Send only the Referer's scheme and host to Sentry (closes #548)

A Sentry event kept the Referer header verbatim, and a Referer can be
an entrypoint URL: a sender that follows the README's http:// to
https:// redirect sends the URL it was redirected from, and a browser
sends the 404 page's URL under /h/ when it fetches that page's
stylesheet. The Sentry hook now keeps only the Referer's scheme and
host. The access log's reduction moves, unchanged, into a new package,
internal/referer, as SchemeAndHost; the access log and the Sentry hook
both call it there. The comment above sentryKeepsHeader and the
README's Sentry passage say so.

Model: opus-5-5

internal/referer/referer.go

@@ -0,0 +1,32 @@
+// Package referer reduces a Referer header to its scheme and host, so
+// that a URL path, which on the receiver route carries the entrypoint
+// UUID, never reaches a log line or an error report.
+package referer
+
+import "net/url"
+
+// redacted stands in for a Referer that cannot be reduced to its
+// scheme and host.
+const redacted = "(redacted)"
+
+// SchemeAndHost returns a Referer's scheme and host, never its path or
+// query. The path can be an entrypoint URL, which carries the UUID: a
+// sender that follows a redirect from http:// to https:// sends the
+// URL it was redirected from, and a browser sends the 404 page's URL
+// under /h/ when it fetches that page's stylesheet. A Referer without
+// both a scheme and a host comes back as "(redacted)".
+//
+// It is the access log's referer field, and the Referer header the
+// Sentry hook sends.
+func SchemeAndHost(referer string) string {
+	if referer == "" {
+		return ""
+	}
+
+	parsed, err := url.Parse(referer)
+	if err != nil || parsed.Scheme == "" || parsed.Host == "" {
+		return redacted
+	}
+
+	return parsed.Scheme + "://" + parsed.Host
+}

internal/referer/referer_test.go

@@ -0,0 +1,67 @@
+package referer_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"sneak.berlin/go/webhooker/internal/referer"
+)
+
+// entrypointUUID is the secret part of an entrypoint URL; no case may
+// return it.
+const entrypointUUID = "0b6f7d3e-6a39-4e3c-9f0a-2c4d8e1b5a77"
+
+// redacted is what SchemeAndHost returns for a Referer it cannot
+// reduce to a scheme and host.
+const redacted = "(redacted)"
+
+func TestSchemeAndHost(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name    string
+		referer string
+		want    string
+	}{
+		{
+			name:    "no Referer",
+			referer: "",
+			want:    "",
+		},
+		{
+			name: "entrypoint URL with a query",
+			referer: "http://webhooker.example/h/" + entrypointUUID +
+				"?src=ci",
+			want: "http://webhooker.example",
+		},
+		{
+			name:    "host with a port",
+			referer: "https://webhooker.example:8443/h/" + entrypointUUID,
+			want:    "https://webhooker.example:8443",
+		},
+		{
+			name:    "path without a scheme or host",
+			referer: "/h/" + entrypointUUID,
+			want:    redacted,
+		},
+		{
+			name:    "host without a scheme",
+			referer: "webhooker.example/h/" + entrypointUUID,
+			want:    redacted,
+		},
+		{
+			name:    "not a URL",
+			referer: "http://[::1/h/" + entrypointUUID,
+			want:    redacted,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := referer.SchemeAndHost(tc.referer)
+
+			assert.Equal(t, tc.want, got)
+			assert.NotContains(t, got, entrypointUUID)
+		})
+	}
+}