subtract_refs() now pass the parent object to visit_decref() which
pass it to _PyObject_ASSERT(). So if the "is freed" assertion fails,
the parent is used in debug trace, rather than the freed object. The
parent object is more likely to contain useful information. Freed
objects cannot be inspected are are displayed as "<object at xxx is
freed>" with no other detail.

Some objects like Py_None are not initialized with conventional means
that prepare the circular linked list pointers, leaving them unlinked
from the rest of the objects. For those objects, NULL pointers does
not mean that they are freed, so we need to skip the check in those
cases.

In debug mode, PyObject_GC_Track() now calls tp_traverse() of the
object type to ensure that the object is valid: test that objects
visited by tp_traverse() are valid.

Fix pyexpat.c: only track the parser in the GC once the parser is
fully initialized.

Add a newline between the verbose object dump and the Py_FatalError()
logs for readability.

Prior to 3.7, re.escape escaped many characters that don't have
special meaning in Python, but that use to require escaping in other
tools and languages. This commit aims to make it clear which characters
were, but are no longer escaped.

Fix _PyBytesWriter API when Python is built in release mode with
assertions.

bpo-36389, bpo-38376: The _PyObject_CheckConsistency() function is
now also available in release mode. For example, it can be used to
debug a crash in the visit_decref() function of the GC.

Modify the following functions to also work in release mode:

* _PyDict_CheckConsistency()
* _PyObject_CheckConsistency()
* _PyType_CheckConsistency()
* _PyUnicode_CheckConsistency()

Other changes:

* _PyMem_IsPtrFreed(ptr) now also returns 1 if ptr is NULL
  (equals to 0).
* _PyBytesWriter_CheckConsistency() now returns 1 and is only used
  with assert().
* Reorder _PyObject_Dump() to write safe fields first, and only
  attempt to render repr() at the end.

Calling _Py_wfopen() is enough to check if filename is an existing
file or not. There is no need to check first isfile().

The "tmp" variable is no longer used.

The `required` argument to `argparse.add_subparsers` was added in #3027. This PR specifies the earliest version of Python where it is available.


https://bugs.python.org/issue26510



Automerge-Triggered-By: @merwok

KeyError should cause a failure in parsing the encoded word and should be caught and raised as a _InvalidEWError instead.

Add SMTPNotSupportedError in the exports of smtplib
Co-Authored-By: Brandt Bucher <brandtbucher@gmail.com>

* _Py_FindEnvConfigValue() now returns a string allocated
  by PyMem_RawMalloc().
* calculate_init() now decodes VPATH macro.
* Add calculate_open_pyenv() function.
* Add substring() and joinpath2() functions.

* Fix add_exe_suffix()

And a few cleanup changes.

* PyCalculatePath: add "_macro" to the 3 fields storing macro values
* Add dynamically allocated strings to PyCalculatePath

Revert the removal of PyThreadState_DeleteCurrent() with documentation.

On Windows use UTF-16 (or UTF-32 for 32-bit Tcl_UniChar) with the
"surrogatepass" error handler for converting to/from Tcl Unicode objects.

On Linux use UTF-8 with the "surrogateescape" error handler for converting
to/from Tcl String objects.

Converting strings from Tcl to Python and back now never fails
(except MemoryError).

* ismodule() no longer modify the input string.
* copy_absolute(), joinpath(): rename parameters and local variables.

Following symbolic links is now limited to 40 attempts, just to
prevent loops.

Add subfunctions:

* Add resolve_symlinks()
* Add calculate_argv0_path_framework()
* Add calculate_which()
* Add calculate_program_macos()

Fix also _Py_wreadlink(): readlink() result type is Py_ssize_t, not
int.

Replace "_PyRuntimeState" with "struct pyruntimestate" to avoid
a warning on typedef re-definition.

Replace "_PyRuntimeState" with "struct pyruntimestate" to avoid
a warning on typedef re-definition.

calculate_argv0_path() must read argv0_path link, not read
program_full_path link.

* Add log() method: add timestamp and load average prefixes
  to main messages.
* WindowsLoadTracker:

  * LOAD_FACTOR_1 is now computed using SAMPLING_INTERVAL
  * Initialize the load to the arithmetic mean of the first 5 values
    of the Processor Queue Length value (so over 5 seconds), rather
    than 0.0.
  * Handle BrokenPipeError and when typeperf exit.

* format_duration(1.5) now returns '1.5 sec', rather than
  '1 sec 500 ms'

The last line is now passed to the parser even if it does not end
with a newline, but only if it's a valid value.

WindowsLoadTracker.read_output() now uses a short buffer for
incomplete line.

Use forward declaration of types to avoid includes in the internal C
API. Add also comment to justify other includes.

test.pythoninfo now logs environment variables used by OpenSSL and
Python ssl modules, and logs attributes of 3 SSL contexts
(SSLContext, default HTTPS context, stdlib context).

* Fix TestWorkerProcess.__repr__(): start_time is only valid
  if _popen is not None.
* Fix _kill(): don't set _killed to True if _popen is None.
* _run_process(): only set _killed to False after calling
  run_test_in_subprocess().