Add support for ActiveHelp

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

README.md

@@ -30,6 +30,7 @@ name a few. [This list](./projects_using_cobra.md) contains a more extensive lis
   * [Suggestions when "unknown command" happens](user_guide.md#suggestions-when-unknown-command-happens)
   * [Generating documentation for your command](user_guide.md#generating-documentation-for-your-command)
   * [Generating shell completions](user_guide.md#generating-shell-completions)
+  * [Providing Active Help](user_guide.md#providing-active-help)
 - [Contributing](CONTRIBUTING.md)
 - [License](#license)
 

active_help.go

@@ -0,0 +1,52 @@
+package cobra
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+const (
+	activeHelpMarker = "_activeHelp_ "
+	// The below values should not be changed: programs will be using them explicitly
+	// in their user documentation, and users will be using them explicitly.
+	activeHelpEnvVarSuffix  = "_ACTIVE_HELP"
+	activeHelpGlobalEnvVar  = "COBRA_ACTIVE_HELP"
+	activeHelpGlobalDisable = "0"
+)
+
+// AppendActiveHelp adds the specified string to the specified array to be used as ActiveHelp.
+// Such strings will be processed by the completion script and will be shown as ActiveHelp
+// to the user.
+// The array parameter should be the array that will contain the completions.
+// This function can be called multiple times before and/or after completions are added to
+// the array.  Each time this function is called with the same array, the new
+// ActiveHelp line will be shown below the previous ones when completion is triggered.
+func AppendActiveHelp(compArray []string, activeHelpStr string) []string {
+	return append(compArray, fmt.Sprintf("%s%s", activeHelpMarker, activeHelpStr))
+}
+
+// activeHelpEnvVar returns the name of the program-specific ActiveHelp environment
+// variable.  It has the format <PROGRAM>_ACTIVE_HELP where <PROGRAM> is the name of the
+// root command in upper case, with all - replaced by _.
+// This format should not be changed: users will be using it explicitly.
+func activeHelpEnvVar(name string) string {
+	activeHelpEnvVar := strings.ToUpper(fmt.Sprintf("%s%s", name, activeHelpEnvVarSuffix))
+	return strings.ReplaceAll(activeHelpEnvVar, "-", "_")
+}
+
+// setActiveHelpConfig first checks the global environment variable
+// of ActiveHelp to see if it is disabling active help, and if it is not,
+// it then looks to the program-specific variable.
+// It then sets the ActiveHelpConfig value to make it available when
+// calling the completion function.  We also set it on the root,
+// just in case users try to access it from there.
+func setActiveHelpConfig(cmd *Command) {
+	activeHelpCfg := os.Getenv(activeHelpGlobalEnvVar)
+	if activeHelpCfg != activeHelpGlobalDisable {
+		activeHelpCfg = os.Getenv(activeHelpEnvVar(cmd.Root().Name()))
+	}
+
+	cmd.ActiveHelpConfig = activeHelpCfg
+	cmd.Root().ActiveHelpConfig = cmd.ActiveHelpConfig
+}

