debugger: leave output screen with single key press

Introduce support code for non-buffered console input (single key press
that doesn't accumulate input until ENTER is pressed). Support Windows
(msvcrt) and POSIX supporting Unix platforms (select and termios). Fall
back to the Python's input() routine for Webassembly and Emscripten.

Avoid the accumulation of repeated prompts on platforms that support
single key presses. Only the backwards compatible fallback on minor
platforms involves a prompt, so users can tell when they face that
situation.

The approach in this commit automatically detects platforms, no config
option is required. All depencencies are provided by Python core, there
are no external dependencies. The implementation is considered suitable
for future maintenance, extension, or adjustment for other platforms.

pudb/debugger.py

@@ -35,7 +35,7 @@
 
 import urwid
 
-from pudb.lowlevel import decode_lines, ui_log
+from pudb.lowlevel import decode_lines, NonBufferedConsole, ui_log
 from pudb.settings import get_save_config_path, load_config, save_config
 
 
@@ -769,10 +769,15 @@ def __init__(self, screen):
 
     def __enter__(self):
         self.screen.stop()
+        return self
 
     def __exit__(self, exc_type, exc_value, exc_traceback):
         self.screen.start()
 
+    def press_key_to_return(self):
+        with NonBufferedConsole() as nbc:
+            key = nbc.get_data()
+
 
 class DebuggerUI(FrameVarInfoKeeper):
     # {{{ constructor
@@ -2082,8 +2087,8 @@ def shrink_sidebar(w, size, key):
         # {{{ top-level listeners
 
         def show_output(w, size, key):
-            with StoppedScreen(self.screen):
-                input("Hit Enter to return:")
+            with StoppedScreen(self.screen) as s:
+                s.press_key_to_return()
 
         def reload_breakpoints_and_redisplay():
             reload_breakpoints()

pudb/lowlevel.py

@@ -283,4 +283,73 @@ def decode_lines(lines):
 
 # }}}
 
+
+# {{{ non-buffered console
+
+# Local platform dependent helper to get single key presses from a
+# terminal in unbuffered mode. Eliminates the necessity to press ENTER
+# before other input also becomes available. Also avoids the accumulation
+# of prompts on the screen as was the case with Python's input() call.
+# Is used in situations where urwid is disabled and curses calls are
+# not available.
+
+_nbc_use_input = True
+_nbc_use_getch = False
+_nbc_use_select = False
+if sys.platform in ("emscripten", "wasi"):
+    pass
+elif sys.platform in ("win32",):
+    import msvcrt
+    _nbc_use_input = False
+    _nbc_use_getch = True
+else:
+    import select
+    import termios
+    import tty
+    _nbc_use_input = False
+    _nbc_use_select = True
+
+
+class NonBufferedConsole(object):
+
+    def __init__(self):
+        pass
+
+    def __enter__(self):
+        if _nbc_use_select:
+            self.prev_settings = termios.tcgetattr(sys.stdin)
+            tty.setcbreak(sys.stdin.fileno())
+        return self
+
+    def __exit__(self, type, value, traceback):
+        if _nbc_use_select:
+            termios.tcsetattr(sys.stdin, termios.TCSADRAIN, self.prev_settings)
+
+    def get_data(self):
+        if _nbc_use_getch:
+            c = msvcrt.getch()
+            if c in ('\x00', '\xe0'):
+                c = msvcrt.getch()
+            return c
+
+        if _nbc_use_select:
+            rset, _, _ = select.select([sys.stdin], [], [], None)
+            if sys.stdin in rset:
+                return sys.stdin.read(1)
+            return None
+
+        # Strictly speaking putting the fallback here which requires
+        # pressing ENTER is not correct, this is the "non buffered"
+        # console support code. But it simplifies call sites. And is
+        # easy to tell by users because a prompt is provided. This is
+        # the most portable approach, and backwards compatible with
+        # earlier PuDB releases. It's a most appropriate default for
+        # otherwise unsupported platforms. Or when users choose to
+        # not accept single key presses, or keys other than ENTER.
+        if _nbc_use_input:
+            input("Hit Enter to return:")
+        return None
+
+# }}}
+
 # vim: foldmethod=marker