TailSpec.hs

module System.TailSpec where

import Control.Concurrent (forkIO, threadDelay)
import Control.Concurrent.STM.CircularBuffer (CircularBuffer)
import Control.Concurrent.STM.CircularBuffer qualified as CB
import GHC.IO.Handle (hClose)
import System.Directory (doesFileExist)
import System.IO.Temp (withSystemTempFile)
import System.Process (system)
import System.Tail qualified as Tail
import Test.Hspec

-- Helper to read all items from CircularBuffer until closed
drainAll :: CircularBuffer a -> IO [a]
drainAll q = do
  let go acc = do
        maybeItems <- atomically $ CB.drain q
        case maybeItems of
          Nothing -> pure acc -- Queue is closed
          Just items -> go (acc ++ toList items)
  go []

spec :: Spec
spec = describe "System.Tail" $ do
  it "hello" $ do
    t <- Tail.tailFile 100 "/dev/null"
    Tail.tailStop t
  it "streams a static file" $ do
    thisFile <- findThisFile
    t <- Tail.tailFile 100 thisFile
    q <- Tail.tailSubscribe t
    threadDelay 1_000_000 -- Wait 1 second for tail to start and read content
    Tail.tailStop t
    ls <- drainAll q
    viaNonEmpty head ls `shouldBe` Just "module System.TailSpec where"
    -- Find the last line from the lines we got
    let lastLine = viaNonEmpty last ls
    lastLine `shouldBe` Just "-- End of file."
  it "streams a log file being appended to by another process" $ do
    -- Create a file under a temp directory. Then spawn an external process that writes to it lines over time.
    withSystemTempFile "tail-spec" $ \tempFile h -> do
      hClose h
      void $ system $ "echo 'Hello' >> " <> tempFile
      t <- Tail.tailFile 100 tempFile
      q <- Tail.tailSubscribe t
      void $ forkIO $ do
        threadDelay 1_000_000
        void $ system $ "echo 'World' >> " <> tempFile
        Tail.tailStop t
      -- Read everything and compare output
      threadDelay 3_000_000
      ls <- drainAll q
      ls `shouldBe` ["Hello", "World"]
  it "ring buffer provides last lines to new subscribers" $ do
    withSystemTempFile "ring-buffer-spec" $ \tempFile h -> do
      hClose h
      void $ system $ "echo 'Line1' >> " <> tempFile
      t <- Tail.tailFile 100 tempFile
      threadDelay 500_000 -- Let tail read existing lines
      q <- Tail.tailSubscribe t
      void $ system $ "echo 'Line2' >> " <> tempFile -- Write *after* subscribing
      Tail.tailStop t
      ls <- drainAll q
      ls `shouldBe` ["Line1", "Line2"]

-- | Find this test file by trying common paths
findThisFile :: IO FilePath
findThisFile = do
  let paths =
        [ "test/System/TailSpec.hs" -- cabal test / nix build
        , "packages/tail/test/System/TailSpec.hs" -- ghcid from root
        ]
  let findFirst [] = error "TailSpec.hs not found"
      findFirst (p : ps) = do
        exists <- doesFileExist p
        if exists then pure p else findFirst ps
  findFirst paths

-- End of file.