Backport PR #20473: _GSConverter: handle stray 'GS' in output gracefully

tacaswell · meeseeksmachine · commit fd55320ed8b1 · 2021-06-21T20:56:08.000Z
diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
@@ -124,7 +124,7 @@ def _read_until(self, terminator):
                 raise _ConverterError
             buf.extend(c)
             if buf.endswith(terminator):
-                return bytes(buf[:-len(terminator)])
+                return bytes(buf)
 
 
 class _GSConverter(_Converter):
@@ -154,15 +154,16 @@ def encode_and_escape(name):
             + b") run flush\n")
         self._proc.stdin.flush()
         # GS> if nothing left on the stack; GS<n> if n items left on the stack.
-        err = self._read_until(b"GS")
-        stack = self._read_until(b">")
+        err = self._read_until((b"GS<", b"GS>"))
+        stack = ""
+        if err.endswith(b"GS<"):
+            stack = self._read_until(b">")
         if stack or not os.path.exists(dest):
-            stack_size = int(stack[1:]) if stack else 0
+            stack_size = int(stack[:-1]) if stack else 0
             self._proc.stdin.write(b"pop\n" * stack_size)
             # Using the systemencoding should at least get the filenames right.
             raise ImageComparisonFailure(
-                (err + b"GS" + stack + b">")
-                .decode(sys.getfilesystemencoding(), "replace"))
+                (err + stack).decode(sys.getfilesystemencoding(), "replace"))
 
 
 class _SVGConverter(_Converter):
