cmd/tailscale: add shell tab-completion

The approach is lifted from cobra: `tailscale completion bash` emits a bash
script for configuring the shell's autocomplete:

    . <( tailscale completion bash )

so that typing:

    tailscale st<TAB>

invokes:

    tailscale completion __complete -- st

RELNOTE=tailscale CLI now supports shell tab-completion

Fixes #3793

Signed-off-by: Paul Scott <paul@tailscale.com>

tempfork/spf13/cobra/cobra.go

@@ -0,0 +1,139 @@
+// Copyright 2013-2023 The Cobra Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package cobra contains shell scripts and constants copied from
+// https://github.com/spf13/cobra for use in our own shell tab-completion logic.
+package cobra
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ShellCompDirective is a bit map representing the different behaviors the shell
+// can be instructed to have once completions have been provided.
+type ShellCompDirective int
+
+const (
+	// ShellCompDirectiveError indicates an error occurred and completions should be ignored.
+	ShellCompDirectiveError ShellCompDirective = 1 << iota
+
+	// ShellCompDirectiveNoSpace indicates that the shell should not add a space
+	// after the completion even if there is a single completion provided.
+	ShellCompDirectiveNoSpace
+
+	// ShellCompDirectiveNoFileComp indicates that the shell should not provide
+	// file completion even when no completion is provided.
+	ShellCompDirectiveNoFileComp
+
+	// ShellCompDirectiveFilterFileExt indicates that the provided completions
+	// should be used as file extension filters.
+	ShellCompDirectiveFilterFileExt
+
+	// ShellCompDirectiveFilterDirs indicates that only directory names should
+	// be provided in file completion.  To request directory names within another
+	// directory, the returned completions should specify the directory within
+	// which to search.
+	ShellCompDirectiveFilterDirs
+
+	// ShellCompDirectiveKeepOrder indicates that the shell should preserve the order
+	// in which the completions are provided
+	ShellCompDirectiveKeepOrder
+
+	// ===========================================================================
+
+	// All directives using iota should be above this one.
+	// For internal use.
+	shellCompDirectiveMaxValue
+
+	// ShellCompDirectiveDefault indicates to let the shell perform its default
+	// behavior after completions have been provided.
+	// This one must be last to avoid messing up the iota count.
+	ShellCompDirectiveDefault ShellCompDirective = 0
+)
+
+// Returns a string listing the different directive enabled in the specified parameter
+func (d ShellCompDirective) String() string {
+	var directives []string
+	if d&ShellCompDirectiveError != 0 {
+		directives = append(directives, "ShellCompDirectiveError")
+	}
+	if d&ShellCompDirectiveNoSpace != 0 {
+		directives = append(directives, "ShellCompDirectiveNoSpace")
+	}
+	if d&ShellCompDirectiveNoFileComp != 0 {
+		directives = append(directives, "ShellCompDirectiveNoFileComp")
+	}
+	if d&ShellCompDirectiveFilterFileExt != 0 {
+		directives = append(directives, "ShellCompDirectiveFilterFileExt")
+	}
+	if d&ShellCompDirectiveFilterDirs != 0 {
+		directives = append(directives, "ShellCompDirectiveFilterDirs")
+	}
+	if d&ShellCompDirectiveKeepOrder != 0 {
+		directives = append(directives, "ShellCompDirectiveKeepOrder")
+	}
+	if len(directives) == 0 {
+		directives = append(directives, "ShellCompDirectiveDefault")
+	}
+
+	if d >= shellCompDirectiveMaxValue {
+		return fmt.Sprintf("ERROR: unexpected ShellCompDirective value: %d", d)
+	}
+	return strings.Join(directives, " | ")
+}
+
+const UsageTemplate = `To load completions:
+
+Bash:
+
+	$ source <(%[1]s completion bash)
+
+	# To load completions for each session, execute once:
+	# Linux:
+	$ %[1]s completion bash > /etc/bash_completion.d/%[1]s
+	# macOS:
+	$ %[1]s completion bash > $(brew --prefix)/etc/bash_completion.d/%[1]s
+
+Zsh:
+
+	# If shell completion is not already enabled in your environment,
+	# you will need to enable it.  You can execute the following once:
+
+	$ echo "autoload -U compinit; compinit" >> ~/.zshrc
+
+	# To load completions for each session, execute once:
+	$ %[1]s completion zsh > "${fpath[1]}/_%[1]s"
+
+	# You will need to start a new shell for this setup to take effect.
+
+fish:
+
+	$ %[1]s completion fish | source
+
+	# To load completions for each session, execute once:
+	$ %[1]s completion fish > ~/.config/fish/completions/%[1]s.fish
+
+PowerShell:
+
+	PS> %[1]s completion powershell | Out-String | Invoke-Expression
+
+	# To load completions for every new session, run:
+	PS> %[1]s completion powershell > %[1]s.ps1
+	# and source this file from your PowerShell profile.
+
+The shell scripts and this help message have been adapted from the
+Cobra project (https://cobra.dev, https://github.com/spf13/cobra)
+under the Apache-2.0 license. Thank you for making these available.
+`