perf(bindercli): defer generated command registration until needed

Built-in commands (service, aidl, camera) and --help no longer pay
the cost of constructing thousands of generated cobra commands at
startup. Generated commands are only registered on retry when the
first execution attempt returns an "unknown command" error.

Before: ~47ms for --help, ~46ms for service --help (x86)
After:  ~6ms for --help,  ~7ms for service --help (x86)

Author: xaionaro@dx.center

cmd/bindercli/main.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"runtime/pprof"
+	"strings"
 
 	"github.com/facebookincubator/go-belt"
 	"github.com/facebookincubator/go-belt/tool/logger"
@@ -109,13 +111,44 @@ and invoking Android Binder services using AIDL-generated Go bindings.`,
 	cmd.AddCommand(newServiceCmd())
 	cmd.AddCommand(newAIDLCmd())
 	cmd.AddCommand(newCameraCmd())
-	addGeneratedCommands(cmd)
 
 	return cmd
 }
 
+// isUnknownCommandError reports whether err is cobra's "unknown command" error,
+// which means the user invoked a subcommand that doesn't exist yet (likely a
+// generated proxy command that hasn't been registered).
+func isUnknownCommandError(err error) bool {
+	return err != nil && strings.HasPrefix(err.Error(), "unknown command ")
+}
+
 func main() {
-	if err := newRootCmd().Execute(); err != nil {
+	// Phase 1: try executing with only the lightweight built-in commands
+	// (service, aidl, camera). Suppress cobra's error output so that an
+	// "unknown command" failure for a generated proxy command is invisible.
+	// Stdout is left alone so --help and normal command output still work.
+	cmd := newRootCmd()
+	cmd.SilenceErrors = true
+	cmd.SilenceUsage = true
+	cmd.SetErr(io.Discard)
+
+	err := cmd.Execute()
+	switch {
+	case err == nil:
+		return
+	case isUnknownCommandError(err):
+		// Phase 2: the user invoked a generated command. Register all
+		// generated commands and retry with normal output.
+		addGeneratedCommands(cmd)
+		cmd.SilenceErrors = false
+		cmd.SilenceUsage = false
+		cmd.SetErr(os.Stderr)
+		if err := cmd.Execute(); err != nil {
+			os.Exit(1)
+		}
+	default:
+		// A built-in command failed. Print the error ourselves since
+		// cobra's error printing was silenced for phase 1.
 		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 		os.Exit(1)
 	}