active_help.md

@@ -0,0 +1,182 @@
+# Active Help
+
+Active Help is a framework provided by Cobra which allows a program to define messages (hints, warnings, etc) that will be printed during program usage.  It aims to make it easier for your users to learn how to use your program.  If configured by the program, Active Help is printed when the user triggers shell completion.
+
+For example, 
+```
+bash-5.1$ helm repo add [tab]
+You must choose a name for the repo you are adding.
+
+bash-5.1$ bin/helm package [tab]
+Please specify the path to the chart to package
+
+bash-5.1$ bin/helm package [tab][tab]
+bin/    internal/    scripts/    pkg/     testdata/
+```
+## Supported shells
+
+Active Help is currently only supported for the following shells:
+- Bash (using [bash completion V2](shell_completions.md#bash-completion-v2) only)
+- Zsh
+
+## Adding Active Help messages
+
+As Active Help uses the shell completion system, the implementation of Active Help messages is done by enhancing custom dynamic completions.  If you are not familiar with dynamic completions, please refer to [Shell Completions](shell_completions.md).
+
+Adding Active Help is done through the use of the `cobra.AppendActiveHelp(...)` function, where the program repeatedly adds Active Help messages to the list of completions.  Keep reading for details.
+
+### Active Help for nouns
+
+Adding Active Help when completing a noun is done within the `ValidArgsFunction(...)` of a command.  Please notice the use of `cobra.AppendActiveHelp(...)` in the following example:
+
+```go
+cmd := &cobra.Command{
+	Use:   "add [NAME] [URL]",
+	Short: "add a chart repository",
+	Args:  require.ExactArgs(2),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		return addRepo(args)
+	},
+	ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		var comps []string
+		if len(args) == 0 {
+			comps = cobra.AppendActiveHelp(comps, "You must choose a name for the repo you are adding")
+		} else if len(args) == 1 {
+			comps = cobra.AppendActiveHelp(comps, "You must specify the URL for the repo you are adding")
+		} else {
+			comps = cobra.AppendActiveHelp(comps, "This command does not take any more arguments")
+		}
+		return comps, cobra.ShellCompDirectiveNoFileComp
+	},
+}
+```
+The example above defines the completions (none, in this specific example) as well as the Active Help messages for the `helm repo add` command.  It yields the following behavior:
+```
+bash-5.1$ helm repo add [tab]
+You must choose a name for the repo you are adding
+
+bash-5.1$ helm repo add grafana [tab]
+You must specify the URL for the repo you are adding
+
+bash-5.1$ helm repo add grafana https://grafana.github.io/helm-charts [tab]
+This command does not take any more arguments
+```
+
+### Active Help for flags
+
+Providing Active Help for flags is done in the same fashion as for nouns, but using the completion function registered for the flag.  For example:
+```go
+_ = cmd.RegisterFlagCompletionFunc("version", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		if len(args) != 2 {
+			return cobra.AppendActiveHelp(nil, "You must first specify the chart to install before the --version flag can be completed"), cobra.ShellCompDirectiveNoFileComp
+		}
+		return compVersionFlag(args[1], toComplete)
+	})
+```
+The example above prints an Active Help message when not enough information was given by the user to complete the `--version` flag.
+```
+bash-5.1$ bin/helm install myrelease --version 2.0.[tab]
+You must first specify the chart to install before the --version flag can be completed
+
+bash-5.1$ bin/helm install myrelease bitnami/solr --version 2.0.[tab][tab]
+2.0.1  2.0.2  2.0.3
+```
+
+## User control of Active Help
+
+You may want to allow your users to disable Active Help or choose between different levels of Active Help.  It is entirely up to the program to define the type of configurability of Active Help that it wants to offer.  
+
+### Configuration using an environment variable
+
+One way to configure Active Help is to use the program's Active Help environment
+variable.  That variable is named `<PROGRAM>_ACTIVE_HELP` where `<PROGRAM>` is the name of your 
+program in uppercase with any `-` replaced by an `_`.  You can find that variable in the generated
+completion scripts of your program.  The variable should be set by the user to whatever Active Help 
+configuration values are supported by the program.
+
+For example, say `helm` supports three levels for Active Help: `on`, `off`, `local`.  Then a user
+would set the desired behavior to `local` by doing `export HELM_ACTIVE_HELP=local` in their shell.
+
