cli: make sure max workers workers are set to at least 2 by default

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/chatty-owls-care.md

@@ -2,4 +2,4 @@
 '@backstage/cli': patch
 ---
 
-The `backstage-cli repo test` command now sets a default Jest `--workerIdleMemoryLimit` of 1GB. This is the recommended workaround for dealing with Jest workers leaking memory and eventually hitting the heap limit.
+The `backstage-cli repo test` command now sets a default Jest `--workerIdleMemoryLimit` of 1GB. If needed to ensure that tests are not run in band, `--maxWorkers=2` is set as well. This is the recommended workaround for dealing with Jest workers leaking memory and eventually hitting the heap limit.

packages/cli/src/commands/repo/test.ts

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import os from 'os';
 import { Command, OptionValues } from 'commander';
 import { PackageGraph } from '../../lib/monorepo';
 import { paths } from '../../lib/paths';
@@ -91,6 +92,19 @@ export async function command(opts: OptionValues, cmd: Command): Promise<void> {
     args.push('--workerIdleMemoryLimit=1000M');
   }
 
+  // In order for the above worker memory limit to work we need to make sure the worker
+  // count is set to at least 2, as the tests will otherwise run in-band.
+  // Depending on the mode tests are run with the default count is either cpus-1, or cpus/2.
+  // This means that if we've got at 4 or more cores we'll always get at least 2 workers, but
+  // otherwise we need to set the worker count explicitly unless already done.
+  if (
+    os.cpus().length <= 3 &&
+    !includesAnyOf(args, '-i', '--runInBand') &&
+    !args.some(arg => arg.match(/^(--maxWorkers|-w)/))
+  ) {
+    args.push('--maxWorkers=2');
+  }
+
   if (opts.since) {
     removeOptionArg(args, '--since');
   }