Work around subprocess.call() issue on Windows

On POSIX-like systems, calling `subprocess.call()` with both
`shell=True` and `executable='...'` has the following behavior:

> If `shell=True`, on POSIX the _executable_ argument specifies a
> replacement shell for the default `/bin/sh`.

(via https://docs.python.org/3/library/subprocess.html?highlight=subprocess#popen-constructor)

This seems to have a similar behavior on Windows, but this is
problematic when a POSIX shell is substituted for cmd.exe. This is
because when `shell=True`, the shell is invoked with a '/c' argument,
which is the correct argument for cmd.exe but not for Bash, which
expects a '-c' argument instead. See here:
https://github.com/python/cpython/blob/1def7754b7a41fe57efafaf5eff24cfa15353444/Lib/subprocess.py#L1407

This is problematic when combined with Dotbot's behavior, where the
`executable` argument is set based on `$SHELL`. For example, when
running in Git Bash, the `$SHELL` environment variable is set to Bash,
so any commands run by Dotbot will fail (because it'll invoke Bash with
a '/c' argument).

This behavior of setting the `executable` argument based on `$SHELL` was
introduced in 7593d8c13479b382357be065c7bf51562a130660. This is the
desired behavior. See discussion in
https://github.com/anishathalye/dotbot/issues/97 and
https://github.com/anishathalye/dotbot/pull/100.

Unfortunately, this doesn't work quite right on Windows. This patch
works around the issue by avoiding setting the `executable` argument
when the platform is Windows, which is tested using
`platform.system() == 'Windows'`. This means that shell commands
executed by Dotbot on this platform will always be run using cmd.exe.
Invocations of single programs or simple commands will probably work
just fine in cmd.exe. If Bash-like behavior is desired, the user will
have to write their command as `bash -c '...'`.

This shouldn't have any implications for backwards-compatibility,
because setting the `executable` argument on Windows didn't do the right
thing anyways. Previous workarounds that users had should continue to
work with the new code.

When using Python from CYGWIN, `platform.system()` returns something
like 'CYGWIN_NT-...', so it won't be detected with the check, but this
is the correct behavior, because CYGWIN Python's `subprocess.call()` has
the POSIX-like behavior.

This patch also refactors the code to factor out the
`subprocess.call()`, which was being called in both `link.py` and
`shell.py`, so the workaround can be applied in a single place.

See the following issues/pull requests for a discussion of this bug:
- https://github.com/anishathalye/dotbot/issues/170
- https://github.com/anishathalye/dotbot/pull/177
- https://github.com/anishathalye/dotbot/issues/219

An issue has also been raised in Python's issue tracker:
- https://bugs.python.org/issue40467

Thanks to @shivapoudel for originally reporting the issue, @SuJiKiNen
for debugging it and submitting a pull request, and @mohkale for
suggesting factoring out the code so that other plugins could use it.

6 files changed, 79 insertions, 55 deletions

diff --git a/dotbot/plugins/clean.py b/dotbot/plugins/clean.py
index 82d77a6..09b11d8 100644
--- a/dotbot/plugins/clean.py
+++ b/dotbot/plugins/clean.py
@@ -1,4 +1,6 @@
-import os, dotbot
+import os
+import dotbot
+
 
 class Clean(dotbot.Plugin):
     '''
diff --git a/dotbot/plugins/create.py b/dotbot/plugins/create.py
index dc119da..015645a 100644
--- a/dotbot/plugins/create.py
+++ b/dotbot/plugins/create.py
@@ -1,8 +1,5 @@
 import os
-import glob
-import shutil
 import dotbot
-import subprocess
 
 
 class Create(dotbot.Plugin):
diff --git a/dotbot/plugins/link.py b/dotbot/plugins/link.py
index 82a61ce..6f2b562 100644
--- a/dotbot/plugins/link.py
+++ b/dotbot/plugins/link.py
@@ -2,6 +2,7 @@ import os
 import glob
 import shutil
 import dotbot
+import dotbot.util
 import subprocess
 
 
@@ -105,14 +106,7 @@ class Link(dotbot.Plugin):
         return success
 
     def _test_success(self, command):
-        with open(os.devnull, 'w') as devnull:
-            ret = subprocess.call(
-                command,
-                shell=True,
-                stdout=devnull,
-                stderr=devnull,
-                executable=os.environ.get('SHELL'),
-            )
+        ret = dotbot.util.shell_command(command, cwd=self._context.base_directory())
         if ret != 0:
             self._log.debug('Test \'%s\' returned false' % command)
         return ret == 0
diff --git a/dotbot/plugins/shell.py b/dotbot/plugins/shell.py
index 06a9a89..3092f20 100644
--- a/dotbot/plugins/shell.py
+++ b/dotbot/plugins/shell.py
@@ -1,4 +1,8 @@
-import os, subprocess, dotbot
+import os
+import subprocess
+import dotbot
+import dotbot.util
+
 
