REPL: use >>>> and .... as prompts to reduce chance of false positives

Technologicat · Technologicat · commit b91e8f8926e9 · 2020-01-14T00:16:45.000+02:00
diff --git a/unpythonic/net/client.py b/unpythonic/net/client.py
@@ -224,8 +224,24 @@ def read_more_input():
                 val = buf.getvalue()
                 while True:  # The "L" in the "REPL"
                     try:
-                        # Wait for the prompt. It's the last thing the console
-                        # sends before listening for more input.
+                        # Wait for the prompt.
+                        #
+                        # We rely on the fact that it's always the last thing the console sends
+                        # before listening for more input.
+                        #
+                        # TODO: The current implementation leaves one race condition that can't be solved easily.
+                        # If the prompt string appears in the data stream, but is not actually a prompt,
+                        # we might interpret it as a prompt, and things will go south from there.
+                        #
+                        # This only happens when partial data recv'd on the socket ends exactly at the end of
+                        # the prompt string (e.g. ">>>"). So it's unlikely, but it may happen.
+                        #
+                        # The at-a-glance-almost-correct hack of prefixing bps1 and bps2 with b"\n" doesn't
+                        # work, because the newline won't be there when we handle the server's response to a
+                        # `KeyboardInterrupt` request. So doing that would cause the client to hang.
+                        #
+                        # As a semi-working hack, our server sets its prompts to ">>>>" and "...." (like PyPy).
+                        # Whereas "..." may appear in Python code or English, these strings usually don't.
                         if val.endswith(bps1) or val.endswith(bps2):
                             # "P"
                             text = val.decode("utf-8")
diff --git a/unpythonic/net/server.py b/unpythonic/net/server.py
@@ -436,11 +436,12 @@ def start(locals, addrspec=("127.0.0.1", 1337), banner=None):
     else:
         _banner = banner
 
-    # Set the prompts.
-    if not hasattr(sys, "ps1"):
-        sys.ps1 = ">>> "
-    if not hasattr(sys, "ps2"):
-        sys.ps2 = "... "
+    # Set the prompts. We use four "." to make semi-sure the prompt string only appears as a prompt.
+    # The client needs to identify the prompts from the data stream in order to know when to switch
+    # between listening and prompting, so "..." is not even semi-safe (it's valid Python, as well as
+    # valid English).
+    sys.ps1 = ">>>> "
+    sys.ps2 = ".... "
 
     # We use a combo of Shim and ThreadLocalBox to redirect attribute lookups
     # to the thread-specific read/write streams.
