core: better logging when spawn() fails

If someone sends us an invalid thing to run, right now we just silently
swallow it, because the logging module doesn't work after we've forked.

Instead, let's just keep around what we're going to exec (being smart about
shell=True invocations so we don't just do a lookup of "/bin/sh") and do a
path lookup on it. If it doesn't exist, we can fail without doing all the
forking and give the user a real error message.

While arguably not logging anything is fine (someone typed an invalid
thing), I've seen a lot of confusion more recently about spawn not working
because of typos and such. Hopefully this will fix things.

Signed-off-by: Tycho Andersen <[email protected]>

libqtile/core/manager.py

@@ -24,6 +24,7 @@
 import os
 import pickle
 import shlex
+import shutil
 import signal
 import subprocess
 import tempfile
@@ -1138,14 +1139,19 @@ def cmd_spawn(self, cmd, shell=False):
 
             spawn(["xterm", "-T", "Temporary terminal"])
         """
-        if shell:
-            if not isinstance(cmd, str):
-                cmd = subprocess.list2cmdline(cmd)
-            args = ["/bin/sh", "-c", cmd]
-        elif isinstance(cmd, str):
+        if isinstance(cmd, str):
             args = shlex.split(cmd)
         else:
             args = list(cmd)
+            cmd = subprocess.list2cmdline(args)
+
+        to_lookup = args[0]
+        if shell:
+            args = ["/bin/sh", "-c", cmd]
+
+        if shutil.which(to_lookup) is None:
+            logger.error("couldn't find `{}`".format(to_lookup))
+            return -1
 
         r, w = os.pipe()
         pid = os.fork()
@@ -1197,8 +1203,9 @@ def cmd_spawn(self, cmd, shell=False):
 
                 try:
                     os.execvp(args[0], args)
-                except OSError as e:
-                    logger.error("failed spawn: \"{0}\"\n{1}".format(cmd, e))
+                except OSError:
+                    # can't log here since we forked :(
+                    pass
 
                 os._exit(1)
             else: