_rules_test.go`:插件自写 billing.json 并重启恢复、
  free/subscription/token/通配符编译语义、1e6+1e6 token = 0.75 端到端、
  非法规则被拒且不覆盖旧文件、reasoning/long_context/peak;
- `internal/lua/billing_test.go`:profile default 策略三态 + 显式 unpriced 优先;
- `internal/lua/billing_ui_test.go`:规则编辑页仍挂在 billing 页内、列宽/列数守卫。

(cherry picked from commit 7941b4110a)
This commit is contained in:
root
2026-10-20 18:57:23 +08:00
parent b24b69719d
commit db1cb7182d
11 changed files with 878 additions and 845 deletions

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@ -1,282 +0,0 @@
package gateway
import (
"encoding/json"
"net/http"
"llmsproxy/internal/billing"
"llmsproxy/internal/config"
)
// Billing rules API.
//
// GET /api/plugins/billing/rules the active profile's rules + all profiles
// PUT /api/plugins/billing/rules replace the whole DSL and re-inject prices
// POST /api/plugins/billing/rules { profile, rule } append one rule
// DELETE /api/plugins/billing/rules { profile, url } remove one rule
//
// Why a separate endpoint instead of the generic plugin state: prices are
// plugin state, but they are also a durable, reviewable CONFIGURATION. Routing
// them through /state would let an admin key PUT arbitrary plugin state and
// silently reset the accumulated accounting. Here the server owns the shape:
// it validates the DSL, writes it back to config.yaml, recompiles, and hands
// the plugin its prices table. The operator edits rules; the plugin keeps
// numbers. The two are never mixed in one payload.
//
// Editing means "what the operator typed is what config.yaml holds". A
// billing mistake found next week must be traceable to a reviewable file, not
// to a blob in the plugin's state sidecar.
func (g *Gateway) handleBillingRules(w http.ResponseWriter, r *http.Request) {
if reqRole(r.Context()) != "admin" {
writeError(w, http.StatusForbidden, "forbidden", "admin role required")
return
}
switch r.Method {
case http.MethodGet:
g.getBillingRules(w)
case http.MethodPut:
g.putBillingRules(w, r)
case http.MethodPost:
g.addBillingRule(w, r)
case http.MethodDelete:
g.delBillingRule(w, r)
default:
writeError(w, http.StatusMethodNotAllowed, "method_not_allowed", "use GET/PUT/POST/DELETE")
}
}
// billingRulesPayload is the UI-facing view: the profiles as declared, plus
// which one is active and the source URLs they can match — the editor needs
// the URL list to offer suggestions, and an operator typing a URL that matches
// no source is exactly the silent-no-pricing failure this feature exists to
// remove.
type billingRulesPayload struct {
DSL *config.BillingDSL `json:"billing"`
Active string `json:"active"`
URLs []string `json:"urls"`
Warnings []string `json:"warnings"`
}
func (g *Gateway) billingRulesPayload() billingRulesPayload {
out := billingRulesPayload{}
cfg := g.core.Config()
if cfg != nil && cfg.BillingDSL != nil {
// Copy so the response cannot be mutated back through the pointer.
cp := *cfg.BillingDSL
out.DSL = &cp
if p := cp.Resolve(""); p != nil {
out.Active = p.ID
}
}
for _, s := range g.sourceURLs() {
out.URLs = append(out.URLs, s)
}
out.Warnings = g.billingRuleWarnings()
return out
}
func (g *Gateway) sourceURLs() []string {
cfg := g.core.Config()
if cfg == nil {
return nil
}
seen := map[string]bool{}
var out []string
for _, s := range cfg.Sources {
if s.BaseURL == "" || seen[s.BaseURL] {
continue
}
seen[s.BaseURL] = true
out = append(out, s.BaseURL)
}
return out
}
// billingRuleWarnings reports rules that match no configured source. This is
// the silent killer of hand-written price tables: the rule parses, validates,
// and prices nothing at all, so every request on that URL lands in unpriced.
