await SSH availability, resolving a race condition

Signed-off-by: Kiara Grouwstra <kiara@procolix.eu>

deployment/run/tf-proxmox/main.tf

@@ -187,8 +187,30 @@ resource "proxmox_virtual_environment_vm" "nix_vm" {
   # }
 }
 
+resource "null_resource" "wait_for_ssh" {
+  depends_on = [
+    proxmox_virtual_environment_vm.nix_vm
+  ]
+  provisioner "local-exec" {
+    command = <<-EOT
+      for i in $(seq 1 30); do
+        if ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 root@${proxmox_virtual_environment_vm.nix_vm.ipv4_addresses[1][0]} "true" 2>/dev/null; then
+          exit 0
+        fi
+        echo "Waiting for SSH (attempt #$i)..."
+        sleep 5
+      done
+      echo "SSH never came up!" >&2
+      exit 1
+    EOT
+  }
+}
+
 # FIXME expose (and handle thru) [`exec`](https://pve.proxmox.com/pve-docs/api-viewer/#/nodes/{node}/qemu/{vmid}/agent/exec) endpoint in proxmox TF provider? wait, what command would i use it for?: https://github.com/bpg/terraform-provider-proxmox/issues/1576
 module "nixos-rebuild" {
+  depends_on = [
+    null_resource.wait_for_ssh
+  ]
   source = "../tf-single-host"
   nixos_conf = var.nixos_conf
   # username = var.ssh_user # refers to the proxmox ssh user, not the VM one

deployment/run/tf-proxmox/tf.nix

@@ -35,6 +35,7 @@ in
 ).withPlugins
   (p: [
     p.external
+    p.null
     (mkProvider {
       owner = "bpg";
       repo = "terraform-provider-proxmox";