Add support for config files in downloader. (#404)

* Add support for config files in downloader.

* Add no-ini for the version flag, too.

* Add config file options in doc to downloader.

cmd/csaf_downloader/config.go

@@ -0,0 +1,66 @@
+// 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: 2022 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2022 Intevation GmbH <https://intevation.de>
+
+package main
+
+import (
+	"log"
+	"net/http"
+	"os"
+
+	"github.com/mitchellh/go-homedir"
+)
+
+const defaultWorker = 2
+
+type config struct {
+	Directory            *string  `short:"d" long:"directory" description:"DIRectory to store the downloaded files in" value-name:"DIR"`
+	Insecure             bool     `long:"insecure" description:"Do not check TLS certificates from provider"`
+	IgnoreSignatureCheck bool     `long:"ignoresigcheck" description:"Ignore signature check results, just warn on mismatch"`
+	Version              bool     `long:"version" description:"Display version of the binary" no-ini:"true"`
+	Verbose              bool     `long:"verbose" short:"v" description:"Verbose output"`
+	Rate                 *float64 `long:"rate" short:"r" description:"The average upper limit of https operations per second (defaults to unlimited)"`
+	Worker               int      `long:"worker" short:"w" description:"NUMber of concurrent downloads" value-name:"NUM"`
+
+	ExtraHeader http.Header `long:"header" short:"H" description:"One or more extra HTTP header fields"`
+
+	RemoteValidator        string   `long:"validator" description:"URL to validate documents remotely" value-name:"URL"`
+	RemoteValidatorCache   string   `long:"validatorcache" description:"FILE to cache remote validations" value-name:"FILE"`
+	RemoteValidatorPresets []string `long:"validatorpreset" description:"One or more presets to validate remotely" default:"mandatory"`
+
+	Config *string `short:"c" long:"config" description:"Path to config ini file" value-name:"INI-FILE" no-ini:"true"`
+}
+
+// iniPaths are the potential file locations of the the config file.
+var iniPaths = []string{
+	"~/.config/csaf/downloader.ini",
+	"~/.csaf_downloader.ini",
+	"csaf_downloader.ini",
+}
+
+// findIniFile looks for a file in the pre-defined paths in "iniPaths".
+// The returned value will be the name of file if found, otherwise an empty string.
+func findIniFile() string {
+	for _, f := range iniPaths {
+		name, err := homedir.Expand(f)
+		if err != nil {
+			log.Printf("warn: %v\n", err)
+			continue
+		}
+		if _, err := os.Stat(name); err == nil {
+			return name
+		}
+	}
+	return ""
+}
+
+// prepare prepares internal state of a loaded configuration.
+func (cfg *config) prepare() error {
+	// TODO: Implement me!
+	return nil
+}