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Update documentation for custom completion in Go
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Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>
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marckhouzam committed Feb 16, 2020
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Expand Up @@ -56,7 +56,146 @@ func main() {

`out.sh` will get you completions of subcommands and flags. Copy it to `/etc/bash_completion.d/` as described [here](https://debian-administration.org/article/316/An_introduction_to_bash_completion_part_1) and reset your terminal to use autocompletion. If you make additional annotations to your code, you can get even more intelligent and flexible behavior.

## Creating your own custom functions
## Have the completions code complete your 'nouns'

### Static completion of nouns

This method allows you to provide a pre-defined list of completion choices for your nouns using the `validArgs` field.
For example, if you want `kubectl get [tab][tab]` to show a list of valid "nouns" you have to set them. Simplified code from `kubectl get` looks like:

```go
validArgs []string = { "pod", "node", "service", "replicationcontroller" }

cmd := &cobra.Command{
Use: "get [(-o|--output=)json|yaml|template|...] (RESOURCE [NAME] | RESOURCE/NAME ...)",
Short: "Display one or many resources",
Long: get_long,
Example: get_example,
Run: func(cmd *cobra.Command, args []string) {
err := RunGet(f, out, cmd, args)
util.CheckErr(err)
},
ValidArgs: validArgs,
}
```

Notice we put the "ValidArgs" on the "get" subcommand. Doing so will give results like

```bash
# kubectl get [tab][tab]
node pod replicationcontroller service
```

### Plural form and shortcuts for nouns

If your nouns have a number of aliases, you can define them alongside `ValidArgs` using `ArgAliases`:

```go
argAliases []string = { "pods", "nodes", "services", "svc", "replicationcontrollers", "rc" }

cmd := &cobra.Command{
...
ValidArgs: validArgs,
ArgAliases: argAliases
}
```

The aliases are not shown to the user on tab completion, but they are accepted as valid nouns by
the completion algorithm if entered manually, e.g. in:

```bash
# kubectl get rc [tab][tab]
backend frontend database
```

Note that without declaring `rc` as an alias, the completion algorithm would show the list of nouns
in this example again instead of the replication controllers.

### Dynamic completion of nouns

In some cases it is not possible to provide a list of possible completions in advance. Instead, the list of completions must be determined at execution-time. Cobra provides two ways of defining such dynamic completion of nouns. Note that both these methods can be used along-side each other as long as they are not both used for the same command.

#### 1. Custom completions of nouns written in Go

In a similar fashion as for static completions, you can use the `validArgsFunction` field to provide a Go function that Cobra will execute when it needs the list of completion choices for the nouns of a command. Simplified code from `helm status` looks like:

```go
cmd := &cobra.Command{
Use: "status RELEASE_NAME",
Short: "Display the status of the named release",
Long: status_long,
RunE: func(cmd *cobra.Command, args []string) {
RunGet(args[0])
},
ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.BashCompDirective) {
if len(args) != 0 {
return nil, cobra.BashCompDirectiveNoFileComp
}
return getReleasesFromCluster(toComplete), cobra.BashCompDirectiveNoFileComp
},
}
```
Where `getReleasesFromCluster()` is a Go function that obtains the list of current Helm releases running on the Kubernetes cluster.
Notice we put the `ValidArgsFunction` on the `status` subcommand. Let's assume the Helm releases on the cluster are: `harbor`, `notary`, `rook` and `thanos` then this dynamic completion will give results like

```bash
# helm status [tab][tab]
harbor notary rook thanos
```
You may have noticed the use of `cobra.BashCompDirective`. These directives are bit fields allowing to control some shell completion behaviors for your particular completion. You can combine them with the bit-or operator such as `cobra.BashCompDirectiveNoSpace | cobra.BashCompDirectiveNoFileComp`
```go
// Indicates an error occurred and completions should be ignored.
BashCompDirectiveError
// Indicates that the shell should not add a space after the completion,
// even if there is a single completion provided.
BashCompDirectiveNoSpace
// Indicates that the shell should not provide file completion even when
// no completion is provided.
// This currently does not work for zsh or bash < 4
BashCompDirectiveNoFileComp
// Indicates that the shell will perform its default behavior after completions
// have been provided (this implies !BashCompDirectiveNoSpace && !BashCompDirectiveNoFileComp).
BashCompDirectiveDefault
```

When using the `ValidArgsFunction`, Cobra will call your registered function after having parsed all flags and arguments provided in the command-line. You therefore don't need to do this parsing yourself. For example, when a user calls `helm status --namespace my-rook-ns [tab][tab]`, Cobra will call your registered `ValidArgsFunction` after having parsed the `--namespace` flag, as it would have done when calling the `RunE` function.

