Simple implementation of file system path completion.

This behaves reasonably similarly to the readline default.

For example, try

$ bin/osh

osh$ ls c<TAB>

core/completion.py

@@ -196,6 +196,34 @@ def Matches(self, words, index, prefix):
         yield name + ' '  # full word
 
 
+class FileSystemAction(CompletionAction):
+  def __init__(self):
+    self.cache = {}  # Do we need this?
+
+  def Matches(self, words, index, prefix):
+    i = prefix.rfind('/')
+    if i == -1:
+      base_dir = '.'
+      base = ''
+    else:
+      base_dir = prefix[:i]
+      base = base_dir
+      #log('base_dir %r', base_dir)
+
+    try:
+      names = os.listdir(base_dir)
+    except OSError as e:
+      return  # nothing
+
+    for name in names:
+      path = os.path.join(base, name)
+      if path.startswith(prefix):
+        if os.path.isdir(path):
+          yield path + '/'
+        else:
+          yield path
+
+
 class ShellFuncAction(CompletionAction):
   def __init__(self, ex, func):
     self.ex = ex
@@ -780,7 +808,7 @@ def Init(readline_mod, pool, builtins, mem, funcs, comp_lookup, progress_f,
 
   # NOTE: Need set_completer_delims to be space here?  Otherwise you complete
   # as --a and --n.  Why?
-  comp_lookup.RegisterName('__default__', WordsAction(['-a', '-n']))
+  comp_lookup.RegisterName('__default__', FileSystemAction())
 
   A1 = WordsAction(['foo.py', 'foo', 'bar.py'])
   A2 = WordsAction(['m%d' % i for i in range(5)], delay=0.1)

core/completion_test.py

@@ -26,6 +26,7 @@
 from osh import parse_lib
 
 assign_op_e = ast.assign_op_e
+log = util.log
 
 
 A1 = completion.WordsAction(['foo.py', 'foo', 'bar.py'])
@@ -71,6 +72,36 @@ def testExternalCommandAction(self):
     a = completion.ExternalCommandAction(mem)
     print(list(a.Matches([], 0, 'f')))
 
+  def testFileSystemAction(self):
+    a = completion.FileSystemAction()
+    # Current dir -- all files and dirs
+    print(list(a.Matches([], 0, '')))
+
+    # This test depends on actual file system content.  But we choose things
+    # that shouldn't go away.
+    CASES = [
+        # Dirs and files
+        ('c', ['core/', 'configure']),
+        ('nonexistent/', []),
+        ('README', ['README.md']),
+        # Directory should be completed to core/ ?
+        ('core', ['core/']),
+        ('asdl/R', ['asdl/README.md']),
+        ('opy/doc', ['opy/doc/']),
+        ('opy/doc/', ['opy/doc/opcodes.md']),
+    ]
+
+    for prefix, expected in CASES:
+      log('')
+      log('-- PREFIX %s', prefix)
+      log('-- expected %s', expected)
+      self.assertEqual(expected, list(a.Matches([], 0, prefix)))
+
+    print(list(a.Matches([], 0, './o')))
+
+    # A bunch of repos in oilshell
+    print(list(a.Matches([], 0, '../o')))
+
   def testShellFuncExecution(self):
     ex = cmd_exec_test.InitExecutor()
     func_node = ast.FuncDef()