// Saying so out loud is the difference between a five-second fix and an
// afternoon wondering why the bill is zero.
func (g *Gateway) billingRuleWarnings() []string {
cfg := g.core.Config()
if cfg == nil || cfg.BillingDSL == nil {
return nil
}
urls := map[string]bool{}
for _, u := range g.sourceURLs() {
urls[u] = true
}
var warns []string
for _, p := range cfg.BillingDSL.Profiles {
for _, r := range p.Rules {
if r.URL == "*" || urls[r.URL] {
continue
}
warns = append(warns, "profile "+p.ID+": rule url "+r.URL+" matches no configured source")
}
}
return warns
}
func (g *Gateway) getBillingRules(w http.ResponseWriter) {
writeJSON(w, http.StatusOK, g.billingRulesPayload())
}
func (g *Gateway) putBillingRules(w http.ResponseWriter, r *http.Request) {
var body struct {
Billing *config.BillingDSL `json:"billing"`
}
if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
writeError(w, http.StatusBadRequest, "invalid_request", "invalid json: "+err.Error())
return
}
if body.Billing == nil {
writeError(w, http.StatusBadRequest, "invalid_request", "billing is required")
return
}
g.applyBillingDSLUpdate(w, body.Billing)
}
func (g *Gateway) addBillingRule(w http.ResponseWriter, r *http.Request) {
var body struct {
Profile string `json:"profile"`
Rule config.BillingRule `json:"rule"`
}
if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
writeError(w, http.StatusBadRequest, "invalid_request", "invalid json: "+err.Error())
return
}
cfg := g.core.Config()
if cfg == nil || cfg.BillingDSL == nil || len(cfg.BillingDSL.Profiles) == 0 {
writeError(w, http.StatusBadRequest, "no_profile", "no billing profiles configured; create one first")
return
}
next := cloneBillingDSL(cfg.BillingDSL)
idx := profileIndex(next, body.Profile)
if idx < 0 {
writeError(w, http.StatusBadRequest, "unknown_profile", "no such profile: "+body.Profile)
return
}
p := &next.Profiles[idx]
// Replace rather than append when the URL is already declared: two rules
// for one URL would silently make the first unreachable (first match wins),
// which is the exact ambiguity an editor should not be able to create.
replaced := false
for i := range p.Rules {
if p.Rules[i].URL == body.Rule.URL {
p.Rules[i] = body.Rule
replaced = true
break
}
}
if !replaced {
p.Rules = append(p.Rules, body.Rule)
}
g.applyBillingDSLUpdate(w, next)
}
func (g *Gateway) delBillingRule(w http.ResponseWriter, r *http.Request) {
var body struct {
Profile string `json:"profile"`
URL string `json:"url"`
}
if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
writeError(w, http.StatusBadRequest, "invalid_request", "invalid json: "+err.Error())
return
}
cfg := g.core.Config()
if cfg == nil || cfg.BillingDSL == nil {
writeError(w, http.StatusBadRequest, "no_profile", "no billing profiles configured")
return
}
next := cloneBillingDSL(cfg.BillingDSL)
idx := profileIndex(next, body.Profile)
if idx < 0 {
writeError(w, http.StatusBadRequest, "unknown_profile", "no such profile: "+body.Profile)
return
}
p := &next.Profiles[idx]
out := p.Rules[:0]
found := false
for _, rule := range p.Rules {
if rule.URL == body.URL {
found = true
continue
}
out = append(out, rule)
}
if !found {
writeError(w, http.StatusNotFound, "not_found", "no rule for url "+body.URL)
return
}
p.Rules = out
g.applyBillingDSLUpdate(w, next)
}
// applyBillingDSLUpdate is the single write path: validate, persist to
// config.yaml, recompile, inject, and only then report success. If the config
// cannot be written the change is NOT applied in memory either — a rules editor
// that says "saved" and loses the edit on the next restart is worse than one
// that refuses.
