Add tests for flag completion registration (#2053)

Different problems have been reported about flag completion registration.
These two tests are the cases that were not being verified but had been
mentioned as problematic.

Ref:
- https://github.com/spf13/cobra/issues/1320
- https://github.com/spf13/cobra/pull/1438#issuecomment-872928669

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>

completions_test.go

@@ -17,7 +17,9 @@ package cobra
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -2040,6 +2042,114 @@ func TestFlagCompletionWorksRootCommandAddedAfterFlags(t *testing.T) {
 	}
 }
 
+func TestFlagCompletionForPersistentFlagsCalledFromSubCmd(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	rootCmd.PersistentFlags().String("string", "", "test string flag")
+	_ = rootCmd.RegisterFlagCompletionFunc("string", func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return []string{"myval"}, ShellCompDirectiveDefault
+	})
+
+	childCmd := &Command{
+		Use: "child",
+		Run: emptyRun,
+		ValidArgsFunction: func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+			return []string{"--validarg", "test"}, ShellCompDirectiveDefault
+		},
+	}
+	childCmd.Flags().Bool("bool", false, "test bool flag")
+	rootCmd.AddCommand(childCmd)
+
+	// Test that persistent flag completion works for the subcmd
+	output, err := executeCommand(rootCmd, ShellCompRequestCmd, "child", "--string", "")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	expected := strings.Join([]string{
+		"myval",
+		":0",
+		"Completion ended with directive: ShellCompDirectiveDefault", ""}, "\n")
+
+	if output != expected {
+		t.Errorf("expected: %q, got: %q", expected, output)
+	}
+}
+
+// This test tries to register flag completion concurrently to make sure the
+// code handles concurrency properly.
+// This was reported as a problem when tests are run concurrently:
+// https://github.com/spf13/cobra/issues/1320
+//
+// NOTE: this test can sometimes pass even if the code were to not handle
+// concurrency properly. This is not great but the important part is that
+// it should never fail.  Therefore, if the tests fails sometimes, we will
+// still be able to know there is a problem.
+func TestFlagCompletionConcurrentRegistration(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	const maxFlags = 50
+	for i := 1; i < maxFlags; i += 2 {
+		flagName := fmt.Sprintf("flag%d", i)
+		rootCmd.Flags().String(flagName, "", fmt.Sprintf("test %s flag on root", flagName))
+	}
+
+	childCmd := &Command{
+		Use: "child",
+		Run: emptyRun,
+	}
+	for i := 2; i <= maxFlags; i += 2 {
+		flagName := fmt.Sprintf("flag%d", i)
+		childCmd.Flags().String(flagName, "", fmt.Sprintf("test %s flag on child", flagName))
+	}
+
+	rootCmd.AddCommand(childCmd)
+
+	// Register completion in different threads to test concurrency.
+	var wg sync.WaitGroup
+	for i := 1; i <= maxFlags; i++ {
+		index := i
+		flagName := fmt.Sprintf("flag%d", i)
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			cmd := rootCmd
+			if index%2 == 0 {
+				cmd = childCmd
+			}
+			_ = cmd.RegisterFlagCompletionFunc(flagName, func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+				return []string{fmt.Sprintf("flag%d", index)}, ShellCompDirectiveDefault
+			})
+		}()
+	}
+
+	wg.Wait()
+
+	// Test that flag completion works for each flag
+	for i := 1; i <= 6; i++ {
+		var output string
+		var err error
+		flagName := fmt.Sprintf("flag%d", i)
+
+		if i%2 == 1 {
+			output, err = executeCommand(rootCmd, ShellCompRequestCmd, "--"+flagName, "")
+		} else {
+			output, err = executeCommand(rootCmd, ShellCompRequestCmd, "child", "--"+flagName, "")
+		}
+
+		if err != nil {
+			t.Errorf("Unexpected error: %v", err)
+		}
+
+		expected := strings.Join([]string{
+			flagName,
+			":0",
+			"Completion ended with directive: ShellCompDirectiveDefault", ""}, "\n")
+
+		if output != expected {
+			t.Errorf("expected: %q, got: %q", expected, output)
+		}
+	}
+}
+
 func TestFlagCompletionInGoWithDesc(t *testing.T) {
 	rootCmd := &Command{
 		Use: "root",