[chore/bugfix] Fix double gzip on prometheus endpoint (#2383)

* [chore] Move "/metrics" into separate API module

* use our own gzip middleware for prom

3 files changed, 95 insertions, 21 deletions

diff --git a/internal/api/metrics.go b/internal/api/metrics.go
new file mode 100644
index 000000000..5d97b9610
--- /dev/null
+++ b/internal/api/metrics.go
@@ -0,0 +1,66 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package api
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api/metrics"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/middleware"
+	"github.com/superseriousbusiness/gotosocial/internal/router"
+)
+
+type Metrics struct {
+	metrics *metrics.Module
+}
+
+func (mt *Metrics) Route(r *router.Router, m ...gin.HandlerFunc) {
+	if !config.GetMetricsEnabled() {
+		// Noop: metrics
+		// not enabled.
+		return
+	}
+
+	// Create new group on top level "metrics" prefix.
+	metricsGroup := r.AttachGroup("metrics")
+	metricsGroup.Use(m...)
+	metricsGroup.Use(
+		middleware.CacheControl(middleware.CacheControlConfig{
+			// Never cache metrics responses.
+			Directives: []string{"no-store"},
+		}),
+	)
+
+	// Attach basic auth if enabled.
+	if config.GetMetricsAuthEnabled() {
+		var (
+			username = config.GetMetricsAuthUsername()
+			password = config.GetMetricsAuthPassword()
+			accounts = gin.Accounts{username: password}
+		)
+		metricsGroup.Use(gin.BasicAuth(accounts))
+	}
+
+	mt.metrics.Route(metricsGroup.Handle)
+}
+
+func NewMetrics() *Metrics {
+	return &Metrics{
+		metrics: metrics.New(),
+	}
+}
diff --git a/internal/web/metrics.go b/internal/api/metrics/metrics.go
index eb5530290..d89e56ad5 100644
--- a/internal/web/metrics.go
+++ b/internal/api/metrics/metrics.go
@@ -15,19 +15,40 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-package web
+package metrics
 
 import (
+	"net/http"
+
 	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const (
-	metricsPath = "/metrics"
-	metricsUser = "metrics"
-)
+type Module struct {
+	handler http.Handler
+}
+
+func New() *Module {
+	// Use our own gzip handler.
+	opts := promhttp.HandlerOpts{
+		DisableCompression: true,
+	}
+
+	// Instrument handler itself.
+	handler := promhttp.InstrumentMetricHandler(
+		prometheus.DefaultRegisterer,
+		promhttp.HandlerFor(prometheus.DefaultGatherer, opts),
+	)
+
+	return &Module{
+		handler: handler,
+	}
+}
 
-func (m *Module) metricsGETHandler(c *gin.Context) {
-	h := promhttp.Handler()
-	h.ServeHTTP(c.Writer, c.Request)
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, "", func(c *gin.Context) {
+		// Defer all "/metrics" handling to prom.
+		m.handler.ServeHTTP(c.Writer, c.Request)
+	})
 }
diff --git a/internal/web/web.go b/internal/web/web.go
index 6a21a754b..86e74d6f8 100644
--- a/internal/web/web.go
+++ b/internal/web/web.go
@@ -110,19 +110,6 @@ func (m *Module) Route(r *router.Router, mi ...gin.HandlerFunc) {
 	r.AttachHandler(http.MethodGet, domainBlockListPath, m.domainBlockListGETHandler)
 	r.AttachHandler(http.MethodGet, tagsPath, m.tagGETHandler)
 
-	// Prometheus metrics export endpoint
-	if config.GetMetricsEnabled() {
-		metricsGroup := r.AttachGroup(metricsPath)
-		metricsGroup.Use(mi...)
-		// Attach basic auth if enabled
-		if config.GetMetricsAuthEnabled() {
-			metricsGroup.Use(gin.BasicAuth(gin.Accounts{
-				config.GetMetricsAuthUsername(): config.GetMetricsAuthPassword(),
-			}))
-		}
-		metricsGroup.Handle(http.MethodGet, "", m.metricsGETHandler)
-	}
-
 	// Attach redirects from old endpoints to current ones for backwards compatibility
 	r.AttachHandler(http.MethodGet, "/auth/edit", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })
 	r.AttachHandler(http.MethodGet, "/user", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })