Add framework for dynamic tab completions

By setting the COBRA_FLAG_COMPLETION environment variable, the normal execution path of the command
is short circuited, and instead the function registered by `MarkCustomFlagCompletion` is executed.

All flags other than the one being completed get parsed according to whatever type they are defined
as, but the flag being completed is parsed as a raw string and passed into the custom compeltion.

command.go

@@ -143,6 +143,10 @@ type Command struct {
 	//FParseErrWhitelist flag parse errors to be ignored
 	FParseErrWhitelist FParseErrWhitelist
 
+	// RunPreRunsDuringCompletion defines if the (Persistent)PreRun functions should be run before calling the
+	// completion functions
+	RunPreRunsDuringCompletion bool
+
 	// commands is the list of commands supported by this program.
 	commands []*Command
 	// parent is a parent command for this command.
@@ -200,6 +204,10 @@ type Command struct {
 	outWriter io.Writer
 	// errWriter is a writer defined by the user that replaces stderr
 	errWriter io.Writer
+
+	// flagCompletions is a map of flag to a function that returns a list of values to suggest during tab completion for
+	// this flag
+	flagCompletions map[*flag.Flag]DynamicFlagCompletion
 }
 
 // SetArgs sets arguments for the command. It is set to os.Args[1:] by default, if desired, can be overridden
@@ -736,6 +744,8 @@ func (c *Command) ArgsLenAtDash() int {
 	return c.Flags().ArgsLenAtDash()
 }
 
+const FlagCompletionEnvVar = "COBRA_FLAG_COMPLETION"
+
 func (c *Command) execute(a []string) (err error) {
 	if c == nil {
 		return fmt.Errorf("Called Execute() on a nil Command")
@@ -851,6 +861,90 @@ func (c *Command) execute(a []string) (err error) {
 	return nil
 }
 
+func (c *Command) complete(flagName string, a []string) (err error) {
+	if c == nil {
+		return fmt.Errorf("Called Execute() on a nil Command")
+	}
+
+	// initialize help and version flag at the last point possible to allow for user
+	// overriding
+	c.InitDefaultHelpFlag()
+	c.InitDefaultVersionFlag()
+
+	var flagToComplete *flag.Flag
+	var currentCompletionValue string
+
+	oldFlags := c.Flags()
+	c.flags = nil
+	oldFlags.VisitAll(func(f *flag.Flag) {
+		if f.Name == flagName {
+			flagToComplete = f
+		} else {
+			c.Flags().AddFlag(f)
+		}
+	})
+	if flagToComplete.Shorthand != "" {
+		c.Flags().StringVarP(&currentCompletionValue, flagName, flagToComplete.Shorthand, "", "")
+	} else {
+		c.Flags().StringVar(&currentCompletionValue, flagName, "", "")
+	}
+	c.Flag(flagName).NoOptDefVal = "_hack_"
+
+	err = c.ParseFlags(a)
+	if err != nil {
+		return c.FlagErrorFunc()(c, err)
+	}
+
+	c.preRun()
+
+	currentCommand := c
+	completionFunc := currentCommand.flagCompletions[flagToComplete]
+	for completionFunc == nil && currentCommand.HasParent() {
+		currentCommand = currentCommand.Parent()
+		completionFunc = currentCommand.flagCompletions[flagToComplete]
+	}
+	if completionFunc == nil {
+		return fmt.Errorf("%s does not have completions enabled", flagName)
+	}
+
+	if c.RunPreRunsDuringCompletion {
+		argWoFlags := c.Flags().Args()
+		if c.DisableFlagParsing {
+			argWoFlags = a
+		}
+
+		for p := c; p != nil; p = p.Parent() {
+			if p.PersistentPreRunE != nil {
+				if err := p.PersistentPreRunE(c, argWoFlags); err != nil {
+					return err
+				}
+				break
+			} else if p.PersistentPreRun != nil {
+				p.PersistentPreRun(c, argWoFlags)
+				break
+			}
+		}
+		if c.PreRunE != nil {
+			if err := c.PreRunE(c, argWoFlags); err != nil {
+				return err
+			}
+		} else if c.PreRun != nil {
+			c.PreRun(c, argWoFlags)
+		}
+	}
+
+	values, err := completionFunc(currentCompletionValue)
+	if err != nil {
+		return err
+	}
+
+	for _, v := range values {
+		c.Print(v + "\x00")
+	}
+
+	return nil
+}
+
 func (c *Command) preRun() {
 	for _, x := range initializers {
 		x()
@@ -911,6 +1005,16 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 		cmd.commandCalledAs.name = cmd.Name()
 	}
 
+	flagName, flagCompletionEnabled := os.LookupEnv(FlagCompletionEnvVar)
+	if flagCompletionEnabled {
+		err = cmd.complete(flagName, flags)
+		if err != nil {
+			c.Println("Error:", err.Error())
+		}
+
+		return cmd, err
+	}
+
 	err = cmd.execute(flags)
 	if err != nil {
 		// Always show help if requested, even if SilenceErrors is in

shell_completions.go

@@ -1,6 +1,8 @@
 package cobra
 
 import (
+	"fmt"
+
 	"github.com/spf13/pflag"
 )
 
