Add utility for parsing shebangs and resolving PATH

pre_commit/parse_shebang.py

@@ -0,0 +1,95 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import io
+import os.path
+import shlex
+import string
+
+from pre_commit import five
+
+
+printable = frozenset(string.printable)
+
+
+def parse_bytesio(bytesio):
+    """Parse the shebang from a file opened for reading binary."""
+    if bytesio.read(2) != b'#!':
+        return ()
+    first_line = bytesio.readline()
+    try:
+        first_line = first_line.decode('US-ASCII')
+    except UnicodeDecodeError:
+        return ()
+
+    # Require only printable ascii
+    for c in first_line:
+        if c not in printable:
+            return ()
+
+    # shlex.split is horribly broken in py26 on text strings
+    cmd = tuple(shlex.split(five.n(first_line)))
+    if cmd[0] == '/usr/bin/env':
+        cmd = cmd[1:]
+    return cmd
+
+
+def parse_filename(filename):
+    """Parse the shebang given a filename."""
+    if not os.path.exists(filename) or not os.access(filename, os.X_OK):
+        return ()
+
+    with io.open(filename, 'rb') as f:
+        return parse_bytesio(f)
+
+
+def find_executable(exe, _environ=None):
+    exe = os.path.normpath(exe)
+    if os.sep in exe:
+        return exe
+
+    environ = _environ if _environ is not None else os.environ
+
+    if 'PATHEXT' in environ:
+        possible_exe_names = (exe,) + tuple(
+            exe + ext.lower() for ext in environ['PATHEXT'].split(os.pathsep)
+        )
+    else:
+        possible_exe_names = (exe,)
+
+    for path in environ.get('PATH', '').split(os.pathsep):
+        for possible_exe_name in possible_exe_names:
+            joined = os.path.join(path, possible_exe_name)
+            if os.path.isfile(joined) and os.access(joined, os.X_OK):
+                return joined
+    else:
+        return None
+
+
+def normexe(orig_exe):
+    if os.sep not in orig_exe:
+        exe = find_executable(orig_exe)
+        if exe is None:
+            raise OSError('Executable {0} not found'.format(orig_exe))
+        return exe
+    else:
+        return orig_exe
+
+
+def normalize_cmd(cmd):
+    """Fixes for the following issues on windows
+    - http://bugs.python.org/issue8557
+    - windows does not parse shebangs
+
+    This function also makes deep-path shebangs work just fine
+    """
+    # Use PATH to determine the executable
+    exe = normexe(cmd[0])
+
+    # Figure out the shebang from the resulting command
+    cmd = parse_filename(exe) + (exe,) + cmd[1:]
+
+    # This could have given us back another bare executable
+    exe = normexe(cmd[0])
+
+    return (exe,) + cmd[1:]