services: use wants dependency where possible

Let A be a service that depends on another service B.
When A can gracefully handle failures and restarts of B, use
```
wants = [ "B.service" ];
after = [ "B.service" ];
```
instead of
```
requires = [ "B.service" ];
after = [ "B.service" ];
```
in the definition of A.

This way, A keeps running when B is stopped or restarted after a failure.
With `requires`, A is instead stopped when B is stopped or restarted due to a failure.

This brings two benefits:

1. Improved uptime
Examples:
- RTL keeps running when one lightning node has failed
- btcpayserver keeps running and accepting on-chain payments when the lightning node has crashed

2. Avoids a systemd bug where depending units (`A.service` in the
above example) are not restarted when their dependency fails
(issue github/systemd#18856, no full link to avoid spamming the issue).
In real world nix-bitcoin deployments, this issue was only likely to
appear when clightning failed during activation, causing depending
units (like `RTL`) to stop and not be restarted.
All services depending on `clightning` have now been changed to use
`wants`, thereby avoiding the bug.

Services `electrs` and `lightning-loop` fail when their respective
dependencies stop, so these services have not been changed.
I also haven't changed services `joinmarket` and
`joinmarket-yieldgenerator`. Further manual testing is needed to
determine if they can be switched to `wants`.

modules/btcpayserver.nix

@@ -171,8 +171,9 @@ in {
       '';
     in rec {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "bitcoind.service" "postgresql.service" ] ++ optional cfg.btcpayserver.lbtc "liquidd.service";
+      requires = [ "postgresql.service" ];
-      after = requires ++ [ "nix-bitcoin-secrets.target" ];
+      wants = [ "bitcoind.service" ] ++ optional cfg.btcpayserver.lbtc "liquidd.service";
+      after = requires ++ wants ++ [ "nix-bitcoin-secrets.target" ];
       preStart = ''
         install -m 600 ${configFile} '${cfg.nbxplorer.dataDir}/settings.config'
         {
@@ -223,11 +224,12 @@ in {
         lbtcexplorerurl=${nbExplorerUrl}
         lbtcexplorercookiefile=${nbExplorerCookie}
       '');
-    in let self = {
+    in rec {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "nbxplorer.service" "postgresql.service" ]
+      requires = [ "postgresql.service" ];
+      wants = [ "nbxplorer.service" ]
               ++ optional (cfg.btcpayserver.lightningBackend != null) "${cfg.btcpayserver.lightningBackend}.service";
-      after = self.requires;
+      after = requires ++ wants;
       serviceConfig = nbLib.defaultHardening // {
         ExecStart = ''
           ${cfg.btcpayserver.package}/bin/btcpayserver --conf=${configFile} \
@@ -244,7 +246,7 @@ in {
       } // nbLib.allowedIPAddresses cfg.btcpayserver.tor.enforce;
       startLimitIntervalSec = 30;
       startLimitBurst = 10;
-    }; in self;
+    };
 
     users.users.${cfg.nbxplorer.user} = {
       isSystemUser = true;

modules/clightning-rest.nix

@@ -75,7 +75,7 @@ in {
 
     systemd.services.clightning-rest = mkIf cfg.enable {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "clightning.service" ];
+      wants = [ "clightning.service" ];
       after = [ "clightning.service" ];
       path = [ pkgs.openssl ];
       environment.CL_REST_STATE_DIR = cfg.dataDir;

modules/fulcrum.nix

@@ -111,7 +111,7 @@ in {
 
     systemd.services.fulcrum = {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "bitcoind.service" ];
+      wants = [ "bitcoind.service" ];
       after = [ "bitcoind.service" "nix-bitcoin-secrets.target" ];
       preStart = ''
         {

modules/liquid.nix

@@ -255,7 +255,7 @@ in {
     ];
 
     systemd.services.liquidd = {
-      requires = [ "bitcoind.service" ];
+      wants = [ "bitcoind.service" ];
       after = [ "bitcoind.service" "nix-bitcoin-secrets.target" ];
       wantedBy = [ "multi-user.target" ];
       preStart = ''

modules/mempool.nix

@@ -275,10 +275,11 @@ in {
       };
     };
 
-    systemd.services.mempool = {
+    systemd.services.mempool = rec {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "${cfg.electrumServer}.service" ];
+      requires = [ "mysql.service" ];
-      after = [ "${cfg.electrumServer}.service" "mysql.service" ];
+      wants = [ "${cfg.electrumServer}.service" ];
+      after = requires ++ wants;
       preStart = ''
         mkdir -p '${cacheDir}/cache'
         <${configFile} sed \

modules/rtl.nix

@@ -190,9 +190,9 @@ in {
 
     systemd.services.rtl = rec {
       wantedBy = [ "multi-user.target" ];
-      requires = optional cfg.nodes.clightning.enable "clightning.service" ++
+      wants = optional cfg.nodes.clightning.enable "clightning.service" ++
               optional cfg.nodes.lnd.enable "lnd.service";
-      after = requires ++ [ "nix-bitcoin-secrets.target" ];
+      after = wants ++ [ "nix-bitcoin-secrets.target" ];
       environment.RTL_CONFIG_PATH = cfg.dataDir;
       environment.DB_DIRECTORY_PATH = cfg.dataDir;
       serviceConfig = nbLib.defaultHardening // {

test/tests.py

@@ -386,7 +386,6 @@ def _():
 @test("restart-bitcoind")
 def _():
     # Sanity-check system by restarting bitcoind.
-    # This also restarts all services depending on bitcoind.
     succeed("systemctl restart bitcoind")
 
 @test("regtest")