[feature] Use X-Robots-Tag headers to instruct scrapers/crawlers (#3737)

* [feature] Use `X-Robots-Tag` headers to instruct scrapers/crawlers

* use switch for RobotsHeaders

internal/middleware/robots.go

@@ -0,0 +1,67 @@
+// 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 middleware
+
+import (
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+)
+
+// RobotsHeaders adds robots directives to the X-Robots-Tag HTTP header.
+// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Robots-Tag
+//
+// If mode == "aiOnly" then only the noai and noimageai values will be set,
+// and other headers will be left alone (for route groups / handlers to set).
+//
+// If mode == "allowSome" then noai, noimageai, and some indexing will be set.
+//
+// If mode == "" then noai, noimageai, noindex, and nofollow will be set
+// (ie., as restrictive as possible).
+func RobotsHeaders(mode string) gin.HandlerFunc {
+	const (
+		key = "X-Robots-Tag"
+		// Some AI scrapers respect the following tags
+		// to opt-out of their crawling and datasets.
+		// We add them regardless of allowSome.
+		noai = "noai, noimageai"
+	)
+
+	switch mode {
+
+	// Just set ai headers and
+	// leave the other headers be.
+	case "aiOnly":
+		return func(c *gin.Context) {
+			c.Writer.Header().Set(key, noai)
+		}
+
+	// Allow some limited indexing.
+	case "allowSome":
+		return func(c *gin.Context) {
+			c.Writer.Header().Set(key, apiutil.RobotsDirectivesAllowSome)
+			c.Writer.Header().Add(key, noai)
+		}
+
+	// Disallow indexing via noindex, nofollow.
+	default:
+		return func(c *gin.Context) {
+			c.Writer.Header().Set(key, apiutil.RobotsDirectivesDisallow)
+			c.Writer.Header().Add(key, noai)
+		}
+	}
+}