internal/pwru: Implement a proper help function

With "./pwru --help" or "./pwru -h", pwru:

  1. displays the help message,
  2. prints "pflag: help requested",
  3. exits with error code 2.

Steps 2 and 3 are not expected, the interactive help should return 0
without displaying an error message. This is because of how
pflags.ParseAll() works. From its documentation [0]: "The return value
will be ErrHelp if -help was set but not defined."

Let's implement a dedicated function for displaying the help message
(reusing pflags.Usage() and what we had in pwru already), and call it
when users pass --help or -h to avoid this error and non-zero return
value.

[0] https://pkg.go.dev/github.com/spf13/pflag#FlagSet.ParseAll

Signed-off-by: Quentin Monnet <[email protected]>

README.md

@@ -60,6 +60,7 @@ Usage: pwru [options] [pcap-filter]
       --filter-netns string       filter netns ("/proc/<pid>/ns/net", "inode:<inode>")
       --filter-trace-tc           trace TC bpf progs
       --filter-track-skb          trace a packet even if it does not match given filters (e.g., after NAT or tunnel decapsulation)
+  -h, --help                      display this message and exit
       --kernel-btf string         specify kernel BTF file
       --kmods strings             list of kernel modules names to attach to
       --output-file string        write traces to file

internal/pwru/types.go

@@ -21,6 +21,7 @@ const (
 
 type Flags struct {
 	ShowVersion bool
+	ShowHelp    bool
 
 	KernelBTF string
 
@@ -49,6 +50,7 @@ type Flags struct {
 }
 
 func (f *Flags) SetFlags() {
+	flag.BoolVarP(&f.ShowHelp, "help", "h", false, "display this message and exit")
 	flag.BoolVar(&f.ShowVersion, "version", false, "show pwru version and exit")
 	flag.StringVar(&f.KernelBTF, "kernel-btf", "", "specify kernel BTF file")
 	flag.StringSliceVar(&f.KMods, "kmods", nil, "list of kernel modules names to attach to")
@@ -82,6 +84,10 @@ func (f *Flags) SetFlags() {
 	}
 }
 
+func (f *Flags) PrintHelp() {
+	flag.Usage()
+}
+
 func (f *Flags) Parse() {
 	flag.Parse()
 	f.FilterPcap = strings.Join(flag.Args(), " ")

main.go

@@ -30,6 +30,10 @@ func main() {
 	flags.SetFlags()
 	flags.Parse()
 
+	if flags.ShowHelp {
+		flags.PrintHelp()
+		os.Exit(0)
+	}
 	if flags.ShowVersion {
 		fmt.Printf("pwru %s\n", pwru.Version)
 		os.Exit(0)