appview: state: add GoImport middleware

This serves the go-import meta tag if the user-agent is
Go-http-client/1.1 and there's a ?go-get=1 query parameter present. This
should run before the router 404's a path that technically doesn't exist
like tangled.sh/foo.com/some-go/v5.

appview/state/middleware.go

···
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"net/http"
 	"strconv"
···
 			if err != nil {
 				// invalid did or handle
 				log.Println("failed to resolve repo")
-				w.WriteHeader(http.StatusNotFound)
+				s.pages.Error404(w)
 				return
 			}
 
···
 		})
 	}
 }
+
+// this should serve the go-import meta tag even if the path is technically
+// a 404 like tangled.sh/oppi.li/go-git/v5
+func GoImport(s *State) middleware.Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			f, err := s.fullyResolvedRepo(r)
+			if err != nil {
+				log.Println("failed to fully resolve repo", err)
+				http.Error(w, "invalid repo url", http.StatusNotFound)
+				return
+			}
+
+			fullName := f.OwnerHandle() + "/" + f.RepoName
+
+			if r.Header.Get("User-Agent") == "Go-http-client/1.1" {
+				if r.URL.Query().Get("go-get") == "1" {
+					html := fmt.Sprintf(
+						`<meta name="go-import" content="tangled.sh/%s git https://tangled.sh/%s"/>`,
+						fullName,
+						fullName,
+					)
+					w.Header().Set("Content-Type", "text/html")
+					w.Write([]byte(html))
+					return
+				}
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}

appview/state/router.go

···
 		r.Get("/", s.Profile)
 
 		r.With(ResolveRepo(s)).Route("/{repo}", func(r chi.Router) {
+			r.Use(GoImport(s))
+
 			r.Get("/", s.RepoIndex)
 			r.Get("/commits/{ref}", s.RepoLog)
 			r.Route("/tree/{ref}", func(r chi.Router) {