func (g *Gateway) applyBillingDSLUpdate(w http.ResponseWriter, next *config.BillingDSL) {
if err := next.Validate(); err != nil {
writeError(w, http.StatusBadRequest, "invalid_rules", err.Error())
return
}
cfg := g.core.Config()
if cfg == nil {
writeError(w, http.StatusInternalServerError, "no_config", "no config loaded")
return
}
// Assign BEFORE saving. The previous order saved first and assigned after,
// so every rule edit reported success while writing a config.yaml with no
// billing section at all — the operator's edit vanished on restart and
// nothing in the API response said so.
prev := cfg.BillingDSL
cfg.BillingDSL = next
// Compile before persisting: a profile that cannot be turned into prices
// must not reach the config file, or the next restart fails to load the
// very rules the editor just accepted.
prices, err := billing.CompileOpts(next.Resolve(next.Active), cfg.Sources, true)
if err != nil {
cfg.BillingDSL = prev // no half-applied state
writeError(w, http.StatusBadRequest, "compile_failed", err.Error())
return
}
if err := cfg.Save(); err != nil {
cfg.BillingDSL = prev
writeError(w, http.StatusInternalServerError, "persist_failed", err.Error())
return
}
ps := g.core.Plugins()
if ps != nil {
if err := ps.SetState("billing", map[string]interface{}{"prices": prices}); err != nil {
writeError(w, http.StatusBadRequest, "plugin_error", err.Error())
return
}
}
p := g.billingRulesPayload()
writeJSON(w, http.StatusOK, map[string]interface{}{
"ok": true, "billing": p.DSL, "active": p.Active, "warnings": p.Warnings,
})
}
func cloneBillingDSL(d *config.BillingDSL) *config.BillingDSL {
out := &config.BillingDSL{Active: d.Active}
out.Profiles = make([]config.BillingProfile, len(d.Profiles))
copy(out.Profiles, d.Profiles)
return out
}
func profileIndex(d *config.BillingDSL, id string) int {
if id == "" {
return -1
}
for i, p := range d.Profiles {
if p.ID == id {
return i
}
}
return -1
}

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@ -1,281 +0,0 @@
package gateway
import (
"encoding/json"
"net/http"
"net/http/httptest"
"os"
"sort"
"strings"
"testing"
"llmsproxy/internal/config"
)
// rulesGateway needs a real billing plugin (the API injects prices into it)
// AND two sources whose base_urls the rules will match, so it cannot reuse
// either existing helper: gatewayWithBilling has no sources, and
// newTestGateway loads no plugins.
func rulesGateway(t *testing.T) *Gateway {
t.Helper()
g := gatewayWithBilling(t)
cfg := g.core.Config()
cfg.Sources = []config.Source{
{Name: "localzen", BaseURL: "https://free.example.com/v1"},
{Name: "commandcode", BaseURL: "https://api.commandcode.ai/v1"},
}
return g
}
func rulesDo(t *testing.T, g *Gateway, method, body string) *httptest.ResponseRecorder {
t.Helper()
return doReq(t, g, method, "/api/plugins/billing/rules", body)
}
// TestBillingRulesCRUD covers the whole editor loop against the real HTTP
// surface: read, add, replace-by-URL, delete. The replace-by-URL case is the
// one worth pinning — two rules for one URL would make the first unreachable
// (first match wins) without any error, which is exactly the kind of silent
// ambiguity an editor must not be able to create.
func TestBillingRulesCRUD(t *testing.T) {
g := rulesGateway(t)
seed := `{"billing":{"active":"p1","profiles":[{"id":"p1","rules":[
{"url":"https://free.example.com/v1","mode":"free"}]}]}}`
if rr := rulesDo(t, g, http.MethodPut, seed); rr.Code != http.StatusOK {
t.Fatalf("seed rules = %d: %s", rr.Code, rr.Body.String())
}
// Add a token-priced rule for the second URL.