@@ -83,3 +85,31 @@ func MarkFlagDirname(flags *pflag.FlagSet, name string) error {
 	zshPattern := "-(/)"
 	return flags.SetAnnotation(name, zshCompDirname, []string{zshPattern})
 }
+
+type DynamicFlagCompletion func(currentValue string) (suggestedValues []string, err error)
+
+// MarkDynamicFlagCompletion provides cobra a function to dynamically suggest values to the user during tab completion
+// for this flag. All (Persistent)PreRun(E) functions will be run accordingly before the provided function is called if
+// RunPreRunsDuringCompletion is set to true. All flags other than the flag currently being completed will be parsed
+// according to their type. The flag being completed is parsed as a raw string with no format requirements
+//
+// Shell Completion compatibility matrix: zsh
+func (c *Command) MarkDynamicFlagCompletion(name string, completion DynamicFlagCompletion) error {
+	flag := c.Flag(name)
+	if flag == nil {
+		return fmt.Errorf("no such flag %s", name)
+	}
+	if flag.NoOptDefVal != "" {
+		return fmt.Errorf("%s takes no parameters", name)
+	}
+
+	if c.flagCompletions == nil {
+		c.flagCompletions = make(map[*pflag.Flag]DynamicFlagCompletion)
+	}
+	c.flagCompletions[flag] = completion
+	if flag.Annotations == nil {
+		flag.Annotations = map[string][]string{}
+	}
+	flag.Annotations[zshCompDynamicCompletion] = []string{zshCompGenFlagCompletionFuncName(c)}
+	return nil
+}

zsh_completions.go

@@ -17,14 +17,18 @@ const (
 	zshCompArgumentFilenameComp = "cobra_annotations_zsh_completion_argument_file_completion"
 	zshCompArgumentWordComp     = "cobra_annotations_zsh_completion_argument_word_completion"
 	zshCompDirname              = "cobra_annotations_zsh_dirname"
+	zshCompDynamicCompletion    = "cobra_annotations_zsh_completion_dynamic_completion"
 )
 
 var (
 	zshCompFuncMap = template.FuncMap{
-		"genZshFuncName":              zshCompGenFuncName,
-		"extractFlags":                zshCompExtractFlag,
-		"genFlagEntryForZshArguments": zshCompGenFlagEntryForArguments,
-		"extractArgsCompletions":      zshCompExtractArgumentCompletionHintsForRendering,
+		"genZshFuncName":                      zshCompGenFuncName,
+		"extractFlags":                        zshCompExtractFlag,
+		"genFlagEntryForZshArguments":         zshCompGenFlagEntryForArguments,
+		"extractArgsCompletions":              zshCompExtractArgumentCompletionHintsForRendering,
+		"genZshFlagDynamicCompletionFuncName": zshCompGenFlagCompletionFuncName,
+		"hasDynamicCompletions":               zshCompHasDynamicCompletions,
+		"flagCompletionsEnvVar":               func() string { return FlagCompletionEnvVar },
 	}
 	zshCompletionText = `
 {{/* should accept Command (that contains subcommands) as parameter */}}
@@ -79,6 +83,21 @@ function {{genZshFuncName .}} {
 {{define "Main" -}}
 #compdef _{{.Name}} {{.Name}}
 
+{{if hasDynamicCompletions . -}}
+function {{genZshFlagDynamicCompletionFuncName .}} {
+  export COBRA_FLAG_COMPLETION="$1"
+  if suggestions="$("$words[@]" 2>&1)" ; then
+    local -a args
+    while read -d $'\0' line ; do
+      args+="$line"
+    done <<< "$suggestions"
+    _values "$1" "$args[@]"
+  else
+    _message "Exception occurred during completion: $suggestions"
+  fi
+  unset COBRA_FLAG_COMPLETION
+}{{- end}}
+
 {{template "selectCmdTemplate" .}}
 {{end}}
 `
@@ -250,6 +269,10 @@ func zshCompGenFuncName(c *Command) string {
 	return "_" + c.Name()
 }
 
+func zshCompGenFlagCompletionFuncName(c *Command) string {
+	return "_" + c.Root().Name() + "-flag-completion"
+}
+
 func zshCompExtractFlag(c *Command) []*pflag.Flag {
 	var flags []*pflag.Flag
 	c.LocalFlags().VisitAll(func(f *pflag.Flag) {
@@ -310,7 +333,7 @@ func zshCompGenFlagEntryExtras(f *pflag.Flag) string {
 		return ""
 	}
 
-	extras := ":" // allow options for flag (even without assistance)
+	extras := ":" + f.Name // allow options for flag (even without assistance)
 	for key, values := range f.Annotations {
 		switch key {
 		case zshCompDirname:
@@ -320,6 +343,8 @@ func zshCompGenFlagEntryExtras(f *pflag.Flag) string {
 			for _, pattern := range values {
 				extras = extras + fmt.Sprintf(` -g "%s"`, pattern)
 			}
+		case zshCompDynamicCompletion:
+			extras += fmt.Sprintf(":{%s %s}", values[0], f.Name)
 		}
 	}
 
@@ -334,3 +359,15 @@ func zshCompFlagCouldBeSpecifiedMoreThenOnce(f *pflag.Flag) bool {
 func zshCompQuoteFlagDescription(s string) string {
 	return strings.Replace(s, "'", `'\''`, -1)
 }
+
+func zshCompHasDynamicCompletions(c *Command) bool {
+	if len(c.flagCompletions) > 0 {
+		return true
+	}
+	for _, subcommand := range c.Commands() {
+		if zshCompHasDynamicCompletions(subcommand) {
+			return true
+		}
+	}
+	return false
+}