sg/msp: improve DX by enforcing repo structure (#56841)

Previously, the command expected the full YAML path, which is a bit annoying to write. Now, we just codify the repo structure of https://github.com/sourcegraph/managed-services and allow users to just use the service ID from anywhere in the repository.

## Test plan

```sh
$ sg msp generate telemetry-gateway dev
Terraform assets generated in "services/telemetry-gateway/terraform/dev"!
$ cd services/telemetry-gateway/terraform/dev/stacks/project
$ sg msp generate telemetry-gateway dev
💡 Using repo root /Users/robert@sourcegraph.com/Projects/sourcegraph/managed-services as working directory
Terraform assets generated in "services/telemetry-gateway/terraform/dev"!
```

dev/sg/msp/repo/repo.go

@@ -0,0 +1,33 @@
+package repo
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/urfave/cli/v2"
+
+	"github.com/sourcegraph/sourcegraph/dev/sg/internal/std"
+)
+
+// UseManagedServicesRepo is a cli.BeforeFunc that enforces that we are in the
+// sourcegraph/managed-services repository by setting the current working
+// directory.
+func UseManagedServicesRepo(c *cli.Context) error {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	repoRoot, err := repositoryRoot(cwd)
+	if err != nil {
+		return err
+	}
+	if repoRoot != cwd {
+		std.Out.WriteSuggestionf("Using repo root %s as working directory", repoRoot)
+		return os.Chdir(repoRoot)
+	}
+	return nil
+}
+
+func ServiceYAMLPath(serviceID string) string {
+	return filepath.Join("services", serviceID, "service.yaml")
+}

dev/sg/msp/repo/root.go

@@ -0,0 +1,50 @@
+package repo
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+)
+
+var once sync.Once
+var repositoryRootValue string
+var repositoryRootError error
+
+var ErrNotInsideManagedServices = errors.New("not running inside sourcegraph/managed-services")
+
+// RepositoryRoot caches and returns the value of findRoot.
+func repositoryRoot(cwd string) (string, error) {
+	once.Do(func() {
+		if forcedRoot := os.Getenv("SG_MSP_FORCE_REPO_ROOT"); forcedRoot != "" {
+			repositoryRootValue = forcedRoot
+		} else {
+			repositoryRootValue, repositoryRootError = findRoot(cwd)
+		}
+	})
+	return repositoryRootValue, repositoryRootError
+}
+
+// findRoot finds the root path of sourcegraph/managed-services from wd
+func findRoot(wd string) (string, error) {
+	for {
+		contents, err := os.ReadFile(filepath.Join(wd, ".repository"))
+		if err == nil {
+			for _, line := range strings.Split(string(contents), "\n") {
+				if strings.HasPrefix(line, "sourcegraph/managed-services") {
+					return wd, nil
+				}
+			}
+		} else if !os.IsNotExist(err) {
+			return "", err
+		}
+
+		if parent := filepath.Dir(wd); parent != wd {
+			wd = parent
+			continue
+		}
+
+		return "", ErrNotInsideManagedServices
+	}
+}

dev/sg/msp/sg_msp.go

@@ -18,6 +18,7 @@ import (
 	"github.com/sourcegraph/sourcegraph/dev/managedservicesplatform/terraformcloud"
 	"github.com/sourcegraph/sourcegraph/dev/sg/internal/secrets"
 	"github.com/sourcegraph/sourcegraph/dev/sg/internal/std"
+	msprepo "github.com/sourcegraph/sourcegraph/dev/sg/msp/repo"
 	"github.com/sourcegraph/sourcegraph/dev/sg/msp/schema"
 	"github.com/sourcegraph/sourcegraph/lib/errors"
 	"github.com/sourcegraph/sourcegraph/lib/output"
@@ -42,6 +43,7 @@ func init() {
 	// Override no-op implementation with our real implementation.
 	Command.Hidden = false
 	Command.Action = nil
+	// All 'sg msp ...' subcommands
 	Command.Subcommands = []*cli.Command{
 		{
 			Name:        "init",
@@ -55,6 +57,7 @@ func init() {
 					Value:   "services",
 				},
 			},
+			Before: msprepo.UseManagedServicesRepo,
 			Action: func(c *cli.Context) error {
 				if c.Args().Len() != 1 {
 					return errors.New("exactly 1 argument required: service ID")
@@ -125,8 +128,9 @@ func init() {
 		},
 		{
 			Name:        "generate",
-			ArgsUsage:   "<service spec file> <environment ID>",
+			ArgsUsage:   "<service ID> <environment ID>",
 			Description: "Generate Terraform assets for a Managed Services Platform service spec.",
+			Before:      msprepo.UseManagedServicesRepo,
 			Flags: []cli.Flag{
 				&cli.StringFlag{
 					Name:    "output",
@@ -142,14 +146,12 @@ func init() {
 			},
 			Action: func(c *cli.Context) error {
 				if c.Args().Len() != 2 {
-					return errors.New("exactly 2 arguments required: service spec file and environment ID")
+					return errors.New("exactly 2 arguments required: service ID and environment ID")
 				}
 
 				// Load specification
-				serviceSpecPath, err := getYAMLPathArg(c, 0)
-				if err != nil {
-					return err
-				}
+				serviceSpecPath := msprepo.ServiceYAMLPath(c.Args().First())
+
 				serviceSpecData, err := os.ReadFile(serviceSpecPath)
 				if err != nil {
 					return err
@@ -205,11 +207,12 @@ func init() {
 			Name:        "terraform-cloud",
 			Aliases:     []string{"tfc"},
 			Description: "Manage Terraform Cloud workspaces for a service",
+			Before:      msprepo.UseManagedServicesRepo,
 			Subcommands: []*cli.Command{
 				{
 					Name:        "sync",
 					Description: "Create or update all required Terraform Cloud workspaces for a service",
-					ArgsUsage:   "<service spec file>",
+					ArgsUsage:   "<service ID>",
 					Flags: []cli.Flag{
 						&cli.StringFlag{
 							Name:  "workspace-run-mode",
@@ -223,10 +226,12 @@ func init() {
 						},
 					},
 					Action: func(c *cli.Context) error {
-						serviceSpecPath, err := getYAMLPathArg(c, 0)
-						if err != nil {
-							return err
+						serviceID := c.Args().First()
+						if serviceID == "" {
+							return errors.New("argument service is required")
 						}
+						serviceSpecPath := msprepo.ServiceYAMLPath(serviceID)
+
 						serviceSpecData, err := os.ReadFile(serviceSpecPath)
 						if err != nil {
 							return err
@@ -355,11 +360,3 @@ func init() {
 		},
 	}
 }
-
-func getYAMLPathArg(c *cli.Context, n int) (string, error) {
-	v := c.Args().Get(n)
-	if strings.HasSuffix(v, ".yaml") || strings.HasSuffix(v, ".yml") {
-		return v, nil
-	}
-	return v, errors.Newf("expected argument %d %q to be a path to a YAML file", n, v)
-}