Fixes issue 27777: cgi.FieldStorage can't parse simple body with Content-Length and no Content-Disposition

- If content length is present, read at most len bytes.
- When reading `read_lines_to_eof` use `read` instead of `readline`.
- Use `self.__write` even when content length is present and > 1000.

Lib/cgi.py

@@ -667,7 +667,7 @@ class FieldStorage:
     def read_single(self):
         """Internal: read an atomic part."""
         if self.length >= 0:
-            self.read_binary()
+            self.read_content()
             self.skip_lines()
         else:
             self.read_lines()
@@ -675,31 +675,30 @@ class FieldStorage:
 
     bufsize = 8*1024            # I/O buffering size for copy to file
 
-    def read_binary(self):
-        """Internal: read binary data."""
-        self.file = self.make_file()
+    def read_content(self):
+        """Internal: read data up until content-length len."""
+        if self.length > 1000:
+            self.file = self.make_file()
+        else:
+            self.file = self.io_object()
         todo = self.length
         if todo >= 0:
             while todo > 0:
                 data = self.fp.read(min(todo, self.bufsize)) # bytes
-                if not isinstance(data, bytes):
-                    raise ValueError("%s should return bytes, got %s"
-                                     % (self.fp, type(data).__name__))
                 self.bytes_read += len(data)
                 if not data:
                     self.done = -1
                     break
-                self.file.write(data)
+                self.__write(data)
                 todo = todo - len(data)
 
     def read_lines(self):
         """Internal: read lines until EOF or outerboundary."""
-        if self._binary_file:
-            self.file = self.__file = BytesIO() # store data as bytes for files
-        else:
-            self.file = self.__file = StringIO() # as strings for other fields
+        self.file = self.__file = self.io_object()
         if self.outerboundary:
             self.read_lines_to_outerboundary()
+        elif self.length >= 0:
+            self.read_content()
         else:
             self.read_lines_to_eof()
 
@@ -721,7 +720,7 @@ class FieldStorage:
     def read_lines_to_eof(self):
         """Internal: read lines until EOF."""
         while 1:
-            line = self.fp.readline(1<<16) # bytes
+            line = self.fp.read(1<<16) # bytes
             self.bytes_read += len(line)
             if not line:
                 self.done = -1
@@ -830,6 +829,12 @@ class FieldStorage:
             return tempfile.TemporaryFile("w+",
                 encoding=self.encoding, newline = '\n')
 
+    def io_object(self):
+        if self._binary_file:
+            return BytesIO() # store data as bytes for files
+        else:
+            return StringIO() # as strings for other fields
+
 
 # Test/debug code
 # ===============

Misc/NEWS.d/next/Library/2019-02-05-16-57-27.bpo-27777.AKUxDR.rst

@@ -0,0 +1,2 @@
+Fixes a bug in :mod:`cgi` when a request has a "Content-Length" header but
+without "Content-Disposition" headers.