+When in `cmd.ValidArgsFunction(...)` or a flag's completion function, the program should read the
+Active Help configuration from the `cmd.ActiveHelpConfig` field and select what Active Help messages
+should or should not be added.
+
+For example:
+```go
+ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	activeHelpLevel := cmd.ActiveHelpConfig
+
+	var comps []string
+	if len(args) == 0 {
+		if activeHelpLevel != "off"  {
+			comps = cobra.AppendActiveHelp(comps, "You must choose a name for the repo you are adding")
+		}
+	} else if len(args) == 1 {
+		if activeHelpLevel != "off" {
+			comps = cobra.AppendActiveHelp(comps, "You must specify the URL for the repo you are adding")
+		}
+	} else {
+		if activeHelpLevel == "local" {
+			comps = cobra.AppendActiveHelp(comps, "This command does not take any more arguments")
+		}
+	}
+	return comps, cobra.ShellCompDirectiveNoFileComp
+},
+```
+**Note 1**: If the string "0" is used for `cmd.Root().ActiveHelpConfig`, it will automatically be handled by Cobra and will completely disable all Active Help output (even if some output was specified by the program using the `cobra.AppendActiveHelp(...)` function).  Using "0" can simplify your code in situations where you want to blindly disable Active Help.
+
+**Note 2**: Cobra transparently passes the `cmd.ActiveHelpConfig` string you specified back to your program when completion is invoked.  You can therefore define any scheme you choose for your program; you are not limited to using integer levels for the configuration of Active Help.  **However, the reserved "0" value can also be sent to you program and you should be prepared for it.**
+
+**Note 3**: If a user wants to disable Active Help for every single program based on Cobra, the global environment variable `COBRA_ACTIVE_HELP` can be used as follows:
+```
+export COBRA_ACTIVE_HELP=0
+```
+
+### Configuration using a flag
+
+Another approach for a user to configure Active Help is for the program to add a flag to the command that generates 
+the completion script.  Using the flag, the user specifies the Active Help configuration that is
+desired.  Then the program should specify that configuration by setting the `rootCmd.ActiveHelpConfig` string
+before calling the Cobra API that generates the shell completion script.
+The ActiveHelp configuration would then be read in `cmd.ValidArgsFunction(...)` or a flag's completion function, in the same 
+fashion as explained above, using the same `cmd.ActiveHelpConfig` field.
+
+For example, a program that uses a `completion` command to generate the shell completion script can add a flag `--activehelp-level` to that command.  The user would then use that flag to choose an Active Help level:
+```
+bash-5.1$ source <(helm completion bash --activehelp-level 1)
+```
+The code to pass that information to Cobra would look something like:
+```go
+cmd.Root().ActiveHelpConfig = strconv.Itoa(activeHelpLevel)
+return cmd.Root().GenBashCompletionV2(out, true)
+```
+
+The advantage of using a flag is that it becomes self-documenting through Cobra's `help` command.  Also, it allows you to
+use Cobra's flag parsing to handle the configuration value, instead of having to deal with a environment variable natively.
+
+## Active Help with Cobra's default completion command
+
+Cobra provides a default `completion` command for programs that wish to use it.
+When using the default `completion` command, Active Help is configurable using the
+environment variable approach described above.  You may wish to document this in more
+details for your users.
+
+## Debugging Active Help
+
+Debugging your Active Help code is done in the same way as debugging the dynamic completion code, which is with Cobra's hidden `__complete` command.  Please refer to [debugging shell completion](shell_completions.md#debugging) for details.
+
+When debugging with the `__complete` command, if you want to specify different Active Help configurations, you should use the active help environment variable (as you can find in the generated completion scripts).  That variable is named `<PROGRAM>_ACTIVE_HELP` where any `-` is replaced by an `_`.  For example, we can test deactivating some Active Help as shown below:
+```
+$ HELM_ACTIVE_HELP=1 bin/helm __complete install wordpress bitnami/h<ENTER>
+bitnami/haproxy
+bitnami/harbor
+_activeHelp_ WARNING: cannot re-use a name that is still in use
+:0
+Completion ended with directive: ShellCompDirectiveDefault
+
+$ HELM_ACTIVE_HELP=0 bin/helm __complete install wordpress bitnami/h<ENTER>
+bitnami/haproxy
+bitnami/harbor
+:0
+Completion ended with directive: ShellCompDirectiveDefault
+```