[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

6 files changed, 148 insertions, 0 deletions

diff --git a/internal/api/util/mime.go b/internal/api/util/mime.go
index 30f0f0d6e..cfdc3b08b 100644
--- a/internal/api/util/mime.go
+++ b/internal/api/util/mime.go
@@ -25,6 +25,7 @@ type MIME string
 const (
 	AppJSON           MIME = `application/json`
 	AppXML            MIME = `application/xml`
+	AppXMLXRD         MIME = `application/xrd+xml`
 	AppRSSXML         MIME = `application/rss+xml`
 	AppActivityJSON   MIME = `application/activity+json`
 	AppActivityLDJSON MIME = `application/ld+json; profile="https://www.w3.org/ns/activitystreams"`
diff --git a/internal/api/util/negotiate.go b/internal/api/util/negotiate.go
index 3a5f21775..06a202815 100644
--- a/internal/api/util/negotiate.go
+++ b/internal/api/util/negotiate.go
@@ -58,6 +58,11 @@ var HTMLOrActivityPubHeaders = []MIME{
 	AppActivityLDJSON,
 }
 
+var HostMetaHeaders = []MIME{
+	AppXMLXRD,
+	AppXML,
+}
+
 // NegotiateAccept takes the *gin.Context from an incoming request, and a
 // slice of Offers, and performs content negotiation for the given request
 // with the given content-type offers. It will return a string representation
diff --git a/internal/api/wellknown.go b/internal/api/wellknown.go
index 7edbb4d7d..a837667fb 100644
--- a/internal/api/wellknown.go
+++ b/internal/api/wellknown.go
@@ -20,6 +20,7 @@ package api
 
 import (
 	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/hostmeta"
 	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/nodeinfo"
 	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/webfinger"
 	"github.com/superseriousbusiness/gotosocial/internal/middleware"
@@ -30,6 +31,7 @@ import (
 type WellKnown struct {
 	nodeInfo  *nodeinfo.Module
 	webfinger *webfinger.Module
+	hostMeta  *hostmeta.Module
 }
 
 func (w *WellKnown) Route(r router.Router, m ...gin.HandlerFunc) {
@@ -45,11 +47,13 @@ func (w *WellKnown) Route(r router.Router, m ...gin.HandlerFunc) {
 
 	w.nodeInfo.Route(wellKnownGroup.Handle)
 	w.webfinger.Route(wellKnownGroup.Handle)
+	w.hostMeta.Route(wellKnownGroup.Handle)
 }
 
 func NewWellKnown(p *processing.Processor) *WellKnown {
 	return &WellKnown{
 		nodeInfo:  nodeinfo.New(p),
 		webfinger: webfinger.New(p),
+		hostMeta:  hostmeta.New(p),
 	}
 }
diff --git a/internal/api/wellknown/hostmeta/hostmeta.go b/internal/api/wellknown/hostmeta/hostmeta.go
new file mode 100644
index 000000000..17eb748ec
--- /dev/null
+++ b/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)
+}
diff --git a/internal/api/wellknown/hostmeta/hostmetaget.go b/internal/api/wellknown/hostmeta/hostmetaget.go
new file mode 100644
index 000000000..f45b2cf9c
--- /dev/null
+++ b/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())
+}
diff --git a/internal/processing/fedi/wellknown.go b/internal/processing/fedi/wellknown.go
index 6f113ac5d..7be75649b 100644
--- a/internal/processing/fedi/wellknown.go
+++ b/internal/processing/fedi/wellknown.go
@@ -28,6 +28,10 @@ import (
 )
 
 const (
+	hostMetaXMLNS                   = "http://docs.oasis-open.org/ns/xri/xrd-1.0"
+	hostMetaRel                     = "lrdd"
+	hostMetaType                    = "application/xrd+xml"
+	hostMetaTemplate                = ".well-known/webfinger?resource={uri}"
 	nodeInfoVersion                 = "2.0"
 	nodeInfoSoftwareName            = "gotosocial"
 	nodeInfoRel                     = "http://nodeinfo.diaspora.software/ns/schema/" + nodeInfoVersion
@@ -96,6 +100,22 @@ func (p *Processor) NodeInfoGet(ctx context.Context) (*apimodel.Nodeinfo, gtserr
 	}, nil
 }
 
+// HostMetaGet returns a host-meta struct in response to a host-meta request.
+func (p *Processor) HostMetaGet() *apimodel.HostMeta {
+	protocol := config.GetProtocol()
+	host := config.GetHost()
+	return &apimodel.HostMeta{
+		XMLNS: hostMetaXMLNS,
+		Link: []apimodel.Link{
+			{
+				Rel:      hostMetaRel,
+				Type:     hostMetaType,
+				Template: fmt.Sprintf("%s://%s/%s", protocol, host, hostMetaTemplate),
+			},
+		},
+	}
+}
+
 // WebfingerGet handles the GET for a webfinger resource. Most commonly, it will be used for returning account lookups.
 func (p *Processor) WebfingerGet(ctx context.Context, requestedUsername string) (*apimodel.WellKnownResponse, gtserror.WithCode) {
 	// Get the local account the request is referring to.