Gracefully handle absence of state

src/Terraform/HttpBackend/Pass/Api.hs

 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeOperators #-}
 
 module Terraform.HttpBackend.Pass.Api where
 
+import Control.Monad (unless)
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
 import GHC.Generics (Generic)
@@ -14,11 +16,11 @@ import Terraform.HttpBackend.Pass.App (AppT)
 import Terraform.HttpBackend.Pass.Crypt (MonadPass (..))
 import Terraform.HttpBackend.Pass.Git (MonadGit (..))
 
-type GetState = "state" :> Capture "name" Text :> Get '[PlainText] Text
+type GetState = "state" :> Capture "name" Text :> UVerb 'GET '[PlainText] [WithStatus 200 Text, WithStatus 404 Text]
 
 type UpdateState = "state" :> Capture "name" Text :> ReqBody '[PlainText] Text :> PostNoContent
 
-type DeleteState = "state" :> Capture "name" Text :> Delete '[PlainText] Text
+type DeleteState = "state" :> Capture "name" Text :> DeleteNoContent
 
 type Api = GetState :<|> UpdateState :<|> DeleteState
 
@@ -31,11 +33,14 @@ server =
     :<|> updateStateImpl
     :<|> purgeStateImpl
 
--- TODO: Gracefully return 404 when the file doesn't exist
-getStateImpl :: (Monad m, MonadGit m, MonadPass m) => Text -> m Text
+getStateImpl :: (Monad m, MonadGit m, MonadPass m) => Text -> m (Union '[WithStatus 200 Text, WithStatus 404 Text])
 getStateImpl name = do
   gitPull
-  decrypt (name <> "/terraform.tfstate")
+  let path = stateFilePath name
+  stateExists <- exists path
+  if stateExists
+    then respond =<< (WithStatus @200 <$> decrypt path)
+    else respond (WithStatus @404 ("Not found!" :: Text))
 
 updateStateImpl :: (Monad m, MonadPass m, MonadGit m) => Text -> Text -> m NoContent
 updateStateImpl name tfstate = do
@@ -47,12 +52,15 @@ updateStateImpl name tfstate = do
   gitPush
   pure NoContent
 
-purgeStateImpl :: (MonadGit m, MonadPass m, Monad m) => Text -> m Text
+purgeStateImpl :: (MonadGit m, MonadPass m, Monad m) => Text -> m NoContent
 purgeStateImpl name = do
-  tfstate <- getStateImpl name
-  purge $ stateFilePath name
-  gitPush
-  pure tfstate
+  gitPull
+  let path = stateFilePath name
+  stateExists <- exists path
+  unless stateExists $ do
+    purge (stateFilePath name)
+    gitPush
+  pure NoContent
 
 stateFilePath :: Text -> Text
 stateFilePath name = name <> "/terraform.tfstate"

src/Terraform/HttpBackend/Pass/App.hs

@@ -6,12 +6,13 @@
 
 module Terraform.HttpBackend.Pass.App where
 
-import Control.Monad.IO.Class (MonadIO)
+import Control.Monad.IO.Class (MonadIO, liftIO)
 import Control.Monad.Reader (MonadReader (ask), ReaderT (runReaderT))
 import Data.Text (Text)
 import qualified Data.Text as Text
 import Shelly (shelly)
 import qualified Shelly
+import System.Directory (doesFileExist)
 import Terraform.HttpBackend.Pass.Crypt (MonadPass (..))
 import Terraform.HttpBackend.Pass.Env (Env (..))
 import Terraform.HttpBackend.Pass.Git (MonadGit (..))
@@ -48,6 +49,9 @@ instance (Monad m, MonadIO m) => MonadPass (AppT m) where
     shelly $ do
       Shelly.setenv "PASSWORD_STORE_DIR" (Text.pack directory)
       Shelly.run_ "pass" ["rm", name]
+  exists name = do
+    Env {..} <- ask
+    liftIO $ doesFileExist (directory <> "/" <> Text.unpack name)
 
 runAppT :: Env -> AppT m a -> m a
 runAppT env (AppT r) = runReaderT r env

src/Terraform/HttpBackend/Pass/Crypt.hs

@@ -6,3 +6,4 @@ class MonadPass m where
   encrypt :: Text -> Text -> m ()
   decrypt :: Text -> m Text
   purge :: Text -> m ()
+  exists :: Text -> m Bool

terraform-http-backend-pass.cabal

@@ -20,6 +20,7 @@ library
   extra-libraries:     z
   build-depends:       base >= 4.14 && <5
                      , bytestring
+                     , directory
                      , mtl
                      , optparse-applicative
                      , optparse-generic