Change the treatment of positions returned by PEP293

error handers in the Unicode codecs: Negative
positions are treated as being relative to the end of
the input and out of bounds positions result in an
IndexError.

Also update the PEP and include an explanation of
this in the documentation for codecs.register_error.

Fixes a small bug in iconv_codecs: if the position
from the callback is negative *add* it to the size
instead of substracting it.

From SF patch #677429.

Lib/test/test_codeccallbacks.py

@@ -1,6 +1,23 @@
 import test.test_support, unittest
 import sys, codecs, htmlentitydefs, unicodedata
 
+class PosReturn:
+    # this can be used for configurable callbacks
+
+    def __init__(self):
+        self.pos = 0
+
+    def handle(self, exc):
+        oldpos = self.pos
+        realpos = oldpos
+        if realpos<0:
+           realpos = len(exc.object) + realpos
+        # if we don't advance this time, terminate on the next call
+        # otherwise we'd get an endless loop
+        if realpos <= exc.start:
+            self.pos = len(exc.object)
+        return (u"<?>", oldpos)
+
 class CodecCallbackTest(unittest.TestCase):
 
     def test_xmlcharrefreplace(self):
@@ -543,18 +560,36 @@ class CodecCallbackTest(unittest.TestCase):
         codecs.register_error("test.baddecodereturn2", baddecodereturn2)
         self.assertRaises(TypeError, "\xff".decode, "ascii", "test.baddecodereturn2")
 
-        pos = [-42]
-        def negposreturn(exc):
-            pos[0] += 1 # use list to work around scoping problem
-            return (u"?", pos[0])
-        codecs.register_error("test.negposreturn", negposreturn)
-        "\xff".decode("ascii", "test.negposreturn")
+        handler = PosReturn()
+        codecs.register_error("test.posreturn", handler.handle)
 
-        def hugeposreturn(exc):
-            return (u"?", 424242)
-        codecs.register_error("test.hugeposreturn", hugeposreturn)
-        "\xff".decode("ascii", "test.hugeposreturn")
-        "\\uyyyy".decode("raw-unicode-escape", "test.hugeposreturn")
+        # Valid negative position
+        handler.pos = -1
+        self.assertEquals("\xff0".decode("ascii", "test.posreturn"), u"<?>0")
+
+        # Valid negative position
+        handler.pos = -2
+        self.assertEquals("\xff0".decode("ascii", "test.posreturn"), u"<?><?>")
+
+        # Negative position out of bounds
+        handler.pos = -3
+        self.assertRaises(IndexError, "\xff0".decode, "ascii", "test.posreturn")
+
+        # Valid positive position
+        handler.pos = 1
+        self.assertEquals("\xff0".decode("ascii", "test.posreturn"), u"<?>0")
+
+        # Largest valid positive position (one beyond end of input
+        handler.pos = 2
+        self.assertEquals("\xff0".decode("ascii", "test.posreturn"), u"<?>")
+
+        # Invalid positive position
+        handler.pos = 3
+        self.assertRaises(IndexError, "\xff0".decode, "ascii", "test.posreturn")
+
+        # Restart at the "0"
+        handler.pos = 6
+        self.assertEquals("\\uyyyy0".decode("raw-unicode-escape", "test.posreturn"), u"<?>0")
 
         class D(dict):
             def __getitem__(self, key):
@@ -579,22 +614,39 @@ class CodecCallbackTest(unittest.TestCase):
         codecs.register_error("test.badencodereturn2", badencodereturn2)
         self.assertRaises(TypeError, u"\xff".encode, "ascii", "test.badencodereturn2")
 
-        pos = [-42]
-        def negposreturn(exc):
-            pos[0] += 1 # use list to work around scoping problem
-            return (u"?", pos[0])
-        codecs.register_error("test.negposreturn", negposreturn)
-        u"\xff".encode("ascii", "test.negposreturn")
+        handler = PosReturn()
+        codecs.register_error("test.posreturn", handler.handle)
 
-        def hugeposreturn(exc):
-            return (u"?", 424242)
-        codecs.register_error("test.hugeposreturn", hugeposreturn)
-        u"\xff".encode("ascii", "test.hugeposreturn")
+        # Valid negative position
+        handler.pos = -1
+        self.assertEquals(u"\xff0".encode("ascii", "test.posreturn"), "<?>0")
+
+        # Valid negative position
+        handler.pos = -2
+        self.assertEquals(u"\xff0".encode("ascii", "test.posreturn"), "<?><?>")
+
+        # Negative position out of bounds
+        handler.pos = -3
+        self.assertRaises(IndexError, u"\xff0".encode, "ascii", "test.posreturn")
+
+        # Valid positive position
+        handler.pos = 1
+        self.assertEquals(u"\xff0".encode("ascii", "test.posreturn"), "<?>0")
+
+        # Largest valid positive position (one beyond end of input
+        handler.pos = 2
+        self.assertEquals(u"\xff0".encode("ascii", "test.posreturn"), "<?>")
+
+        # Invalid positive position
+        handler.pos = 3
+        self.assertRaises(IndexError, u"\xff0".encode, "ascii", "test.posreturn")
+
+        handler.pos = 0
 
         class D(dict):
             def __getitem__(self, key):
                 raise ValueError
-        for err in ("strict", "replace", "xmlcharrefreplace", "backslashreplace", "test.hugeposreturn"):
+        for err in ("strict", "replace", "xmlcharrefreplace", "backslashreplace", "test.posreturn"):
             self.assertRaises(UnicodeError, codecs.charmap_encode, u"\xff", err, {0xff: None})
             self.assertRaises(ValueError, codecs.charmap_encode, u"\xff", err, D())
             self.assertRaises(TypeError, codecs.charmap_encode, u"\xff", err, {0xff: 300})