argparse: do not rely on os.Exit() inside FlagSet.Parse()

When the internal 'FlagSet.Parse()' of an 'argparser' encounters an invalid
flag, the current 'ExitOnError' error handling causes it to invoke the
'os.Exit(2)' syscall and exit the program abruptly. In later patches, we're
going to want to "catch" that exit status in our logs, so change the error
handling to 'ContinueOnError' and explicitly call 'os.Exit(2)' in
'argparse.Parse()' if 'FlagSet.Parse()' returns an error.

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

Author: vdye

internal/argparse/argparse.go

@@ -7,6 +7,9 @@ import (
 	"strings"
 )
 
+// For consistency with 'flag', use 2 as the usage-related error code
+const usageExitCode int = 2
+
 type positionalArg struct {
 	name        string
 	description string
@@ -30,7 +33,7 @@ type argParser struct {
 }
 
 func NewArgParser(usageString string) *argParser {
-	flagSet := flag.NewFlagSet("", flag.ExitOnError)
+	flagSet := flag.NewFlagSet("", flag.ContinueOnError)
 
 	a := &argParser{
 		isTopLevel:  false,
@@ -136,7 +139,9 @@ 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")
+		// The error was already printed (via a.FlagSet.Usage()), so we
+		// just need to exit
+		os.Exit(usageExitCode)
 	}
 
 	if len(a.subcommands) > 0 {
@@ -212,6 +217,5 @@ 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)
+	os.Exit(usageExitCode)
 }