resources: Add responseHeaders option to resources.GetRemote

* These response headers will be included in `.Data.Headers` if found.
* The header name matching is case insensitive.
* `Data.Headers` is of type `map[string][]string`
* In most cases there will be only one value per header key, but e.g. `Set-Cookie` commonly has multiple values.

Fixes #12521

resources/resource_factories/create/create_integration_test.go

@@ -56,7 +56,42 @@ func TestGetRemoteHead(t *testing.T) {
 
 	b.AssertFileContent("public/index.html",
 		"Head Content: .",
-		"Head Data: map[ContentLength:18210 ContentType:image/png Status:200 OK StatusCode:200 TransferEncoding:[]]",
+		"Head Data: map[ContentLength:18210 ContentType:image/png Headers:map[] Status:200 OK StatusCode:200 TransferEncoding:[]]",
+	)
+}
+
+func TestGetRemoteResponseHeaders(t *testing.T) {
+	files := `
+-- config.toml --
+[security]
+  [security.http]
+    methods = ['(?i)GET|POST|HEAD']
+    urls = ['.*gohugo\.io.*']
+-- layouts/index.html --
+{{ $url := "https://gohugo.io/img/hugo.png" }}
+{{ $opts := dict "method" "head" "responseHeaders" (slice "X-Frame-Options" "Server") }}
+{{ with try (resources.GetRemote $url $opts) }}
+  {{ with .Err }}
+    {{ errorf "Unable to get remote resource: %s" . }}
+  {{ else with .Value }}
+    Response Headers: {{ .Data.Headers }}
+  {{ else }}
+  {{ errorf "Unable to get remote resource: %s" $url }}
+  {{ end }}
+{{ end }}
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	)
+
+	b.Build()
+
+	b.AssertFileContent("public/index.html",
+		"Response Headers: map[Server:[Netlify] X-Frame-Options:[DENY]]",
 	)
 }
 

resources/resource_factories/create/remote.go

@@ -30,6 +30,7 @@ import (
 
 	"github.com/gohugoio/httpcache"
 	"github.com/gohugoio/hugo/common/hashing"
+	"github.com/gohugoio/hugo/common/hstrings"
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/common/maps"
@@ -51,18 +52,28 @@ type HTTPError struct {
 	Body       string
 }
 
-func responseToData(res *http.Response, readBody bool) map[string]any {
+func responseToData(res *http.Response, readBody bool, includeHeaders []string) map[string]any {
 	var body []byte
 	if readBody {
 		body, _ = io.ReadAll(res.Body)
 	}
 
+	responseHeaders := make(map[string][]string)
+	if true || len(includeHeaders) > 0 {
+		for k, v := range res.Header {
+			if hstrings.InSlicEqualFold(includeHeaders, k) {
+				responseHeaders[k] = v
+			}
+		}
+	}
+
 	m := map[string]any{
 		"StatusCode":       res.StatusCode,
 		"Status":           res.Status,
 		"TransferEncoding": res.TransferEncoding,
 		"ContentLength":    res.ContentLength,
 		"ContentType":      res.Header.Get("Content-Type"),
+		"Headers":          responseHeaders,
 	}
 
 	if readBody {
@@ -72,7 +83,7 @@ func responseToData(res *http.Response, readBody bool) map[string]any {
 	return m
 }
 
-func toHTTPError(err error, res *http.Response, readBody bool) *HTTPError {
+func toHTTPError(err error, res *http.Response, readBody bool, responseHeaders []string) *HTTPError {
 	if err == nil {
 		panic("err is nil")
 	}
@@ -85,7 +96,7 @@ func toHTTPError(err error, res *http.Response, readBody bool) *HTTPError {
 
 	return &HTTPError{
 		error: err,
-		Data:  responseToData(res, readBody),
+		Data:  responseToData(res, readBody, responseHeaders),
 	}
 }
 
@@ -213,7 +224,7 @@ func (c *Client) FromRemote(uri string, optionsm map[string]any) (resource.Resou
 		}
 
 		if res.StatusCode < 200 || res.StatusCode > 299 {
-			return nil, toHTTPError(fmt.Errorf("failed to fetch remote resource from '%s': %s", uri, http.StatusText(res.StatusCode)), res, !isHeadMethod)
+			return nil, toHTTPError(fmt.Errorf("failed to fetch remote resource from '%s': %s", uri, http.StatusText(res.StatusCode)), res, !isHeadMethod, options.ResponseHeaders)
 		}
 
 		var (
@@ -280,7 +291,7 @@ func (c *Client) FromRemote(uri string, optionsm map[string]any) (resource.Resou
 		}
 
 		userKey = filename[:len(filename)-len(path.Ext(filename))] + "_" + userKey + mediaType.FirstSuffix.FullSuffix
-		data := responseToData(res, false)
+		data := responseToData(res, false, options.ResponseHeaders)
 
 		return c.rs.NewResource(
 			resources.ResourceSourceDescriptor{
@@ -345,9 +356,10 @@ func hasHeaderKey(m http.Header, key string) bool {
 }
 
 type fromRemoteOptions struct {
-	Method  string
-	Headers map[string]any
-	Body    []byte
+	Method          string
+	Headers         map[string]any
+	Body            []byte
+	ResponseHeaders []string
 }
 
 func (o fromRemoteOptions) BodyReader() io.Reader {
@@ -432,7 +444,7 @@ func (t *transport) RoundTrip(req *http.Request) (resp *http.Response, err error
 				if resp != nil {
 					msg = resp.Status
 				}
-				err := toHTTPError(fmt.Errorf("retry timeout (configured to %s) fetching remote resource: %s", t.Cfg.Timeout(), msg), resp, req.Method != "HEAD")
+				err := toHTTPError(fmt.Errorf("retry timeout (configured to %s) fetching remote resource: %s", t.Cfg.Timeout(), msg), resp, req.Method != "HEAD", nil)
 				return resp, err
 			}
 			time.Sleep(nextSleep)