##### Debugging

Cobra achieves dynamic completions written in Go through the use of a hidden command called by the completion script. To debug your Go completion code, you can call this hidden command directly:
```bash
# helm __complete status har<ENTER>
harbor
:4
Completion ended with directive: BashCompDirectiveNoFileComp # This is on stderr
```
***Important:*** If the noun to complete is empty, you must pass an empty parameter to the `__complete` command:
```bash
# helm __complete status ""<ENTER>
harbor
notary
rook
thanos
:4
Completion ended with directive: BashCompDirectiveNoFileComp # This is on stderr
```
Calling the `__complete` command directly allows you to run the Go debugger to troubleshoot your code. You can also add printouts to your code; Cobra provides the following functions to use for printouts in Go completion code:
```go
// Prints to the completion script debug file (if BASH_COMP_DEBUG_FILE
// is set to a file path) and optionally prints to stderr.
cobra.CompDebug(msg string, printToStdErr bool) {
cobra.CompDebugln(msg string, printToStdErr bool)

// Prints to the completion script debug file (if BASH_COMP_DEBUG_FILE
// is set to a file path) and to stderr.
cobra.CompError(msg string)
cobra.CompErrorln(msg string)
```
***Important:*** You should **not** leave traces that print to stdout in your completion code as they will be interpreted as completion choices by the completion script. Instead, use the cobra-provided debugging traces functions mentioned above.
#### 2. Custom completions of nouns written in Bash
This method allows you to inject bash functions into the completion script. Those bash functions are responsible for providing the completion choices for your own completions.
Some more actual code that works in kubernetes:
Expand Down Expand Up @@ -111,58 +250,6 @@ Find more information at https://github.com/GoogleCloudPlatform/kubernetes.`,
The `BashCompletionFunction` option is really only valid/useful on the root command. Doing the above will cause `__kubectl_custom_func()` (`__<command-use>_custom_func()`) to be called when the built in processor was unable to find a solution. In the case of kubernetes a valid command might look something like `kubectl get pod [mypod]`. If you type `kubectl get pod [tab][tab]` the `__kubectl_customc_func()` will run because the cobra.Command only understood "kubectl" and "get." `__kubectl_custom_func()` will see that the cobra.Command is "kubectl_get" and will thus call another helper `__kubectl_get_resource()`. `__kubectl_get_resource` will look at the 'nouns' collected. In our example the only noun will be `pod`. So it will call `__kubectl_parse_get pod`. `__kubectl_parse_get` will actually call out to kubernetes and get any pods. It will then set `COMPREPLY` to valid pods!
## Have the completions code complete your 'nouns'
In the above example "pod" was assumed to already be typed. But if you want `kubectl get [tab][tab]` to show a list of valid "nouns" you have to set them. Simplified code from `kubectl get` looks like:
```go
validArgs []string = { "pod", "node", "service", "replicationcontroller" }
cmd := &cobra.Command{
Use: "get [(-o|--output=)json|yaml|template|...] (RESOURCE [NAME] | RESOURCE/NAME ...)",
Short: "Display one or many resources",
Long: get_long,
Example: get_example,
Run: func(cmd *cobra.Command, args []string) {
err := RunGet(f, out, cmd, args)
util.CheckErr(err)
},
ValidArgs: validArgs,
}
```
Notice we put the "ValidArgs" on the "get" subcommand. Doing so will give results like
```bash
# kubectl get [tab][tab]
node pod replicationcontroller service
```
## Plural form and shortcuts for nouns
If your nouns have a number of aliases, you can define them alongside `ValidArgs` using `ArgAliases`:
```go
argAliases []string = { "pods", "nodes", "services", "svc", "replicationcontrollers", "rc" }
cmd := &cobra.Command{
...
ValidArgs: validArgs,
ArgAliases: argAliases
}
```
The aliases are not shown to the user on tab completion, but they are accepted as valid nouns by
the completion algorithm if entered manually, e.g. in:
```bash
# kubectl get rc [tab][tab]
backend frontend database
```
Note that without declaring `rc` as an alias, the completion algorithm would show the list of nouns
in this example again instead of the replication controllers.
## Mark flags as required
Most of the time completions will only show subcommands. But if a flag is required to make a subcommand work, you probably want it to show up when the user types [tab][tab]. Marking a flag as 'Required' is incredibly easy.
Expand Down Expand Up @@ -211,8 +298,43 @@ So while there are many other files in the CWD it only shows me subdirs and thos
# Specify custom flag completion
Similar to the filename completion and filtering using cobra.BashCompFilenameExt, you can specify
a custom flag completion function with cobra.BashCompCustom:
As for nouns, Cobra provides two ways of defining dynamic completion of flags. Note that both these methods can be used along-side each other as long as they are not both used for the same flag.
## 1. Custom completions of flags written in Go
To provide a Go function that Cobra will execute when it needs the list of completion choices for a flag, you must register the function in the following manner:
```go
flagName := "output"
cmd.RegisterFlagCompletionFunc(flagName, func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.BashCompDirective) {
return []string{"json", "table", "yaml"}, cobra.BashCompDirectiveDefault
})
```
Notice that calling `RegisterFlagCompletionFunc()` is done through the `command` with which the flag is associated. In our example this dynamic completion will give results like so:
```bash
# helm status --output [tab][tab]
json table yaml
```
### Debugging
You can also easily debug your Go completion code for flags:
```bash
# helm __complete status --output ""
json
table
yaml
:4
Completion ended with directive: BashCompDirectiveNoFileComp # This is on stderr
```
***Important:*** You should **not** leave traces that print to stdout in your completion code as they will be interpreted as completion choices by the completion script. Instead, use the cobra-provided debugging traces functions mentioned in the above section.
## 2. Custom completions of flags written in Bash
Alternatively, you can use bash code for flag custom completion. Similar to the filename
completion and filtering using `cobra.BashCompFilenameExt`, you can specify
a custom flag completion bash function with `cobra.BashCompCustom`:
```go
annotation := make(map[string][]string)
Expand All @@ -226,7 +348,7 @@ a custom flag completion function with cobra.BashCompCustom:
cmd.Flags().AddFlag(flag)
```
In addition add the `__handle_namespace_flag` implementation in the `BashCompletionFunction`
In addition add the `__kubectl_get_namespaces` implementation in the `BashCompletionFunction`
value, e.g.:
```bash
Expand Down

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