fix(cmd): print only the error on CLI usage errors

cli v3.10 dumps the full command help to stdout on a usage error (unknown
flag, missing required flag, etc.). Take over that reporting via
OnUsageError so usage errors print only "Incorrect Usage: <err>" to
stderr and nothing to stdout, covering the auto-built completion subtree
through ConfigureShellCompletionCommand. Also stop RunMainApp from
printing the error a second time for non-unknown-flag usage errors.

Assisted-by: Claude:Opus-4.8

cmd/cmdtest/cmd_test.go

@@ -202,8 +202,8 @@ func TestCliCmdError(t *testing.T) {
 	r, err = runTestApp(app, "./gitea", "test-cmd", "--no-such")
 	assert.Error(t, err)
 	assert.Equal(t, 1, r.ExitCode)
-	assert.NotEmpty(t, r.Stdout) // since cli v3.10, the usage help is printed to stdout on a usage error
-	assert.Equal(t, "Incorrect Usage: flag provided but not defined: -no-such\n\n", r.Stderr)
+	assert.Empty(t, r.Stdout)
+	assert.Equal(t, "Incorrect Usage: flag provided but not defined: -no-such\n", r.Stderr)
 
 	app = newTestApp(cli.Command{Action: func(ctx context.Context, cmd *cli.Command) error { return nil }})
 	r, err = runTestApp(app, "./gitea", "test-cmd")

cmd/main.go

@@ -5,10 +5,10 @@ package cmd
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
-	"strings"
 
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
@@ -154,16 +154,37 @@ func NewMainApp(appVer AppVersion) *cli.Command {
 	return app
 }
 
+// usageErr marks a usage error already reported by cliOnUsageError, so RunMainApp does not print it again.
+type usageErr struct{ err error }
+
+func (e usageErr) Error() string { return e.err.Error() }
+func (e usageErr) Unwrap() error { return e.err }
+
+// cliOnUsageError reports usage errors itself instead of letting cli dump the full help to stdout (since cli v3.10).
+func cliOnUsageError(_ context.Context, cmd *cli.Command, err error, _ bool) error {
+	_, _ = fmt.Fprintf(cmd.Root().ErrWriter, "Incorrect Usage: %s\n", err.Error())
+	return usageErr{err}
+}
+
+func setCLIOnUsageError(cmd *cli.Command) {
+	_ = cmd.Walk(func(c *cli.Command) error {
+		c.OnUsageError = cliOnUsageError
+		return nil
+	})
+}
+
 func RunMainApp(app *cli.Command, args ...string) error {
 	ctx, cancel := installSignals()
 	defer cancel()
+	setCLIOnUsageError(app)
+	// the completion subcommands are built during app.Run, after the Walk above, so cover them via this hook
+	app.ConfigureShellCompletionCommand = setCLIOnUsageError
 	err := app.Run(ctx, args)
 	if err == nil {
 		return nil
 	}
-	if strings.HasPrefix(err.Error(), "flag provided but not defined:") {
-		// the cli package should already have output the error message, so just exit
-		cli.OsExiter(1)
+	if ue := (usageErr{}); errors.As(err, &ue) {
+		cli.OsExiter(1) // cliOnUsageError already reported it
 		return err
 	}
 	_, _ = fmt.Fprintf(app.ErrWriter, "Command error: %v\n", err)