argparse: create basic argument parser

Add a basic argument parser for arbitrary series of string arguments. For
now, it is little more than a wrapper of 'flag.FlagSet' (a builtin structure
that handles parsing of flags preceded by a '-'). In future patches, the arg
parser will be extended to handle subcommands and positional arguments to
simplify 'git-bundle-server' command setup.

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

internal/argparse/argparse.go

@@ -0,0 +1,41 @@
+package argparse
+
+import (
+	"flag"
+	"fmt"
+	"os"
+)
+
+type argParser struct {
+	flag.FlagSet
+}
+
+func NewArgParser(usageString string) *argParser {
+	flagSet := flag.NewFlagSet("", flag.ExitOnError)
+
+	a := &argParser{
+		FlagSet: *flagSet,
+	}
+
+	a.FlagSet.Usage = func() {
+		out := a.FlagSet.Output()
+		fmt.Fprintf(out, "usage: %s\n\n", usageString)
+	}
+
+	return a
+}
+
+func (a *argParser) Parse(args []string) {
+	err := a.FlagSet.Parse(args)
+	if err != nil {
+		panic("argParser FlagSet error handling should be 'ExitOnError', but error encountered")
+	}
+}
+
+func (a *argParser) Usage(errFmt string, args ...any) {
+	fmt.Fprintf(a.FlagSet.Output(), errFmt+"\n", args...)
+	a.FlagSet.Usage()
+
+	// Exit with error code 2 to match flag.Parse() behavior
+	os.Exit(2)
+}