Fix waitForProcess errors when configuration is recompiled

Previously, xmobar would ignore SIGCHLD, but only when the configuration
is recompiled. This means child processes would not leave zombies, so
that waiting for them to exit (either directly by calling
waitForProcess, or indirectly through another function from
System.Process, like system or readProcessWithExitCode) would produce an
error. This breaks the Alsa, Com (#657) and NotmuchMail plugins, as well
as the <action> tag (#687) and low battery action (#688).

As far as I can tell, bracketing the recompilation in
uninstallSignalHandlers and installSignalHandlers was inherited from
xmonad. In xmonad, SIGPIPE and SIGCHLD are always ignored
(installSignalHandlers is called at the start of xmonad and launch), so
the bracket is necessary to be able to wait for the compiler or build
script to exit. Since xmobar does not otherwise ignore the signals, it
is not necessary to change signal handlers at all during recompilation.
Removing it leaves the default action for SIGCHLD, which fixes the
issues described above.

Fixes #657, #687 and #688.

2 files changed, 4 insertions, 25 deletions

diff --git a/changelog.md b/changelog.md
index 6410188..e7893e9 100644
--- a/changelog.md
+++ b/changelog.md
@@ -2,6 +2,8 @@
 
 - The `Kbd` monitor is not clickable (thanks, Enrico Maria)
 - Fix zombie processes left by `<action>` tag and low battery action
+- Fix plugins such as `Alsa` and `Com` not working when configuration is
+  recompiled (#657)
 
 ## Version 0.47.4 (March, 2024)
 
diff --git a/src/Xmobar/App/Compile.hs b/src/Xmobar/App/Compile.hs
index 368c3e6..5d1f48d 100644
--- a/src/Xmobar/App/Compile.hs
+++ b/src/Xmobar/App/Compile.hs
@@ -20,20 +20,17 @@
 module Xmobar.App.Compile(recompile, trace, xmessage) where
 
 import Control.Monad.IO.Class
-import Control.Monad.Fix (fix)
-import Control.Exception.Extensible (try, bracket, SomeException(..))
+import Control.Exception.Extensible (bracket, SomeException(..))
 import qualified Control.Exception.Extensible as E
 import Control.Monad (filterM, when)
 import Data.List ((\\))
-import Data.Maybe (isJust)
 import System.FilePath((</>), takeExtension)
 import System.IO
 import System.Directory
 import System.Process
 import System.Exit
-import System.Posix.Process(executeFile, forkProcess, getAnyProcessStatus)
+import System.Posix.Process(executeFile, forkProcess)
 import System.Posix.Types(ProcessID)
-import System.Posix.Signals
 
 isExecutable :: FilePath -> IO Bool
 isExecutable f =
@@ -144,14 +141,12 @@ recompile confDir dataDir execName force verb = liftIO $ do
           else shouldRecompile verb src bin lib
     if sc
       then do
-        uninstallSignalHandlers
         status <- bracket (openFile err WriteMode) hClose $
                     \errHandle ->
                       waitForProcess =<<
                         if useScript
                         then runScript script bin confDir errHandle
                         else runGHC bin confDir errHandle
-        installSignalHandlers
         if status == ExitSuccess
             then trace verb "Xmobar recompilation process exited with success!"
             else do
@@ -174,21 +169,3 @@ recompile confDir dataDir execName force verb = liftIO $ do
                   ++ ["-o", bin]
        runGHC bin = runProc "ghc" (opts bin)
        runScript script bin = runProc script [bin]
-
--- | Ignore SIGPIPE to avoid termination when a pipe is full, and SIGCHLD to
--- avoid zombie processes, and clean up any extant zombie processes.
-installSignalHandlers :: MonadIO m => m ()
-installSignalHandlers = liftIO $ do
-    installHandler openEndedPipe Ignore Nothing
-    installHandler sigCHLD Ignore Nothing
-    (try :: IO a -> IO (Either SomeException a))
-      $ fix $ \more -> do
-        x <- getAnyProcessStatus False False
-        when (isJust x) more
-    return ()
-
-uninstallSignalHandlers :: MonadIO m => m ()
-uninstallSignalHandlers = liftIO $ do
-    installHandler openEndedPipe Default Nothing
-    installHandler sigCHLD Default Nothing
-    return ()