Allow enabling gitlab-shell "discover"-feature

This adds the possibility to enable features for GitLab shell.

The first feature being recognized is "Discover": It's the command
that is executed when running `ssh git@gitlab.example.com` and is
called without a command.

The gitlab key id or username is already parsed from the command line
arguments.

Currently we only support communicating with GitLab-rails using unix
sockets. So features will not be enabled if the GitLab-url is using a
different protocol. The url for this read from the config yaml.

Pending ruby-specs have been added for the gitlab-shell command.

Refactor to have separate command packages

6 files changed, 297 insertions, 0 deletions

diff --git a/go/internal/command/command.go b/go/internal/command/command.go
new file mode 100644
index 0000000..cb2acdc
--- /dev/null
+++ b/go/internal/command/command.go
@@ -0,0 +1,35 @@
+package command
+
+import (
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/commandargs"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/discover"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/fallback"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/config"
+)
+
+type Command interface {
+	Execute() error
+}
+
+func New(arguments []string, config *config.Config) (Command, error) {
+	args, err := commandargs.Parse(arguments)
+
+	if err != nil {
+		return nil, err
+	}
+
+	if config.FeatureEnabled(string(args.CommandType)) {
+		return buildCommand(args, config), nil
+	}
+
+	return &fallback.Command{}, nil
+}
+
+func buildCommand(args *commandargs.CommandArgs, config *config.Config) Command {
+	switch args.CommandType {
+	case commandargs.Discover:
+		return &discover.Command{Config: config, Args: args}
+	}
+
+	return nil
+}
diff --git a/go/internal/command/command_test.go b/go/internal/command/command_test.go
new file mode 100644
index 0000000..02fc0d0
--- /dev/null
+++ b/go/internal/command/command_test.go
@@ -0,0 +1,71 @@
+package command
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/discover"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/fallback"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/config"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/testhelper"
+)
+
+func TestNew(t *testing.T) {
+	testCases := []struct {
+		desc         string
+		arguments    []string
+		config       *config.Config
+		environment  map[string]string
+		expectedType interface{}
+	}{
+		{
+			desc:      "it returns a Discover command if the feature is enabled",
+			arguments: []string{},
+			config: &config.Config{
+				GitlabUrl: "http+unix://gitlab.socket",
+				Migration: config.MigrationConfig{Enabled: true, Features: []string{"discover"}},
+			},
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "",
+			},
+			expectedType: &discover.Command{},
+		},
+		{
+			desc:      "it returns a Fallback command no feature is enabled",
+			arguments: []string{},
+			config: &config.Config{
+				GitlabUrl: "http+unix://gitlab.socket",
+				Migration: config.MigrationConfig{Enabled: false},
+			},
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "",
+			},
+			expectedType: &fallback.Command{},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			restoreEnv := testhelper.TempEnv(tc.environment)
+			defer restoreEnv()
+
+			command, err := New(tc.arguments, tc.config)
+
+			assert.NoError(t, err)
+			assert.IsType(t, tc.expectedType, command)
+		})
+	}
+}
+
+func TestFailingNew(t *testing.T) {
+	t.Run("It returns an error when SSH_CONNECTION is not set", func(t *testing.T) {
+		restoreEnv := testhelper.TempEnv(map[string]string{})
+		defer restoreEnv()
+
+		_, err := New([]string{}, &config.Config{})
+
+		assert.Error(t, err, "Only ssh allowed")
+	})
+}
diff --git a/go/internal/command/commandargs/command_args.go b/go/internal/command/commandargs/command_args.go
new file mode 100644
index 0000000..7bc13b6
--- /dev/null
+++ b/go/internal/command/commandargs/command_args.go
@@ -0,0 +1,82 @@
+package commandargs
+
+import (
+	"errors"
+	"os"
+	"regexp"
+)
+
+type CommandType string
+
+const (
+	Discover CommandType = "discover"
+)
+
+var (
+	whoKeyRegex      = regexp.MustCompile(`\bkey-(?P<keyid>\d+)\b`)
+	whoUsernameRegex = regexp.MustCompile(`\busername-(?P<username>\S+)\b`)
+)
+
+type CommandArgs struct {
+	GitlabUsername string
+	GitlabKeyId    string
+	SshCommand     string
+	CommandType    CommandType
+}
+
+func Parse(arguments []string) (*CommandArgs, error) {
