Fix deployer checking stale state instead of actual running services

The deployer compared its in-memory stateServices map to decide if a
service needed restarting. When the deployer restarted, this state was
lost, causing it to think services were 'already at' the desired path
when they were actually running old code.

Changes:
- Add getRunningStorePath to Systemd module to read actual store path
- Update deployService to query systemd instead of stale in-memory state
- Add DerivingStrategies extension to Deployer.hs

1 files changed, 23 insertions, 0 deletions

diff --git a/Omni/Deploy/Systemd.hs b/Omni/Deploy/Systemd.hs
index d7af1cd..7b64d1f 100644
--- a/Omni/Deploy/Systemd.hs
+++ b/Omni/Deploy/Systemd.hs
@@ -13,6 +13,7 @@ module Omni.Deploy.Systemd
     reloadAndRestart,
     stopAndDisable,
     removeUnit,
+    getRunningStorePath,
     servicesDir,
     main,
     test,
@@ -141,6 +142,28 @@ removeUnit baseDir sysDir serviceName' = do
   _ <- Process.readProcessWithExitCode "systemctl" ["daemon-reload"] ""
   pure ()
 
+-- | Get the store path of the currently running service by reading its unit file.
+getRunningStorePath :: Text -> IO (Maybe Text)
+getRunningStorePath serviceName' = do
+  let unitPath = servicesDir </> Text.unpack serviceName' <> ".service"
+  exists <- Dir.doesFileExist unitPath
+  if not exists
+    then pure Nothing
+    else do
+      content <- Text.IO.readFile unitPath
+      pure <| extractStorePath content
+  where
+    -- Extract /nix/store/...-service-name from ExecStart=/nix/store/.../bin/...
+    extractStorePath content =
+      content
+        |> Text.lines
+        |> find (Text.isPrefixOf "ExecStart=")
+        |> fmap (Text.drop (Text.length "ExecStart="))
+        |> fmap (Text.dropWhile (/= '/'))
+        |> fmap (Text.drop 1)
+        |> fmap (Text.takeWhile (/= '/'))
+        |> fmap ("/nix/store/" <>)
+
 test :: Test.Tree
 test =
   Test.group