pureboot: the info block is what proves a knock landed

A prompt byte alone does not: one left over from a previous session can
still be in the pipeline while the port opening resets the device into a
fresh window, where the bare command that follows is discarded. Each
attempt is now the whole handshake, retried until the block comes back.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pureboot/pureboot.py

@@ -364,26 +364,37 @@ class Loader:
         self.info = None
 
     def connect(self, wait):
-        """Knock until the window answers, then read the info block. Also
-        converges into a live session: the knock bytes are ignored there and
-        the drain absorbs whatever they produced."""
-        self.port.flush_input()
+        """Knock until the info block comes back. The block is what proves the
+        loader is listening — a prompt byte alone does not, since one left over
+        from a previous session can still be in the pipeline while the port
+        opening resets the device into a fresh activation window, where a
+        command without its knock is discarded. Each attempt is therefore the
+        whole handshake, retried until it produces the block or the window
+        closes. Also converges into a live session: the knock bytes are ignored
+        there and the drain absorbs whatever they produced."""
         deadline = time.monotonic() + wait
         knocks = 0
         while True:
+            self.port.flush_input()
             self.port.write(b"pb")
             knocks += 1
             if PROMPT in self.port.read_available(0.4):
-                break
+                while self.port.read_available(0.3):
+                    pass
+                self.port.write(b"b")
+                try:
+                    block = self.port.read_exact(12, 2.0)
+                except Error:
+                    block = b""
+                # A version the tool cannot speak is the loader's own answer,
+                # not a failed knock: Info reports it rather than retrying.
+                if block[0:2] == b"PB":
+                    self.info = Info(block)
+                    self._expect_prompt()
+                    verbose(f"loader answered knock {knocks}; info block read")
+                    return self.info
             if time.monotonic() > deadline:
                 raise Error("no answer — reset the device within its activation window")
-        while self.port.read_available(0.3):
-            pass
-        self.port.write(b"b")
-        self.info = Info(self.port.read_exact(12, 2.0))
-        self._expect_prompt()
-        verbose(f"loader answered knock {knocks}; info block read")
-        return self.info
 
     def _expect_prompt(self, timeout=2.0):
         byte = self.port.read_exact(1, timeout)