compact: run fstrim by its path; WSL 3.0's --exec does not find /sbin tools

CHANGELOG.md

@@ -14,6 +14,10 @@ useful than saying where the real one is.
 
 ### Fixed
 
+- **`skrog compact` no longer fails at `fstrim` on WSL 3.0** ([#560](https://github.com/wslkit/skrog/issues/560)).
+  WSL 3.0's `wsl --exec` does not find tools in `/sbin` by name, so the first step failed
+  with `execvpe(fstrim) failed`. It now runs `/sbin/fstrim`. On WSL 3.0.3 compaction still
+  fails at the next step, because the disk is not released; #560 tracks that.
 - **`wsl.auto-memory-reclaim` now takes effect** ([#550](https://github.com/wslkit/skrog/issues/550)).
   `skrog wsl-config apply` wrote `autoMemoryReclaim` under `[wsl2]`, but WSL reads it only
   as `experimental.autoMemoryReclaim`: it ignored the line and printed `Unknown key

internal/compact/compact.go

@@ -195,7 +195,9 @@ func (r *Runner) Run(ctx context.Context, opts Options) (Report, error) {
 	// 1. Trim: the guest tells the disk what it no longer uses. Without this,
 	//    compaction has nothing to reclaim.
 	if !opts.NoTrim {
-		out, err := r.WSL.Exec(ctx, opts.Distro, "root", "fstrim", "-v", "/")
+		// By path: WSL 3.0's `wsl --exec` does not find /sbin tools by name
+		// (#560). The rootfs is ours, so the path is fixed.
+		out, err := r.WSL.Exec(ctx, opts.Distro, "root", "/sbin/fstrim", "-v", "/")
 		if err != nil {
 			return rep, fmt.Errorf("compact: fstrim in %s: %w: %s", opts.Distro, err, strings.TrimSpace(out))
 		}

internal/compact/compact_test.go

@@ -23,6 +23,11 @@ type fakeWSL struct {
 
 func (f *fakeWSL) Exec(_ context.Context, distro, user string, args ...string) (string, error) {
 	f.calls = append(f.calls, "exec:"+distro+":"+strings.Join(args, " "))
+	// WSL 3.0's relay does not search /sbin for `wsl --exec`, though /sbin is
+	// on the PATH the process gets (#560).
+	if args[0] == "fstrim" {
+		return "execvpe(fstrim) failed: No such file or directory", errors.New("exit status 1")
+	}
 	return f.fstrimOut, f.fstrimErr
 }
 
@@ -94,7 +99,7 @@ func TestHappyPathOrdersTrimStopTerminateCompact(t *testing.T) {
 	if !started {
 		t.Error("--restart did not start the engine again")
 	}
-	if got := strings.Join(w.calls, "|"); !strings.Contains(got, "exec:skrog-engine:fstrim -v /") ||
+	if got := strings.Join(w.calls, "|"); !strings.Contains(got, "exec:skrog-engine:/sbin/fstrim -v /") ||
 		!strings.Contains(got, "terminate:skrog-engine") {
 		t.Errorf("calls = %s", got)
 	}