State the absent-input note per backend semantics

The shared note claimed a script waiting for input runs to the
timeout, which is wrong for the backends that hand end-of-file at
spawn and steered the patcher toward nonexistent stdin reads.
ConPTY, which cannot deliver an end-of-file, states its own note.

Author: gorandodig

render_machine/_conpty.py

@@ -56,9 +56,6 @@
     DRAIN_DEADLINE_SECONDS,
     GRACE_TICK_SECONDS,
     HANDSHAKE_TIMEOUT_SECONDS,
-)
-from render_machine.terminal_process import NO_INPUT_NOTE as DEFAULT_NO_INPUT_NOTE
-from render_machine.terminal_process import (
     OWNER_PARENT,
     OWNER_READER,
     POLL_INTERVAL_SECONDS,
@@ -162,11 +159,12 @@
 # reaches only processes sharing the caller's console and the target is on the pseudoconsole.
 CONTROL_C_BYTE = b"\x03"
 
-# The absent-input note this backend adds to, stated where the asymmetry is documented: a
-# script that reads input blocks until the execution timeout rather than seeing end-of-file.
-NO_INPUT_NOTE = DEFAULT_NO_INPUT_NOTE + (
-    " On Windows the terminal carries no synthetic end-of-file, so such a script blocks until the "
-    "timeout instead of reading end-of-file."
+# The absent-input note this backend states itself, where the asymmetry is documented:
+# unlike the other backends it cannot hand the target end-of-file, so a script that reads
+# terminal input really does block until the execution timeout.
+NO_INPUT_NOTE = (
+    " No input driver was attached to the script's terminal, and on Windows the terminal carries "
+    "no synthetic end-of-file, so a script that waits for terminal input blocks until the timeout."
 )
 
 

render_machine/terminal_process.py

@@ -83,12 +83,12 @@
 OWNER_PARENT = "parent"
 OWNER_READER = "reader"
 
-# What a timeout diagnostic says when no input driver was attached. A backend that gives
-# the target end-of-file at spawn needs nothing more; ConPTY, which cannot, appends its own
-# clause to this one.
+# What a timeout diagnostic says when no input driver was attached. A backend that hands
+# the target end-of-file at spawn needs nothing more; ConPTY, which cannot deliver an
+# end-of-file, states its own note instead.
 NO_INPUT_NOTE = (
-    " No input driver was attached to the script's terminal, so a script that waits for input "
-    "never receives any and runs to the timeout."
+    " No input driver was attached to the script's terminal; a script that reads terminal "
+    "input is handed end-of-file rather than left waiting for it."
 )
 
 

tests/test_conpty.py

@@ -382,10 +382,12 @@ def test_windows_selects_the_conpty_backend(monkeypatch):
 
 def test_the_backend_notes_that_it_has_no_synthetic_end_of_file():
     """The timeout diagnostic asks the backend that ran, so this one has to state the
-    asymmetry itself: a script reading input blocks instead of seeing end-of-file."""
+    asymmetry itself: a script reading input blocks instead of seeing end-of-file — the
+    base note describes backends that hand end-of-file at spawn, which this one cannot."""
     note = ConPtyProcess().no_input_note()
 
-    assert note.startswith(NO_INPUT_NOTE)
+    assert note != NO_INPUT_NOTE
+    assert "blocks until the timeout" in note
     assert "end-of-file" in note