Fix labs panic on empty release list and make update check best-effort (#5503)

## Why

Found during a full-repo review of the CLI. Two related problems in the
`databricks labs` update check that runs before every command of an
installed project:

1. The CLI panics with an index-out-of-range error when the release list
is empty. This happens for repositories with zero GitHub releases, and
also offline: the cache falls back to an empty list when the network is
unreachable.
2. The check is fail-closed. It exists only to print an upgrade hint,
yet any error aborts the command. The unauthenticated GitHub API allows
only 60 requests per hour per IP, so a 403 from rate limiting blocks
every `databricks labs <project> ...` invocation until the quota resets.

## Changes

Before, an empty release list crashed the CLI and any update-check
failure aborted the command; now the upgrade hint is skipped and the
command runs normally in both cases.

- `cmd/labs/project/project.go`: `checkUpdates` returns early when the
loaded release list is empty instead of indexing into it.
- `cmd/labs/project/proxy.go`: a failing update check is logged at debug
level and the command proceeds, instead of returning the error.

## Test plan

- [x] New unit test `TestCheckUpdatesWithEmptyReleaseList`: panics with
"index out of range [0] with length 0" before the fix, passes after.
- [x] New test `TestRunningCommandWhenUpdateCheckFails`: runs `labs
blueprint echo` with the GitHub API returning HTTP 500; fails with
"github request failed: 500 Internal Server Error" before the fix,
passes after.
- [x] `go test ./cmd/labs/...` passes.
- [x] `./task fmt-q`, `./task lint-q`, and `./task checks` pass.

This pull request and its description were written by Isaac.

Author: simonfaltum

cmd/labs/project/command_test.go

@@ -2,15 +2,20 @@ package project_test
 
 import (
 	"context"
+	"net/http"
+	"net/http/httptest"
+	"os"
 	"path/filepath"
 	"testing"
 	"time"
 
+	"github.com/databricks/cli/cmd/labs/github"
 	"github.com/databricks/cli/internal/testcli"
 	"github.com/databricks/cli/libs/env"
 	"github.com/databricks/cli/libs/python"
 	"github.com/databricks/databricks-sdk-go"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type echoOut struct {
@@ -67,3 +72,26 @@ func TestRenderingTable(t *testing.T) {
 	Third  Fourth
 	`)
 }
+
+func TestRunningCommandWhenUpdateCheckFails(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer server.Close()
+
+	home := copyTestdata(t, "testdata/installed-in-home")
+	// Drop the cached releases so the update check has to hit the API.
+	err := os.Remove(filepath.Join(home, ".databricks", "labs", "blueprint", "cache", "databrickslabs-blueprint-releases.json"))
+	require.NoError(t, err)
+
+	ctx := github.WithApiOverride(t.Context(), server.URL)
+	ctx = env.WithUserHomeDir(ctx, home)
+	py, _ := python.DetectExecutable(ctx)
+	py, _ = filepath.Abs(py)
+	ctx = env.Set(ctx, "PYTHON_BIN", py)
+
+	r := testcli.NewRunner(t, ctx, "labs", "blueprint", "echo")
+	var out echoOut
+	r.RunAndParseJSON(&out)
+	assert.Equal(t, "echo", out.Command)
+}

cmd/labs/project/project.go

@@ -308,6 +308,10 @@ func (p *Project) checkUpdates(cmd *cobra.Command) error {
 	if err != nil {
 		return err
 	}
+	// Repositories without releases (and the offline fallback) yield no versions; nothing to advise on.
+	if len(versions) == 0 {
+		return nil
+	}
 	installed, err := p.InstalledVersion(ctx)
 	if err != nil {
 		return err

cmd/labs/project/project_test.go

@@ -2,10 +2,14 @@ package project
 
 import (
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
+	"github.com/databricks/cli/libs/env"
+	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func assertEqualPaths(t *testing.T, expected, actual string) {
@@ -19,3 +23,21 @@ func TestLoad(t *testing.T) {
 	assert.NoError(t, err)
 	assertEqualPaths(t, "testdata/installed-in-home/.databricks/labs/blueprint/lib", prj.folder)
 }
+
+func TestCheckUpdatesWithEmptyReleaseList(t *testing.T) {
+	ctx := env.WithUserHomeDir(t.Context(), t.TempDir())
+	rootDir, err := PathInLabs(ctx, "blueprint")
+	require.NoError(t, err)
+	prj := &Project{Name: "blueprint", rootDir: rootDir}
+	require.NoError(t, prj.EnsureFoldersExist())
+
+	// The far-future refreshed_at keeps the cache valid so no network call is made.
+	cache := []byte(`{"refreshed_at": "2033-01-01T00:00:00Z", "data": []}`)
+	require.NoError(t, os.WriteFile(filepath.Join(prj.CacheDir(), "databrickslabs-blueprint-releases.json"), cache, ownerRW))
+	version := []byte(`{"version": "v0.3.15", "date": "2023-10-24T15:04:05+01:00"}`)
+	require.NoError(t, os.WriteFile(filepath.Join(prj.StateDir(), "version.json"), version, ownerRW))
+
+	cmd := &cobra.Command{}
+	cmd.SetContext(ctx)
+	assert.NoError(t, prj.checkUpdates(cmd))
+}

cmd/labs/project/proxy.go

@@ -38,9 +38,9 @@ func (cp *proxy) register(parent *cobra.Command) {
 }
 
 func (cp *proxy) runE(cmd *cobra.Command, _ []string) error {
-	err := cp.checkUpdates(cmd)
-	if err != nil {
-		return err
+	// The update check only prints an upgrade hint, so a failure must not block the command.
+	if err := cp.checkUpdates(cmd); err != nil {
+		log.Debugf(cmd.Context(), "Failed to check for newer release of %s: %s", cp.Project.Name, err)
 	}
 	args, err := cp.commandInput(cmd)
 	if err != nil {