Merge fort-nix/nix-bitcoin#587: Fulcrum: Fix available memory detection

86dc7e2669 fulcrum: allow access to `/proc/meminfo` (Erik Arvstedt)
c948af2e18 dev/dev-features: add `enter_service` helper (Erik Arvstedt)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 86dc7e2669

Tree-SHA512: 5c2b7bc5e2247a7fb45e6c805162c02d87b4c917e4a1306134d634f418534b03e3152e402d17e054c410d3d72f3f5eb3d270fcb53019b2f96ea6b27ecae53755

dev/dev-features.sh

@@ -53,6 +53,15 @@ ls -al /var/lib/nixos-containers/nb-test
 # The container root filesystem on NixOS systems with stateVersion < 22.05
 ls -al /var/lib/containers/nb-test
 
+# Start a shell in the context of a service process.
+# Must be run inside the container (enter with cmd `c`).
+enter_service() {
+    local name=$1
+    nsenter --all -t "$(systemctl show -p MainPID --value "$name")" \
+      --setuid "$(id -u "$name")" --setgid "$(id -g "$name")" bash
+}
+enter_service clightning
+
 #―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――
 # bitcoind
 run-tests.sh -s bitcoind container

modules/fulcrum.nix

@@ -126,6 +126,7 @@ in {
         Restart = "on-failure";
         RestartSec = "10s";
         ReadWritePaths = cfg.dataDir;
+        ProcSubset = "all"; # Fulcrum requires read access to /proc/meminfo
       } // nbLib.allowedIPAddresses cfg.tor.enforce;
     };