[feature] Provide .well-known/host-meta endpoint (#1604)

* [feature] Provide .well-known/host-meta endpoint

This adds the host-meta endpoint as Mastodon clients use this to
discover the API domain to use when the host and account domains aren't
the same.

* Address review comments

internal/api/wellknown/hostmeta/hostmeta.go

@@ -0,0 +1,45 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
+
+   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 hostmeta
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	HostMetaContentType = "application/xrd+xml"
+	HostMetaPath        = "/host-meta"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, HostMetaPath, m.HostMetaGETHandler)
+}

internal/api/wellknown/hostmeta/hostmetaget.go

@@ -0,0 +1,73 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
+
+   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 hostmeta
+
+import (
+	"bytes"
+	"encoding/xml"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+// HostMetaGETHandler swagger:operation GET /.well-known/host-meta hostMetaGet
+//
+// Returns a compliant hostmeta response to web host metadata queries.
+//
+// See: https://www.rfc-editor.org/rfc/rfc6415.html
+//
+//	---
+//	tags:
+//	- .well-known
+//
+//	produces:
+//	- application/xrd+xml"
+//
+//	responses:
+//		'200':
+//			schema:
+//				"$ref": "#/definitions/hostmeta"
+func (m *Module) HostMetaGETHandler(c *gin.Context) {
+	if _, err := apiutil.NegotiateAccept(c, apiutil.HostMetaHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	hostMeta := m.processor.Fedi().HostMetaGet()
+
+	// this setup with a separate buffer we encode into is used because
+	// xml.Marshal does not emit xml.Header by itself
+	var buf bytes.Buffer
+
+	// Preallocate buffer of reasonable length.
+	buf.Grow(len(xml.Header) + 64)
+
+	// No need to check for error on write to buffer.
+	_, _ = buf.WriteString(xml.Header)
+
+	// Encode host-meta as XML to in-memory buffer.
+	if err := xml.NewEncoder(&buf).Encode(hostMeta); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorInternalError(err), m.processor.InstanceGetV1)
+		return
+	}
+
+	c.Data(http.StatusOK, HostMetaContentType, buf.Bytes())
+}