add := `{"profile":"p1","rule":{"url":"https://api.commandcode.ai/v1","mode":"token",
"models":{"deepseek-v4.1-flash":{"prompt":"0.15","completion":"0.60"}}}}`
if rr := rulesDo(t, g, http.MethodPost, add); rr.Code != http.StatusOK {
t.Fatalf("add rule = %d: %s", rr.Code, rr.Body.String())
}
var got billingRulesPayload
if err := json.Unmarshal(rulesDo(t, g, http.MethodGet, "").Body.Bytes(), &got); err != nil {
t.Fatal(err)
}
if len(got.DSL.Profiles[0].Rules) != 2 {
t.Fatalf("after add there are %d rules, want 2", len(got.DSL.Profiles[0].Rules))
}
// Adding the SAME url again must replace, not append.
if rr := rulesDo(t, g, http.MethodPost, `{"profile":"p1","rule":{"url":"https://api.commandcode.ai/v1","mode":"free"}}`); rr.Code != http.StatusOK {
t.Fatalf("re-add same url = %d: %s", rr.Code, rr.Body.String())
}
if err := json.Unmarshal(rulesDo(t, g, http.MethodGet, "").Body.Bytes(), &got); err != nil {
t.Fatal(err)
}
if n := len(got.DSL.Profiles[0].Rules); n != 2 {
t.Fatalf("re-adding an existing url appended instead of replacing: %d rules", n)
}
var mode string
for _, r := range got.DSL.Profiles[0].Rules {
if r.URL == "https://api.commandcode.ai/v1" {
mode = r.Mode
}
}
if mode != "free" {
t.Errorf("re-added rule mode = %q, want free (the replacement must win)", mode)
}
// Delete by URL.
if rr := rulesDo(t, g, http.MethodDelete, `{"profile":"p1","url":"https://api.commandcode.ai/v1"}`); rr.Code != http.StatusOK {
t.Fatalf("delete rule = %d: %s", rr.Code, rr.Body.String())
}
if err := json.Unmarshal(rulesDo(t, g, http.MethodGet, "").Body.Bytes(), &got); err != nil {
t.Fatal(err)
}
if len(got.DSL.Profiles[0].Rules) != 1 {
t.Errorf("after delete there are %d rules, want 1", len(got.DSL.Profiles[0].Rules))
}
}
// TestBillingRulesRejectUnknownURLWithWarning is the silent-failure guard: a
// rule whose URL matches no source prices nothing, so every request on it
// falls into unpriced. The API must say so instead of accepting it silently.
func TestBillingRulesRejectUnknownURLWithWarning(t *testing.T) {
g := rulesGateway(t)
if rr := rulesDo(t, g, http.MethodPut, `{"billing":{"active":"p","profiles":[{"id":"p",
"rules":[{"url":"https://typo.example.com/v1","mode":"free"}]}]}}`); rr.Code != http.StatusOK {
t.Fatalf("seed = %d: %s", rr.Code, rr.Body.String())
}
var got billingRulesPayload
if err := json.Unmarshal(rulesDo(t, g, http.MethodGet, "").Body.Bytes(), &got); err != nil {
t.Fatal(err)
}
if len(got.Warnings) == 0 {
t.Fatal("a rule matching no configured source was accepted with no warning — " +
"it will price nothing and every request will be recorded as unpriced")
}
if !strings.Contains(got.Warnings[0], "typo.example.com") {
t.Errorf("warning does not name the offending url: %v", got.Warnings)
}
// The editor needs the real URL list to offer suggestions.
if len(got.URLs) != 2 {
t.Errorf("urls offered to the editor = %v, want the 2 configured base_urls", got.URLs)
}
}
// TestBillingRulesInvalidIsRejectedNotPersisted checks the write path refuses
// bad input and leaves the previous rules in place — an editor that clears the
// table on a typo loses the price table.
func TestBillingRulesInvalidIsRejectedNotPersisted(t *testing.T) {
g := rulesGateway(t)
seed := `{"billing":{"active":"p","profiles":[{"id":"p","rules":[
{"url":"https://free.example.com/v1","mode":"free"}]}]}}`
if rr := rulesDo(t, g, http.MethodPut, seed); rr.Code != http.StatusOK {
t.Fatalf("seed = %d: %s", rr.Code, rr.Body.String())
}
// An unknown mode must be refused.
