wdcli: Update handling of templates

This commit updates the handling of expanding of templates. This is to
prepare embeding templates directly as opposed to embedding a single
resource directory.

embed/template.go

@@ -0,0 +1,60 @@
+package embed
+
+import (
+	"os"
+
+	"github.com/FAU-CDI/wisski-distillery/internal/unpack"
+	"github.com/pkg/errors"
+)
+
+// InstallTemplates open the resource src, and installs it into dst.
+// the template resource must fit into memory.
+//
+// For each variable ${THING} inside dest, a key 'THING' must exist in context.
+// Extra or missing template keys are an error.
+func InstallTemplate(dst, src string, context map[string]string) error {
+	// open the source file!
+	srcFile, err := resourceEmbed.Open(src)
+	if err != nil {
+		return errors.Wrapf(err, "Error opening source file %s", src)
+	}
+	defer srcFile.Close()
+
+	// write the template
+	bytes, srcMode, err := unpack.UnpackTemplate(context, srcFile)
+	if err != nil {
+		return err
+	}
+
+	// determine if we need to create the destination file, or if it already exists
+	dstStat, dstErr := os.Stat(dst)
+	switch {
+	case os.IsNotExist(dstErr):
+	case dstErr != nil:
+		return errors.Wrapf(dstErr, "Error calling stat on destination %s", dst)
+	case dstStat.IsDir():
+		return errors.Wrapf(errExpectedFileButGotDirectory, "Error processing destination %s", dst)
+	}
+
+	// open and write the destination file
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, srcMode)
+	if err != nil {
+		return errors.Wrapf(err, "Unable to open file %s", dst)
+	}
+	_, err = dstFile.Write(bytes)
+	return errors.Wrapf(err, "Unable to write destination %s", dst)
+}
+
+// ReadTemplate is like InstallTemplate, except that it writes template into a byte slice and returns it.
+func ReadTemplate(src string, context map[string]string) ([]byte, error) {
+	// open the source file!
+	srcFile, err := resourceEmbed.Open(src)
+	if err != nil {
+		return nil, errors.Wrapf(err, "Error opening source file %s", src)
+	}
+	defer srcFile.Close()
+
+	// and return it
+	bytes, _, err := unpack.UnpackTemplate(context, srcFile)
+	return bytes, err
+}