internal/handlers/export_test.go

@@ -1,6 +1,19 @@
 package handlers
 
-import "net/http"
+import (
+	"html/template"
+	"net/http"
+)
+
+// AddTemplateForTest registers a template under a page name so that
+// the handlers_test package can drive the render path with a
+// template of its own.
+func (s *Handlers) AddTemplateForTest(
+	pageTemplate string,
+	tmpl *template.Template,
+) {
+	s.templates[pageTemplate] = tmpl
+}
 
 // RenderTemplateForTest exposes renderTemplate for use in the
 // handlers_test package.

internal/handlers/handlers.go

@@ -3,6 +3,7 @@
 package handlers
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -224,13 +225,20 @@ func (s *Handlers) renderTemplate(
 	s.executeTemplate(w, tmpl, wrapper)
 }
 
-// executeTemplate runs the template and handles errors.
+// executeTemplate renders the template into a buffer and writes to
+// the response only once rendering has fully succeeded. Executing
+// straight into the ResponseWriter commits a partial body and a 200
+// status before a mid-render error can be reported, leaving no way
+// to serve a 500. These pages are small, so holding one in memory is
+// the right trade.
 func (s *Handlers) executeTemplate(
 	w http.ResponseWriter,
 	tmpl *template.Template,
 	data any,
 ) {
-	err := tmpl.Execute(w, data)
+	var buf bytes.Buffer
+
+	err := tmpl.Execute(&buf, data)
 	if err != nil {
 		s.log.Error(
 			"failed to execute template", "error", err,
@@ -239,5 +247,16 @@ func (s *Handlers) executeTemplate(
 			w, "Internal server error",
 			http.StatusInternalServerError,
 		)
+
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+
+	_, err = buf.WriteTo(w)
+	if err != nil {
+		s.log.Error(
+			"failed to write rendered page", "error", err,
+		)
 	}
 }

internal/handlers/handlers_test.go

@@ -2,6 +2,8 @@ package handlers_test
 
 import (
 	"context"
+	"errors"
+	"html/template"
 	"net/http"
 	"net/http/httptest"
 	"sync"
@@ -220,6 +222,68 @@ func TestRenderTemplate(t *testing.T) {
 	)
 }
 
+// errMidRender is the failure a test template raises partway through
+// rendering.
+var errMidRender = errors.New("deliberate mid-render failure")
+
+// midRenderFailure is template data whose first method renders and
+// whose second fails, so the template aborts after output has
+// already been produced.
+type midRenderFailure struct{}
+
+// Prefix is the output a streaming renderer would flush before the
+// failure below aborts the template.
+func (midRenderFailure) Prefix() string { return partialPageMarker }
+
+// Boom aborts template execution.
+func (midRenderFailure) Boom() (string, error) {
+	return "", errMidRender
+}
+
+// partialPageMarker is content the failing template emits before it
+// aborts.
+const partialPageMarker = "PARTIAL PAGE CONTENT"
+
+// TestRenderTemplateMidRenderErrorSendsNoPartialBody proves the
+// renderer does not commit output it cannot finish: a template that
+// fails partway through must yield a 500 and a body carrying none of
+// the content emitted before the failure. Against a renderer that
+// executes straight into the ResponseWriter this fails on both
+// counts, returning 200 with the prefix already flushed.
+func TestRenderTemplateMidRenderErrorSendsNoPartialBody(t *testing.T) {
+	t.Parallel()
+
+	var h *handlers.Handlers
+
+	app := newTestApp(t, &h)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	h.AddTemplateForTest("failing.html", template.Must(
+		template.New("failing").Parse(
+			`{{.Data.Prefix}}{{.Data.Boom}}TAIL`,
+		),
+	))
+
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/", nil)
+	w := httptest.NewRecorder()
+
+	h.RenderTemplateForTest(
+		w, req, "failing.html", midRenderFailure{},
+	)
+
+	assert.Equal(
+		t, http.StatusInternalServerError, w.Code,
+		"a failed render must report a 500",
+	)
+	assert.Equal(
+		t, "Internal server error\n", w.Body.String(),
+		"the response must carry no part of the aborted page",
+	)
+}
+
 func TestBuildDatabaseTargetConfig_Valid(t *testing.T) {
 	t.Parallel()