bpo-34226: fix cgi.parse_multipart without content_length (GH-8530)

In Python 3.7 the behavior of parse_multipart changed requiring CONTENT-LENGTH
header, this fix remove this header as required and fix FieldStorage
read_lines_to_outerboundary, by not using limit when it's negative,
since by default it's -1 if not content-length and keeps substracting what
was read from the file object.

Also added a test case for this problem.
(cherry picked from commit d8cf3514dd4682419a66f6e834bb384ee34afc95)

Co-authored-by: roger <rogerduran@gmail.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/Lib/cgi.py b/Lib/cgi.py
index c22c71b387..77ab703cc0 100755
--- a/Lib/cgi.py
+++ b/Lib/cgi.py
@@ -200,7 +200,10 @@ def parse_multipart(fp, pdict, encoding="utf-8", errors="replace"):
     ctype = "multipart/form-data; boundary={}".format(boundary)
     headers = Message()
     headers.set_type(ctype)
-    headers['Content-Length'] = pdict['CONTENT-LENGTH']
+    try:
+        headers['Content-Length'] = pdict['CONTENT-LENGTH']
+    except KeyError:
+        pass
     fs = FieldStorage(fp, headers=headers, encoding=encoding, errors=errors,
         environ={'REQUEST_METHOD': 'POST'})
     return {k: fs.getlist(k) for k in fs}
@@ -736,7 +739,8 @@ class FieldStorage:
         last_line_lfend = True
         _read = 0
         while 1:
-            if self.limit is not None and _read >= self.limit:
+
+            if self.limit is not None and 0 <= self.limit <= _read:
                 break
             line = self.fp.readline(1<<16) # bytes
             self.bytes_read += len(line)