Python: Fix header field values character encoding.

On GitHub, @RomainMou reported an issue whereby HTTP header field values
where being incorrectly reported as non-ascii by the Python .isacii()
method.

For example, using the following test application

  def application(environ, start_response):
      t = environ['HTTP_ASCIITEST']

      t = "'" + t + "'" +  " (" + str(len(t)) + ")"

      if t.isascii():
          t = t + " [ascii]"
      else:
          t = t + " [non-ascii]"

      resp = t + "\n\n"

      start_response("200 OK", [("Content-Type", "text/plain")])
      return (bytes(resp, 'latin1'))

You would see the following

  $ curl -H "ASCIITEST: $" http://localhost:8080/
  '$' (1) [non-ascii]

'$' has an ASCII code of 0x24 (36).

The initial idea was to adjust the second parameter to the
PyUnicode_New() call from 255 to 127. This unfortunately had the
opposite effect.

  $ curl -H "ASCIITEST: $" http://localhost:8080/
  '$' (1) [ascii]

Good. However...

  $ curl -H "ASCIITEST: £" http://localhost:8080/
  '£' (2) [ascii]

Not good. Let's take a closer look at this.

'£' is not in basic ASCII, but is in extended ASCII with a value of 0xA3
(163). Its UTF-8 encoding is 0xC2 0xA3, hence the length of 2 bytes
above.

  $ strace -s 256 -e sendto,recvfrom curl -H "ASCIITEST: £" http://localhost:8080/
  sendto(5, "GET / HTTP/1.1\r\nHost: localhost:8080\r\nUser-Agent: curl/8.0.1\r\nAccept: */*\r\nASCIITEST: \302\243\r\n\r\n", 92, MSG_NOSIGNAL, NULL, 0) = 92
  recvfrom(5, "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nServer: Unit/1.30.0\r\nDate: Mon, 22 May 2023 12:44:11 GMT\r\nTransfer-Encoding: chunked\r\n\r\n12\r\n'\302\243' (2) [ascii]\n\n\r\n0\r\n\r\n", 102400, 0, NULL, NULL) = 160
  '£' (2) [ascii]

So we can see curl sent it UTF-8 encoded '\302\243\' which is C octal
escaped UTF-8 for 0xC2 0xA3, and we got the same back. But it should not
be marked as ASCII.

When doing PyUnicode_New(size, 127) it sets the buffer as ASCII. So we
need to use another function and that function would appear to be

  PyUnicode_DecodeCharmap()

Which creates an Unicode object with the correct ascii/non-ascii
properties based on the character encoding.

With this function we now get

  $ curl -H "ASCIITEST: $" http://localhost:8080/
  '$' (1) [ascii]

  $ curl -H "ASCIITEST: £" http://localhost:8080/
  '£' (2) [non-ascii]

and for good measure

  $ curl -H "ASCIITEST: $ £" http://localhost:8080/
  '$ £' (4) [non-ascii]

  $ curl -H "ASCIITEST: $" -H "ASCIITEST: £" http://localhost:8080/
  '$, £' (5) [non-ascii]

PyUnicode_DecodeCharmap() does require having the full string upfront so
we need to build up the potentially comma separated header field values
string before invoking this function.

I did not want to touch the Python 2.7 code (which may or may not even
be affected by this) so kept these changes completely isolated from
that, hence a slight duplication with the for () loop.

Python 2.7 was sunset on January 1st 2020[0], so this code will
hopefully just disappear soon anyway.

I also purposefully didn't touch other code that may well have similar
issues (such as the HTTP header field names) if we ever get issue
reports about them, we'll deal with them then.

[0]: <https://www.python.org/doc/sunset-python-2/>

Link: <https://docs.python.org/3/c-api/unicode.html>
Closes: <#868>
Reported-by: RomainMou <https://github.com/RomainMou>
Tested-by: RomainMou <https://github.com/RomainMou>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

src/python/nxt_python_wsgi.c

@@ -866,10 +866,35 @@ nxt_python_field_value(nxt_unit_field_t *f, int n, uint32_t vl)
     src = nxt_unit_sptr_get(&f->value);
 
 #if PY_MAJOR_VERSION == 3
-    res = PyUnicode_New(vl, 255);
+    if (nxt_slow_path(n > 1)) {
+        char  *ptr;
+
+        p = nxt_unit_malloc(NULL, vl + 1);
+        if (nxt_slow_path(p == NULL)) {
+            return NULL;
+        }
+
+        ptr = p;
+        p = nxt_cpymem(p, src, f->value_length);
+
+        for (i = 1; i < n; i++) {
+            p = nxt_cpymem(p, ", ", 2);
+
+            src = nxt_unit_sptr_get(&f[i].value);
+            p = nxt_cpymem(p, src, f[i].value_length);
+        }
+        *p = '\0';
+
+        src = ptr;
+    }
+
+    res = PyUnicode_DecodeCharmap(src, vl, NULL, NULL);
+
+    if (nxt_slow_path(n > 1)) {
+        nxt_unit_free(NULL, src);
+    }
 #else
     res = PyString_FromStringAndSize(NULL, vl);
-#endif
 
     if (nxt_slow_path(res == NULL)) {
         return NULL;
@@ -885,6 +910,7 @@ nxt_python_field_value(nxt_unit_field_t *f, int n, uint32_t vl)
         src = nxt_unit_sptr_get(&f[i].value);
         p = nxt_cpymem(p, src, f[i].value_length);
     }
+#endif
 
     return res;
 }