tests: Avoid races when building/using the NSS library

* Lock the target directory to avoid multiple tests running in parallel
  trying to build the library at the same time (possibly with different
  set of features or other options).
* Copy the resulting library to a temporary directory only used by the
  current test, to allow other tests to reuse the target directory (so
  that they don't have to rebuild it from scratch).

Author: adombeck

internal/testutils/rust.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"github.com/ubuntu/authd/internal/fileutils"
 )
 
 func getCargoPath() (path string, isNightly bool, err error) {
@@ -76,6 +77,9 @@ func BuildRustNSSLib(t *testing.T, disableCoverage bool, features ...string) (li
 
 	// Store the build artifacts in a common temp directory, so that they can be reused between tests.
 	target := filepath.Join(os.TempDir(), "authd-tests-rust-build-artifacts")
+	// Ensure the target directory exists.
+	err = os.MkdirAll(target, 0700)
+	require.NoError(t, err, "Setup: could not create Rust target dir")
 
 	rustDir := filepath.Join(projectRoot, "nss")
 	if !disableCoverage {
@@ -84,6 +88,12 @@ func BuildRustNSSLib(t *testing.T, disableCoverage bool, features ...string) (li
 
 	features = append([]string{"integration_tests", "custom_socket"}, features...)
 
+	unlock, err := fileutils.LockDir(target)
+	require.NoError(t, err, "Setup: could not lock Rust target dir")
+	defer func() {
+		require.NoError(t, unlock(), "Setup: could not unlock Rust target dir")
+	}()
+
 	// Builds the nss library.
 	// #nosec:G204 - we control the command arguments in tests
 	cmd := exec.Command(cargo, "build", "--features", strings.Join(features, ","), "--target-dir", target)
@@ -106,9 +116,10 @@ func BuildRustNSSLib(t *testing.T, disableCoverage bool, features ...string) (li
 	// If the env is not set, the target stays the same.
 	target = filepath.Join(target, os.Getenv("DEB_HOST_RUST_TYPE"))
 
-	// Creates a symlink for the compiled library with the expected versioned name.
-	libPath = filepath.Join(target, "libnss_authd.so.2")
-	if err = os.Symlink(filepath.Join(target, "debug", "libnss_authd.so"), libPath); err != nil {
+	// Copy the library to a temporary directory, so that we can safely use it from there after unlocking
+	// the target directory, which allows other tests to rebuild the library.
+	libPath = filepath.Join(t.TempDir(), "libnss_authd.so.2")
+	if err = fileutils.CopyFile(filepath.Join(target, "debug", "libnss_authd.so"), libPath); err != nil {
 		require.ErrorIs(t, err, os.ErrExist, "Setup: failed to create versioned link to the library")
 	}