if rr := rulesDo(t, g, http.MethodPost, `{"profile":"p","rule":{"url":"https://x/v1","mode":"banana"}}`); rr.Code != http.StatusBadRequest {
t.Errorf("invalid mode accepted with %d, want 400", rr.Code)
}
var got billingRulesPayload
if err := json.Unmarshal(rulesDo(t, g, http.MethodGet, "").Body.Bytes(), &got); err != nil {
t.Fatal(err)
}
if len(got.DSL.Profiles[0].Rules) != 1 {
t.Errorf("a rejected rule was persisted anyway: %d rules", len(got.DSL.Profiles[0].Rules))
}
// An unparseable price must be refused too, not silently priced at zero.
if rr := rulesDo(t, g, http.MethodPost, `{"profile":"p","rule":{"url":"https://y/v1","mode":"token",
"models":{"m":{"prompt":"free","completion":"0.6"}}}}`); rr.Code != http.StatusBadRequest {
t.Errorf("non-numeric price accepted with %d, want 400", rr.Code)
}
}
// TestBillingRulesPersistAndReachThePlugin is the point of the whole
// endpoint, and mutation verification showed the earlier CRUD tests missed it
// twice: dropping cfg.Save() and dropping the SetState injection both left
// every test green. Both failures are invisible in a GET — the response echoes
// the in-memory rules either way — so this test checks the two consequences
// that actually matter:
//
// 1. persistence: the rule must be in the config file on disk, because an
// edit that evaporates on restart is a lie the UI told the operator;
// 2. effect: the billing plugin must now PRICE the source (its published
// prices table has a non-zero entry), because a rule that is stored but
// never injected prices nothing at all.
func TestBillingRulesPersistAndReachThePlugin(t *testing.T) {
g := rulesGateway(t)
cfg := g.core.Config()
seed := `{"billing":{"active":"p","profiles":[{"id":"p","rules":[
{"url":"https://free.example.com/v1","mode":"free"},
{"url":"https://api.commandcode.ai/v1","mode":"token",
"models":{"deepseek-v4.1-flash":{"prompt":"0.15","completion":"0.60"}}}]}]}}`
if rr := rulesDo(t, g, http.MethodPut, seed); rr.Code != http.StatusOK {
t.Fatalf("seed = %d: %s", rr.Code, rr.Body.String())
}
// 1. persisted to the file the process loaded from.
raw, err := os.ReadFile(cfg.Path)
if err != nil {
t.Fatalf("read config: %v", err)
}
if !strings.Contains(string(raw), "commandcode.ai") {
t.Errorf("the rule is not in config.yaml on disk — an edit that does not "+
"survive a restart:\n%s", raw)
}
reloaded, err := config.Load(cfg.Path)
if err != nil {
t.Fatalf("reload saved config: %v", err)
}
if reloaded.BillingDSL == nil || len(reloaded.BillingDSL.Profiles) == 0 {
t.Fatalf("saved config has no billing DSL: %s", raw)
}
// A round-trip through YAML must not lose the price strings.
found := false
for _, r := range reloaded.BillingDSL.Profiles[0].Rules {
if strings.Contains(r.URL, "commandcode") {
found = true
if m, ok := r.Models["deepseek-v4.1-flash"]; !ok || m.Prompt != "0.15" {
t.Errorf("price did not survive the YAML round-trip: %+v", m)
}
}
}
if !found {
t.Errorf("the token rule is missing after reload:\n%s", raw)
}
// 2. reached the plugin. Prices live in a separate Lua field from `state`,
// and the GET /state endpoint returns the published state together with
// the prices sidecar — which is exactly what the plugin's own UI reads.
