fix(freebsd): cap the VM's ZFS ARC so big test crates aren't OOM-killed (#190)

* fix(freebsd): give the VM a swap file and cap the ZFS ARC

photocraft's FreeBSD job was SIGKILLed compiling photocraft-ui-egui's lib tests late in the run (release run 42609972735, dispatch 42805955303). Add an 8 GB swap file when the guest has none, cap the ARC at 2 GB when the image uses ZFS, and print disk, swap and memory so the next OOM is diagnosable.

* fix(freebsd): only touch swap and the ARC inside a FreeBSD guest

The workflow tests run the prepare script on Linux under sh -eu, where swapctl doesn't exist. Run the ARC cap, swap file and diagnostics only when uname is FreeBSD, and don't let the diagnostics fail the step.

* fix(freebsd): add the swap file only with room to spare, and remove a partial one

Per review: a dd that runs out of space must not leave a partial /var/swap0 that then starves the build. Require 16 GB free on /var first and delete the file if any step fails.

* fix(freebsd): keep only the ZFS ARC cap; drop the swap-file fallback

Per review: swap on a ZFS-backed file can hang FreeBSD under memory pressure, and today's image already has a swap partition, so the fallback never ran. The verified run passed on the ARC cap alone.

.github/workflows/freebsd.yml

@@ -106,6 +106,19 @@ jobs:
           prepare: |
             # Keep rustup/proxies outside the Cargo data cache.
             unset CARGO_HOME CARGO_TARGET_DIR
+            # A large test crate's rustc (photocraft-ui-egui's lib tests) was SIGKILLed when the
+            # 12 GB guest ran out of memory late in a run: the image's root is ZFS with no ARC
+            # limit, so the ARC kept the target dir's pages. Cap it. (No swap file: swap on a
+            # ZFS-backed file can hang FreeBSD under memory pressure; the image has a 1 GB swap
+            # partition.) Print disk, swap and memory so the next OOM is easy to diagnose.
+            if [ "$(uname -s)" = FreeBSD ]; then
+              if sysctl -n vfs.zfs.arc_max >/dev/null 2>&1; then
+                sysctl vfs.zfs.arc_max=2147483648 || echo "::warning::could not cap the ZFS ARC"
+              fi
+              df -h / /var/tmp || true
+              swapctl -l || true
+              sysctl -n hw.physmem || true
+            fi
             if [ "${{ matrix.arch }}" = aarch64 ]; then
               # The host compiler and the aarch64 standard library come from the amd64 and
               # aarch64 lang/rust packages, which must be the same build. "latest" can build