+	if sshConnection := os.Getenv("SSH_CONNECTION"); sshConnection == "" {
+		return nil, errors.New("Only ssh allowed")
+	}
+
+	info := &CommandArgs{}
+
+	info.parseWho(arguments)
+	info.parseCommand(os.Getenv("SSH_ORIGINAL_COMMAND"))
+
+	return info, nil
+}
+
+func (info *CommandArgs) parseWho(arguments []string) {
+	for _, argument := range arguments {
+		if keyId := tryParseKeyId(argument); keyId != "" {
+			info.GitlabKeyId = keyId
+			break
+		}
+
+		if username := tryParseUsername(argument); username != "" {
+			info.GitlabUsername = username
+			break
+		}
+	}
+}
+
+func tryParseKeyId(argument string) string {
+	matchInfo := whoKeyRegex.FindStringSubmatch(argument)
+	if len(matchInfo) == 2 {
+		// The first element is the full matched string
+		// The second element is the named `keyid`
+		return matchInfo[1]
+	}
+
+	return ""
+}
+
+func tryParseUsername(argument string) string {
+	matchInfo := whoUsernameRegex.FindStringSubmatch(argument)
+	if len(matchInfo) == 2 {
+		// The first element is the full matched string
+		// The second element is the named `username`
+		return matchInfo[1]
+	}
+
+	return ""
+}
+
+func (c *CommandArgs) parseCommand(commandString string) {
+	c.SshCommand = commandString
+
+	if commandString == "" {
+		c.CommandType = Discover
+	}
+}
diff --git a/go/internal/command/commandargs/command_args_test.go b/go/internal/command/commandargs/command_args_test.go
new file mode 100644
index 0000000..10c46fe
--- /dev/null
+++ b/go/internal/command/commandargs/command_args_test.go
@@ -0,0 +1,73 @@
+package commandargs
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/testhelper"
+)
+
+func TestParseSuccess(t *testing.T) {
+	testCases := []struct {
+		desc         string
+		arguments    []string
+		environment  map[string]string
+		expectedArgs *CommandArgs
+	}{
+		// Setting the used env variables for every case to ensure we're
+		// not using anything set in the original env.
+		{
+			desc: "It sets discover as the command when the command string was empty",
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "",
+			},
+			expectedArgs: &CommandArgs{CommandType: Discover},
+		},
+		{
+			desc: "It passes on the original ssh command from the environment",
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "hello world",
+			},
+			expectedArgs: &CommandArgs{SshCommand: "hello world"},
+		}, {
+			desc: "It finds the key id in any passed arguments",
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "",
+			},
+			arguments:    []string{"hello", "key-123"},
+			expectedArgs: &CommandArgs{CommandType: Discover, GitlabKeyId: "123"},
+		}, {
+			desc: "It finds the username in any passed arguments",
+			environment: map[string]string{
+				"SSH_CONNECTION":       "1",
+				"SSH_ORIGINAL_COMMAND": "",
+			},
+			arguments:    []string{"hello", "username-jane-doe"},
+			expectedArgs: &CommandArgs{CommandType: Discover, GitlabUsername: "jane-doe"},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			restoreEnv := testhelper.TempEnv(tc.environment)
+			defer restoreEnv()
+
+			result, err := Parse(tc.arguments)
+
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expectedArgs, result)
+		})
+	}
+}
+
+func TestParseFailure(t *testing.T) {
+	t.Run("It fails if SSH connection is not set", func(t *testing.T) {
+		_, err := Parse([]string{})
+
+		assert.Error(t, err, "Only ssh allowed")
+	})
+
+}
diff --git a/go/internal/command/discover/discover.go b/go/internal/command/discover/discover.go
new file mode 100644
index 0000000..63a7a32
--- /dev/null
+++ b/go/internal/command/discover/discover.go
@@ -0,0 +1,17 @@
+package discover
+
+import (
+	"fmt"
+
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/command/commandargs"
+	"gitlab.com/gitlab-org/gitlab-shell/go/internal/config"
+)
+
+type Command struct {
+	Config *config.Config
+	Args   *commandargs.CommandArgs
+}
+
+func (c *Command) Execute() error {
+	return fmt.Errorf("No feature is implemented yet")
+}
diff --git a/go/internal/command/fallback/fallback.go b/go/internal/command/fallback/fallback.go
new file mode 100644
index 0000000..a136657
--- /dev/null
+++ b/go/internal/command/fallback/fallback.go
@@ -0,0 +1,19 @@
+package fallback
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+type Command struct{}
+
+var (
+	binDir = filepath.Dir(os.Args[0])
+)
+
+func (c *Command) Execute() error {
+	rubyCmd := filepath.Join(binDir, "gitlab-shell-ruby")
+	execErr := syscall.Exec(rubyCmd, os.Args, os.Environ())
+	return execErr
+}