Improve API to get flag completion function

The new API is simpler and matches the `c.RegisterFlagCompletionFunc()`
API.  By removing the global function `GetFlagCompletion()` we are more
future proof if we ever move from a global map of flag completion
functions to something associated with the command.

The commit also makes this API work with persistent flags by using
`c.Flag(flagName)` instead of `c.Flags().Lookup(flagName)`.

The commit also adds unit tests.

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

completions.go

@@ -145,25 +145,20 @@ func (c *Command) RegisterFlagCompletionFunc(flagName string, f func(cmd *Comman
 	return nil
 }
 
-// GetFlagCompletion returns the completion function for the given flag, if available.
-func GetFlagCompletion(flag *pflag.Flag) (func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective), bool) {
+// GetFlagCompletionFunc returns the completion function for the given flag of the command, if available.
+func (c *Command) GetFlagCompletionFunc(flagName string) (func(*Command, []string, string) ([]string, ShellCompDirective), bool) {
+	flag := c.Flag(flagName)
+	if flag == nil {
+		return nil, false
+	}
+
 	flagCompletionMutex.RLock()
 	defer flagCompletionMutex.RUnlock()
 
 	completionFunc, exists := flagCompletionFunctions[flag]
 	return completionFunc, exists
 }
 
-// GetFlagCompletionByName returns the completion function for the given flag in the command by name, if available.
-func (c *Command) GetFlagCompletionByName(flagName string) (func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective), bool) {
-	flag := c.Flags().Lookup(flagName)
-	if flag == nil {
-		return nil, false
-	}
-
-	return GetFlagCompletion(flag)
-}
-
 // Returns a string listing the different directive enabled in the specified parameter
 func (d ShellCompDirective) string() string {
 	var directives []string

completions_test.go

@@ -3427,3 +3427,93 @@ Completion ended with directive: ShellCompDirectiveNoFileComp
 		})
 	}
 }
+
+func TestGetFlagCompletion(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+
+	rootCmd.Flags().String("rootflag", "", "root flag")
+	_ = rootCmd.RegisterFlagCompletionFunc("rootflag", func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return []string{"rootvalue"}, ShellCompDirectiveKeepOrder
+	})
+
+	rootCmd.PersistentFlags().String("persistentflag", "", "persistent flag")
+	_ = rootCmd.RegisterFlagCompletionFunc("persistentflag", func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return []string{"persistentvalue"}, ShellCompDirectiveDefault
+	})
+
+	childCmd := &Command{Use: "child", Run: emptyRun}
+
+	childCmd.Flags().String("childflag", "", "child flag")
+	_ = childCmd.RegisterFlagCompletionFunc("childflag", func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return []string{"childvalue"}, ShellCompDirectiveNoFileComp | ShellCompDirectiveNoSpace
+	})
+
+	rootCmd.AddCommand(childCmd)
+
+	testcases := []struct {
+		desc      string
+		cmd       *Command
+		flagName  string
+		exists    bool
+		comps     []string
+		directive ShellCompDirective
+	}{
+		{
+			desc:      "get flag completion function for command",
+			cmd:       rootCmd,
+			flagName:  "rootflag",
+			exists:    true,
+			comps:     []string{"rootvalue"},
+			directive: ShellCompDirectiveKeepOrder,
+		},
+		{
+			desc:      "get persistent flag completion function for command",
+			cmd:       rootCmd,
+			flagName:  "persistentflag",
+			exists:    true,
+			comps:     []string{"persistentvalue"},
+			directive: ShellCompDirectiveDefault,
+		},
+		{
+			desc:      "get flag completion function for child command",
+			cmd:       childCmd,
+			flagName:  "childflag",
+			exists:    true,
+			comps:     []string{"childvalue"},
+			directive: ShellCompDirectiveNoFileComp | ShellCompDirectiveNoSpace,
+		},
+		{
+			desc:      "get persistent flag completion function for child command",
+			cmd:       childCmd,
+			flagName:  "persistentflag",
+			exists:    true,
+			comps:     []string{"persistentvalue"},
+			directive: ShellCompDirectiveDefault,
+		},
+		{
+			desc:     "cannot get flag completion function for local parent flag",
+			cmd:      childCmd,
+			flagName: "rootflag",
+			exists:   false,
+		},
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.desc, func(t *testing.T) {
+			compFunc, exists := tc.cmd.GetFlagCompletionFunc(tc.flagName)
+			if tc.exists != exists {
+				t.Errorf("Unexpected result looking for flag completion function")
+			}
+
+			if exists {
+				comps, directive := compFunc(tc.cmd, []string{}, "")
+				if strings.Join(tc.comps, " ") != strings.Join(comps, " ") {
+					t.Errorf("Unexpected completions %q", comps)
+				}
+				if tc.directive != directive {
+					t.Errorf("Unexpected directive %q", directive)
+				}
+			}
+		})
+	}
+}