Downloader: Add forwarding to HTTP endpoint (#442)

* started with forwarding support in downloader

* Add missing files.

* Add missing files.

* Raise needed Go version

* More Go version bumping.

* Fix forwarding

* Go 1.21+ needed

* Make terminating forwarder more robust.

* Better var naming

* Remove dead code. Improve commentary.

* Prepare validation status adjustment.

* Move validations to functions to make them executable in a loop.

* Introduce validation mode flag (strict, unsafe)

internal/misc/doc.go

@@ -0,0 +1,10 @@
+// This file is Free Software under the MIT License
+// without warranty, see README.md and LICENSES/MIT.txt for details.
+//
+// SPDX-License-Identifier: MIT
+//
+// SPDX-FileCopyrightText: 2023 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2023 Intevation GmbH <https://intevation.de>
+
+// Package misc implements miscellaneous helper functions.
+package misc

internal/misc/mime.go

@@ -0,0 +1,31 @@
+// This file is Free Software under the MIT License
+// without warranty, see README.md and LICENSES/MIT.txt for details.
+//
+// SPDX-License-Identifier: MIT
+//
+// SPDX-FileCopyrightText: 2023 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2023 Intevation GmbH <https://intevation.de>
+
+package misc
+
+import (
+	"fmt"
+	"io"
+	"mime/multipart"
+	"net/textproto"
+	"strings"
+)
+
+var escapeQuotes = strings.NewReplacer("\\", "\\\\", `"`, "\\\"").Replace
+
+// CreateFormFile creates an [io.Writer] like [mime/multipart.Writer.CreateFromFile].
+// This version allows to set the mime type, too.
+func CreateFormFile(w *multipart.Writer, fieldname, filename, mimeType string) (io.Writer, error) {
+	// Source: https://cs.opensource.google/go/go/+/refs/tags/go1.20:src/mime/multipart/writer.go;l=140
+	h := make(textproto.MIMEHeader)
+	h.Set("Content-Disposition",
+		fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
+			escapeQuotes(fieldname), escapeQuotes(filename)))
+	h.Set("Content-Type", mimeType)
+	return w.CreatePart(h)
+}