Log information about xmonad compile + avoid unnecessary recompile

Particularly with the addition of build scripts, it can be tricky to figure out
what XMonad is doing when attempting recompilation.  This makes it clearer by
adding some logging.

Due to this logging, I noticed that the lag of xmonad start was because it was
always recompiling!  When I startup my computer, I do not want it to delay
rebuilding my window manager. This also fixes that issue such that it only
recompiles XMonad if it is going to reinvoke due to getProgName not being the
expected string.

CHANGES.md

@@ -23,6 +23,18 @@
   * Restored compatability with GHC version prior to 8.0.1 by removing the
     dependency on directory version 1.2.3.
 
+  * xmonad no longer always recompile on startup. Now it only does so if the
+    executable does not have the name that would be used for the compilation
+    output. The purpose of recompiling and executing the results in this case is
+    so that the `xmonad` executable in the package can be used with custom
+    configurations.
+
+### Enhancements
+
+  * Whenever xmonad recompiles, it now explains how it is attempting to
+    recompile, by outputting logs to stderr. If you are using xmonad as a custom
+    X session, then this will end up in a `.xsession-errors` file.
+
 ## 0.13 (February 10, 2017)
 
 ### Breaking Changes

src/XMonad/Core.hs

@@ -606,9 +606,36 @@ recompile force = io $ do
 
     useBuildscript <- do
       exists <- doesFileExist buildscript
-      if exists then isExecutable buildscript else return False
+      if exists
+        then do
+          isExe <- isExecutable buildscript
+          if isExe
+            then do
+              trace $ "XMonad will use build script at " ++ show buildscript ++ " to recompile."
+              return True
+            else do
+              trace $ unlines
+                [ "XMonad will not use build script, because " ++ show buildscript ++ " is not executable."
+                , "Suggested resolution to use it: chmod u+x " ++ show buildscript
+                ]
+              return False
+        else do
+          trace $
+            "XMonad will use ghc to recompile, because " ++ show buildscript ++ " does not exist."
+          return False
 
-    if force || useBuildscript || any (binT <) (srcT : libTs)
+    shouldRecompile <-
+      if useBuildscript || force
+        then return True
+        else if any (binT <) (srcT : libTs)
+          then do
+            trace "XMonad doing recompile because some files have changed."
+            return True
+          else do
+            trace "XMonad skipping recompile because it is not forced (e.g. via --recompile), and neither xmonad.hs nor any *.hs / *.lhs / *.hsc files in lib/ have been changed."
+            return False
+
+    if shouldRecompile
       then do
         -- temporarily disable SIGCHLD ignoring:
         uninstallSignalHandlers
@@ -621,7 +648,9 @@ recompile force = io $ do
         installSignalHandlers
 
         -- now, if it fails, run xmessage to let the user know:
-        when (status /= ExitSuccess) $ do
+        if status == ExitSuccess
+            then trace "XMonad recompilation process exited with success!"
+            else do
                 ghcErr <- readFile err
                 let msg = unlines $
                         ["Error detected while loading xmonad configuration file: " ++ src]

src/XMonad/Main.hs

@@ -113,12 +113,18 @@ usage = do
 --
 buildLaunch ::  IO ()
 buildLaunch = do
+    whoami <- getProgName
+    let compiledConfig = "xmonad-"++arch++"-"++os
+    unless (whoami == compiledConfig) $ do
+      trace $ concat
+        [ "XMonad is recompiling and replacing itself another XMonad process because the current process is called "
+        , show whoami
+        , " but the compiled configuration should be called "
+        , show compiledConfig
+        ]
       recompile False
       dir  <- getXMonadDataDir
       args <- getArgs
-    whoami <- getProgName
-    let compiledConfig = "xmonad-"++arch++"-"++os
-    unless (whoami == compiledConfig) $
       executeFile (dir </> compiledConfig) False args Nothing
 
 sendRestart :: IO ()