Refactor DiffMatchPatch diff handler (#16483)

Follow up to #16027
Fixes #15850

Summary of the issue:
Diff Match Patch proxy crashes and the calling thread deadlocks

Description of user facing changes
Diff Match Patch proxy will become more stable

Description of development approach
Refactored DiffMatchPatch diff handler.

Now, when reading from stdout of a proxy process, if not enough bytes are read, the return code is checked.

If a return code was received, an exception is raised and a fallback to difflib occurs.

source/diffHandler.py

@@ -62,6 +62,21 @@ def _initialize(self):
 	def _getText(self, ti: TextInfo) -> str:
 		return ti.text
 
+	@classmethod
+	def _readData(cls, size: int) -> bytes:
+		"""Reads from stdout, raises exception on EOF."""
+		buffer = b""
+		while (remainingLength := size - len(buffer)) > 0:
+			chunk = cls._proc.stdout.read(remainingLength)
+			if chunk:
+				buffer += chunk
+				continue
+			return_code = cls._proc.poll()
+			if return_code is None:
+				continue
+			raise RuntimeError(f'Diff-match-patch proxy process died! Return code {return_code}')
+		return buffer
+
 	def diff(self, newText: str, oldText: str) -> List[str]:
 		try:
 			if not newText and not oldText:
@@ -70,31 +85,23 @@ def diff(self, newText: str, oldText: str) -> List[str]:
 				return []
 			with DiffMatchPatch._lock:
 				self._initialize()
-				old = oldText.encode("utf-8")
-				new = newText.encode("utf-8")
+				oldEncodedText = oldText.encode()
+				newEncodedText = newText.encode()
 				# Sizes are packed as 32-bit ints in native byte order.
 				# Since nvda and nvda_dmp are running on the same Python
 				# platform/version, this is okay.
-				tl = struct.pack("=II", len(old), len(new))
-				DiffMatchPatch._proc.stdin.write(tl)
-				DiffMatchPatch._proc.stdin.write(old)
-				DiffMatchPatch._proc.stdin.write(new)
-				buf = b""
-				sizeb = b""
-				SIZELEN = 4
-				while len(sizeb) < SIZELEN:
-					try:
-						sizeb += DiffMatchPatch._proc.stdout.read(SIZELEN - len(sizeb))
-					except TypeError:
-						pass
-				(size,) = struct.unpack("=I", sizeb)
-				while len(buf) < size:
-					buf += DiffMatchPatch._proc.stdout.read(size - len(buf))
+				packedTextLength = struct.pack("=II", len(oldEncodedText), len(newEncodedText))
+				DiffMatchPatch._proc.stdin.write(packedTextLength)
+				DiffMatchPatch._proc.stdin.write(oldEncodedText)
+				DiffMatchPatch._proc.stdin.write(newEncodedText)
 				DiffMatchPatch._proc.stdin.flush()
-				DiffMatchPatch._proc.stdout.flush()
+				DIFF_LENGTH_BUFFER_SIZE = 4
+				diffLengthBuffer = DiffMatchPatch._readData(DIFF_LENGTH_BUFFER_SIZE)
+				(diff_length,) = struct.unpack("=I", diffLengthBuffer)
+				diffBuffer = DiffMatchPatch._readData(diff_length)
 				return [
 					line
-					for line in buf.decode("utf-8").splitlines()
+					for line in diffBuffer.decode("utf-8").splitlines()
 					if line and not line.isspace()
 				]
 		except Exception:
@@ -107,11 +114,15 @@ def _terminate(self):
 			if DiffMatchPatch._proc:
 				log.debug("Terminating diff-match-patch proxy")
 				# nvda_dmp exits when it receives two zero-length texts.
-				try:
-					DiffMatchPatch._proc.stdin.write(struct.pack("=II", 0, 0))
-					DiffMatchPatch._proc.wait(timeout=5)
-				except Exception:
-					log.exception("Exception during DMP termination")
+				returnCode = DiffMatchPatch._proc.poll()
+				if returnCode is None:
+					try:
+						DiffMatchPatch._proc.stdin.write(struct.pack("=II", 0, 0))
+						DiffMatchPatch._proc.wait(timeout=5)
+					except Exception:
+						log.exception("Exception during DMP termination")
+				else:
+					log.debug(f"Diff-match-patch proxy already terminated, return code is {returnCode}")
 				DiffMatchPatch._proc = None
 
 

user_docs/en/changes.md

@@ -20,6 +20,7 @@
 ### Bug Fixes
 
 * NVDA will announce correctly the autocomplete suggestions in Eclipse and other Eclipse-based environments on Windows 11. (#16416, @thgcode)
+* Improved reliability of automatic text readout, particularly in terminal applications. (#15850, #16027, @Danstiv)
 * Braille cursor routing is now much more reliable when a line contains one or more Unicode variation selectors or decomposed characters. (#10960, #16477, @mltony, @LeonarddeR)
 * NVDA will correctly announce selection changes when editing a cell's text in Microsoft Excel. (#15843)