Issue 21469: Mitigate risk of false positives with robotparser.

* Repair the broken link to norobots-rfc.txt.

* HTTP response codes >= 500 treated as a failed read rather than as a not
found.  Not found means that we can assume the entire site is allowed.  A 5xx
server error tells us nothing.

* A successful read() or parse() updates the mtime (which is defined to be "the
  time the robots.txt file was last fetched").

* The can_fetch() method returns False unless we've had a read() with a 2xx or
4xx response.  This avoids false positives in the case where a user calls
can_fetch() before calling read().

* I don't see any easy way to test this patch without hitting internet
resources that might change or without use of mock objects that wouldn't
provide must reassurance.

Lib/robotparser.py

@@ -7,7 +7,8 @@
     2) PSF license for Python 2.2
 
     The robots.txt Exclusion Protocol is implemented as specified in
-    http://info.webcrawler.com/mak/projects/robots/norobots-rfc.html
+    http://www.robotstxt.org/norobots-rfc.txt
+
 """
 import urlparse
 import urllib
@@ -60,7 +61,7 @@ class RobotFileParser:
         self.errcode = opener.errcode
         if self.errcode in (401, 403):
             self.disallow_all = True
-        elif self.errcode >= 400:
+        elif self.errcode >= 400 and self.errcode < 500:
             self.allow_all = True
         elif self.errcode == 200 and lines:
             self.parse(lines)
@@ -86,6 +87,7 @@ class RobotFileParser:
         linenumber = 0
         entry = Entry()
 
+        self.modified()
         for line in lines:
             linenumber += 1
             if not line:
@@ -131,6 +133,14 @@ class RobotFileParser:
             return False
         if self.allow_all:
             return True
+
+        # Until the robots.txt file has been read or found not
+        # to exist, we must assume that no url is allowable.
+        # This prevents false positives when a user erronenously
+        # calls can_fetch() before calling read().
+        if not self.last_checked:
+            return False
+
         # search for given user agent matches
         # the first match counts
         parsed_url = urlparse.urlparse(urllib.unquote(url))

Misc/NEWS

@@ -52,6 +52,10 @@ Library
 - Issue #21306: Backport hmac.compare_digest from Python 3. This is part of PEP
   466.
 
+- Issue #21469:  Reduced the risk of false positives in robotparser by
+  checking to make sure that robots.txt has been read or does not exist
+  prior to returning True in can_fetch().
+
 - Issue #21321: itertools.islice() now releases the reference to the source
   iterator when the slice is exhausted.  Patch by Anton Afanasyev.