 class Shell(dotbot.Plugin):
     '''
@@ -19,48 +23,40 @@ class Shell(dotbot.Plugin):
     def _process_commands(self, data):
         success = True
         defaults = self._context.defaults().get('shell', {})
-        with open(os.devnull, 'w') as devnull:
-            for item in data:
-                stdin = stdout = stderr = devnull
-                quiet = False
-                if defaults.get('stdin', False) == True:
-                    stdin = None
-                if defaults.get('stdout', False) == True:
-                    stdout = None
-                if defaults.get('stderr', False) == True:
-                    stderr = None
-                if defaults.get('quiet', False) == True:
-                    quiet = True
-                if isinstance(item, dict):
-                    cmd = item['command']
-                    msg = item.get('description', None)
-                    if 'stdin' in item:
-                        stdin = None if item['stdin'] == True else devnull
-                    if 'stdout' in item:
-                        stdout = None if item['stdout'] == True else devnull
-                    if 'stderr' in item:
-                        stderr = None if item['stderr'] == True else devnull
-                    if 'quiet' in item:
-                        quiet = True if item['quiet'] == True else False
-                elif isinstance(item, list):
-                    cmd = item[0]
-                    msg = item[1] if len(item) > 1 else None
-                else:
-                    cmd = item
-                    msg = None
-                if msg is None:
-                    self._log.lowinfo(cmd)
-                elif quiet:
-                    self._log.lowinfo('%s' % msg)
-                else:
-                    self._log.lowinfo('%s [%s]' % (msg, cmd))
-                executable = os.environ.get('SHELL')
-                ret = subprocess.call(cmd, shell=True, stdin=stdin, stdout=stdout,
-                    stderr=stderr, cwd=self._context.base_directory(),
-                    executable=executable)
-                if ret != 0:
-                    success = False
-                    self._log.warning('Command [%s] failed' % cmd)
+        for item in data:
+            stdin = defaults.get('stdin', False)
+            stdout = defaults.get('stdout', False)
+            stderr = defaults.get('stderr', False)
+            quiet = defaults.get('quiet', False)
+            if isinstance(item, dict):
+                cmd = item['command']
+                msg = item.get('description', None)
+                stdin = item.get('stdin', stdin)
+                stdout = item.get('stdout', stdout)
+                stderr = item.get('stderr', stderr)
+                quiet = item.get('quiet', quiet)
+            elif isinstance(item, list):
+                cmd = item[0]
+                msg = item[1] if len(item) > 1 else None
+            else:
+                cmd = item
+                msg = None
+            if msg is None:
+                self._log.lowinfo(cmd)
+            elif quiet:
+                self._log.lowinfo('%s' % msg)
+            else:
+                self._log.lowinfo('%s [%s]' % (msg, cmd))
+            ret = dotbot.util.shell_command(
+                cmd,
+                cwd=self._context.base_directory(),
+                enable_stdin=stdin,
+                enable_stdout=stdout,
+                enable_stderr=stderr
+            )
+            if ret != 0:
+                success = False
+                self._log.warning('Command [%s] failed' % cmd)
         if success:
             self._log.info('All commands have been executed')
         else:
diff --git a/dotbot/util/__init__.py b/dotbot/util/__init__.py
index e69de29..0c5a8f5 100644
--- a/dotbot/util/__init__.py
+++ b/dotbot/util/__init__.py
@@ -0,0 +1 @@
+from .common import shell_command
diff --git a/dotbot/util/common.py b/dotbot/util/common.py
new file mode 100644
index 0000000..d1e2000
--- /dev/null
+++ b/dotbot/util/common.py
@@ -0,0 +1,34 @@
+import os
+import subprocess
+import platform
+
+
+def shell_command(command, cwd=None, enable_stdin=False, enable_stdout=False, enable_stderr=False):
+    with open(os.devnull, 'w') as devnull_w, open(os.devnull, 'r') as devnull_r:
+        stdin = None if enable_stdin else devnull_r
+        stdout = None if enable_stdout else devnull_w
+        stderr = None if enable_stderr else devnull_w
+        executable = os.environ.get('SHELL')
+        if platform.system() == 'Windows':
+            # We avoid setting the executable kwarg on Windows because it does
+            # not have the desired effect when combined with shell=True. It
+            # will result in the correct program being run (e.g. bash), but it
+            # will be invoked with a '/c' argument instead of a '-c' argument,
+            # which it won't understand.
+            #
+            # See https://github.com/anishathalye/dotbot/issues/219 and
+            # https://bugs.python.org/issue40467.
+            #
+            # This means that complex commands that require Bash's parsing
+            # won't work; a workaround for this is to write the command as
+            # `bash -c "..."`.
+            executable = None
+        return subprocess.call(
+            command,
+            shell=True,
+            executable=executable,
+            stdin=stdin,
+            stdout=stdout,
+            stderr=stderr,
+            cwd=cwd
+        )