state := g.core.Plugins().State("billing")
if state == nil {
t.Fatalf("billing plugin published no state")
}
rr := doReq(t, g, http.MethodGet, "/api/plugins/billing/state", "")
if rr.Code != http.StatusOK {
t.Fatalf("GET state = %d: %s", rr.Code, rr.Body.String())
}
var envelope struct {
Prices map[string]interface{} `json:"prices"`
}
if err := json.Unmarshal(rr.Body.Bytes(), &envelope); err != nil {
t.Fatalf("decode state envelope: %v", err)
}
prices := envelope.Prices
if prices == nil {
t.Fatalf("state payload carries no prices")
}
sources, ok := prices["sources"].(map[string]interface{})
if !ok {
t.Fatalf("prices has no sources map: %#v", prices)
}
cc, ok := sources["commandcode"]
if !ok {
t.Fatalf("the rule was saved but never injected: prices has %v, "+
"no commandcode entry. Every request on it stays unpriced.", keysOf(sources))
}
if cc == nil {
t.Error("commandcode price entry is nil")
}
// The free rule must price localzen, and the two must not be conflated.
if _, ok := sources["localzen"]; !ok {
t.Errorf("the free rule did not reach the plugin: %v", keysOf(sources))
}
// The injected price must be per-token, i.e. 0.15 USD/M => 1.5e-7.
// Asserting the exact magnitude catches a unit error (per-million vs
// per-token), which would be off by a factor of a million and still look
// like "a number".
models, ok := cc.(map[string]interface{})["models"].(map[string]interface{})
if ok {
if m, ok := models["deepseek-v4.1-flash"].(map[string]interface{}); ok {
p, _ := m["prompt"].(float64)
if p <= 0 || p > 1e-6 {
t.Errorf("injected prompt price = %v, want per-token (~1.5e-7); "+
"a per-million value here would over-bill by 1e6x", p)
}
}
}
}
func keysOf(m map[string]interface{}) []string {
out := make([]string, 0, len(m))
for k := range m {
out = append(out, k)
}
sort.Strings(out)
return out
}
// TestBillingRulesRequireAdmin keeps price rewriting behind the admin role,
// like every other plugin write: prices are configuration, not user data.
func TestBillingRulesRequireAdmin(t *testing.T) {
g := rulesGateway(t)
rec, err := g.core.CreateKey("viewer", "user", nil, "")
if err != nil {
t.Fatal(err)
}
userKey := rec.Key
for _, m := range []string{http.MethodPut, http.MethodPost, http.MethodDelete} {
req, _ := http.NewRequest(m, "/api/plugins/billing/rules", strings.NewReader(`{}`))
req.Header.Set("Authorization", "Bearer "+userKey)
req.Header.Set("Content-Type", "application/json")
if rr := newRecorderFor(t, g, req); rr.Code != http.StatusForbidden {
t.Errorf("user %s rules = %d, want 403", m, rr.Code)
}
}
}

View File

@ -247,11 +247,6 @@ func (g *Gateway) routes(w http.ResponseWriter, r *http.Request) {
g.handleSourceTemplatesAPI(w, r)
case r.URL.Path == "/api/ui-inject" || strings.HasPrefix(r.URL.Path, "/api/ui-inject/"):
g.handlePluginUI(w, r)
case r.URL.Path == "/api/plugins/billing/rules":
// Checked BEFORE the generic /api/plugins/ prefix: the plugin router
// treats anything after the name as a plugin sub-resource and would
// 404 on "rules" instead of reaching the billing editor.
g.handleBillingRules(w, r)
case strings.HasPrefix(r.URL.Path, pluginHTTPPrefix):
// Before the generic plugin router: these paths belong to a PLUGIN, not to
// the plugin manager, and "http" is a valid sub-resource name otherwise.

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@ -0,0 +1,15 @@
package gateway
import "sort"
// keysOf returns the sorted keys of a map, for readable failure messages.
// (It used to live in the deleted billing-rules test file; other gateway tests
// still use it.)
func keysOf(m map[string]interface{}) []string {
out := make([]string, 0, len(m))
for k := range m {
out = append(out, k)
}
sort.Strings(out)
return out
}