rework template rendering, cache parsed templates

adapt error wrapping principle

templating/templates.go

@@ -0,0 +1,138 @@
+package templating
+
+import (
+	"fmt"
+	"html/template"
+	"io"
+	"log"
+	"net/http"
+	"officeHours/models"
+	"os"
+	"time"
+)
+
+// parsed templates available for execution
+var templates map[string]*template.Template = map[string]*template.Template{}
+
+// Initialise and parse templates.
+//
+// Should only be called once.
+//
+// Since this is something which may error and feels like
+// it should not be done automatically on import,
+// put it into a function.
+func InitTemplates() error {
+	const templateDir = "templating/templates/"
+	var funcs = template.FuncMap{
+		"DayName": models.DayName,
+		"divide":  func(i int, j int) int { return i / j },
+	}
+	var emptyTemplate = template.New("").Funcs(funcs)
+	var baseTemplate, err = template.ParseFiles(templateDir + "base.html")
+	if err != nil {
+		return fmt.Errorf("parsing base template failed:\n%w", err)
+	}
+	baseTemplate.Funcs(funcs)
+
+	type toCache struct {
+		filename   string
+		standalone bool
+	}
+	var toParse = map[string]toCache{
+		// full html templates
+		"addFailure":      {"addFailure.html", false},
+		"addMask":         {"addMask.html", false},
+		"addSuccess":      {"addSuccess.html", false},
+		"deleteSuccess":   {"deleteSuccess.html", false},
+		"executeFailure":  {"executeFailure.html", false},
+		"executeSuccess":  {"executeSuccess.html", false},
+		"index":           {"index.html", false},
+		"requestNotFound": {"requestNotFound.html", false},
+		// standalone templates
+		"confirmationMail": {"confirmationMail", true},
+		"officeHourTable":  {"officeHourTable.html", true},
+		"td":               {"td.html", true},
+	}
+
+	// parse templates and add to global mapping
+	for key, tmpl := range toParse {
+		if _, exists := templates[key]; exists {
+			return fmt.Errorf("template '%s' already parsed", key)
+		}
+		fullName := templateDir + tmpl.filename
+		// check that template file
+		info, err := os.Stat(fullName)
+		if err != nil {
+			return fmt.Errorf("adding template %s failed:\n%w", tmpl.filename, err)
+		}
+		if info.IsDir() {
+			return fmt.Errorf("adding template %s failed: is a directory", tmpl.filename)
+		}
+		// parse
+		var parsed *template.Template
+		if tmpl.standalone {
+			parsed, err = emptyTemplate.Clone()
+			parsed = parsed.New(tmpl.filename)
+		} else {
+			parsed, err = baseTemplate.Clone()
+		}
+		if err != nil {
+			return fmt.Errorf("cloning base template failed:\n%w", err)
+		}
+		parsed, err = parsed.ParseFiles(fullName)
+		if err != nil {
+			return fmt.Errorf("parsing template %s failed:\n%w", tmpl.filename, err)
+		}
+		templates[key] = parsed
+	}
+	return nil
+}
+
+// Execute a template and write it to the given writer.
+//
+// Parameters:
+//   - name: name of the template to execute
+//   - data: passed through to the template
+func WriteTemplate(w io.Writer, name string, data any) error {
+	tmpl, exists := templates[name]
+	if !exists {
+		return fmt.Errorf("template %s not available", name)
+	}
+	tmpl, err := tmpl.Clone()
+	if err != nil {
+		return fmt.Errorf("cloning template failed:\n%w", err)
+	}
+	err = tmpl.Execute(w, data)
+	if err != nil {
+		return fmt.Errorf("template execution failed:\n%w", err)
+	}
+	return nil
+}
+
+// Execute a template and write it to a http writer.
+//
+// Similar to WriteTemplate, but in error case this adds a corresponding
+// status code and writes an error message to the writer.
+//
+// Typically, this is the final action in handling an http request.
+//
+// Parameters:
+//   - name: name of the template to execute
+//   - data: passed through to the template
+func ServeTemplate(w http.ResponseWriter, name string, data any) {
+	// TODO: make this return an error, handle on top of every request handler
+	err := WriteTemplate(w, name, data)
+	if err != nil {
+		err = fmt.Errorf("writing template failed:\n%w", err)
+		log.Println(err.Error())
+		w.WriteHeader(http.StatusInternalServerError)
+		io.WriteString(w, `Internal Server Error.
+
+Du kannst uns helfen, indem du folgende Fehlermeldung per Mail
+an sprechstunden@mathebau.de sendest:
+
+Fehler um\n
+`+time.Now().String()+"\n"+err.Error())
+		return
+	}
+}