internal/frontend: allow HEAD requests

Fixes golang/go#43739

Change-Id: I887fd3c1949a9f26aff75122f455246dc802d6de
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/284912
Trust: Jonathan Amsterdam <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>

Author: jba

cmd/frontend/main.go

@@ -189,7 +189,7 @@ func main() {
 	}
 	mw := middleware.Chain(
 		middleware.RequestLog(cmdconfig.Logger(ctx, cfg, "frontend-log")),
-		middleware.AcceptRequests(http.MethodGet, http.MethodPost), // accept only GETs and POSTs
+		middleware.AcceptRequests(http.MethodGet, http.MethodPost, http.MethodHead), // accept only GETs, POSTs and HEADs
 		middleware.Quota(cfg.Quota, cacheClient),
 		middleware.GodocURL(), // potentially redirects so should be early in chain
 		middleware.RedirectedFrom(),

internal/frontend/details.go

@@ -25,7 +25,7 @@ import (
 func (s *Server) serveDetails(w http.ResponseWriter, r *http.Request, ds internal.DataSource) (err error) {
 	defer middleware.ElapsedStat(r.Context(), "serveDetails")()
 
-	if r.Method != http.MethodGet {
+	if r.Method != http.MethodGet && r.Method != http.MethodHead {
 		return &serverError{status: http.StatusMethodNotAllowed}
 	}
 	if r.URL.Path == "/" {

internal/frontend/search.go

@@ -122,7 +122,7 @@ const (
 // /search?q=<query>. If <query> is an exact match for a package path, the user
 // will be redirected to the details page.
 func (s *Server) serveSearch(w http.ResponseWriter, r *http.Request, ds internal.DataSource) error {
-	if r.Method != http.MethodGet {
+	if r.Method != http.MethodGet && r.Method != http.MethodHead {
 		return &serverError{status: http.StatusMethodNotAllowed}
 	}
 	db, ok := ds.(*postgres.DB)