This is a very convenient feature introduced in Python 3.6 by PEP 515.

This calls finalisers of things like files and sockets to cleanly close
them.

This patch adds the gc_sweep_all() function which does a garbage collection
without tracing any root pointers, so frees all the memory, and most
importantly runs any remaining finalisers.

This helps primarily for soft reset: it will close any open files, any open
sockets, and help to get the system back to a clean state upon soft reset.

The ST DFU bootloader supports a transfer size up to 2048 bytes, so send
that much data on each download (to device) packet.  This almost halves
total download time.

The DFU USB config descriptor returns 0x0800=2048 for the supported
transfer size, and this applies to both TX (IN) and RX (OUT).  So increase
the rx_buf to support this size without having a buffer overflow on
received data.

With this patch mboot in USB DFU mode now works with dfu-util.

Currently <WLAN>.isconnected() always returns True if a static IP is set,
regardless of the state of the connection.

This patch introduces a new flag 'wifi_sta_connected' which is set in
event_handler() when GOT_IP event is received and reset when DISCONNECTED
event is received (unless re-connect is successful).  isconnected() now
simply returns the status of this flag (for STA_IF).

The pre-existing flag misleadingly named 'wifi_sta_connected" is also
renamed to 'wifi_sta_connect_requested'.

Fixes issue #3837

For i2c.py: the accelerometer now uses the new I2C driver so need to
explicitly init the legacy i2c object to get the test working.

For pyb1.py: the legacy pyb.hid() call will crash if the USB_HID object is
not initialised.

MICROPY_PY_DELATTR_SETATTR can now be enabled without a performance hit for
classes that don't use this feature.

MICROPY_PY_BUILTINS_NOTIMPLEMENTED is a minor addition that improves
compatibility with CPython.

They are now efficient (in runtime performance) and provide a useful
feature that's hard to obtain without them enabled.

See issue #3644 and PR #3826 for background.

This patch is a code optimisation, trading text bytes for speed.  On
pyboard it's an increase of 0.06% in code size for a gain (in pystone
performance) of roughly 6.5%.

The patch optimises load/store/delete of attributes in user defined classes
by not looking up special accessors (@property, __get__, __delete__,
__set__, __setattr__ and __getattr_) if they are guaranteed not to exist in
the class.

Currently, if you do my_obj.foo() then the runtime has to do a few checks
to see if foo is a property or has __get__, and if so delegate the call.
And for stores things like my_obj.foo = 1 has to first check if foo is a
property or has __set__ defined on it.

Doing all those checks each and every time the attribute is accessed has a
performance penalty.  This patch eliminates all those checks for cases when
it's guaranteed that the checks will always fail, ie no attributes are
properties nor have any special accessor methods defined on them.

To make this guarantee it checks all attributes of a user-defined class
when it is first created.  If any of the attributes of the user class are
properties or have special accessors, or any of the base classes of the
user class have them, then it sets a flag in the class to indicate that
special accessors must be checked for.  Then in the load/store/delete code
it checks this flag to see if it can take the shortcut and optimise the
lookup.

It's an optimisation that's pretty widely applicable because it improves
lookup performance for all methods of user defined classes, and stores of
attributes, at least for those that don't have special accessors.  And, it
allows to enable descriptors with minimal additional runtime overhead if
they are not used for a particular user class.

There is one restriction on dynamic class creation that has been introduced
by this patch: a user-defined class cannot go from zero special accessors
to one special accessor (or more) after that class has been subclassed.  If
the script attempts this an AttributeError is raised (see addition to
tests/misc/non_compliant.py for an example of this case).

The cost in code space bytes for the optimisation in this patch is:

   unix x64:  +528
unix nanbox:  +508
      stm32:  +192
     cc3200:  +200
    esp8266:  +332
      esp32:  +244

Performance tests that were done:

- on unix x86-64, pystone improved by about 5%
- on pyboard, pystone improved by about 6.5%, from 1683 up to 1794
- on pyboard, bm_chaos (from CPython benchmark suite) improved by about 5%
- on esp32, pystone improved by about 30% (but there are caching effects)
- on esp32, bm_chaos improved by about 11%

mp_obj_is_instance_type() can be used instead to check for instance types.

Coveralls requires a "recent" version of urllib3, whereas requests requires
a "not so recent" version, less than 1.23.  So force urllib3 v1.22 to get
it all working.

At least to document it's existence.

Following other C-level protocols, this VFS protocol is added to help
abstract away implementation details of the underlying VFS in an efficient
way.  As a starting point, the import_stat function is put into this
protocol so that the VFS sub-system does not need to know about every VFS
implementation in order to do an efficient stat for importing files.

In the future it might be worth adding other functions to this protocol.

Now that the coverage build has fully switched to the VFS sub-system these
functions were no longer available, so add them to the uos_vfs module.

Also, vfs_open is no longer needed, it's available as the built-in open.

This conditional import was only used to get the tests working on the unix
coverage build, which has now switched to use VFS by default so the uos
module alone has the required functionality.

The unix coverage build is now switched fully to the VFS implementation, ie
the uos module is the uos_vfs module.  For example, one can now sandbox uPy
to their home directory via:

    $ ./micropython_coverage

    >>> import uos
    >>> uos.umount('/') # unmount existing root VFS
    >>> vfs = uos.VfsPosix('/home/user') # create new POSIX VFS
    >>> uos.mount(vfs, '/') # mount new POSIX VFS at root

Some filesystem/OS features may no longer work with the coverage build due
to this change, and these need to be gradually fixed.

The standard unix port remains unchanged, it still uses the traditional uos
module which directly accesses the underlying host filesystem.

This VFS component allows to mount a host POSIX filesystem within the uPy
VFS sub-system.  All traditional POSIX file access then goes through the
VFS, allowing to sandbox a uPy process to a certain sub-dir of the host
system, as well as mount other filesystem types alongside the host
filesystem.

So they don't clash with other VFS implementations.

It should be up to the user if they want to print the new time out or not.

Fixes issue #3766.

This patch adds support for building the firmware with external SPI RAM
enabled.  It is disabled by default because it adds overhead (due to
silicon workarounds) and reduces performance (because it's slower to have
bytecode and objects stored in external RAM).

To enable it, either use "make CONFIG_SPIRAM_SUPPORT=1", or add this line
to you custom makefile/GNUmakefile (before "include Makefile"):

    CONFIG_SPIRAM_SUPPORT = 1

When this option is enabled the MicroPython heap is automatically allocated
in external SPI RAM.

Thanks to Angus Gratton for help with the compiler and linker settings.

Since a long time now, mp_obj_type_t no longer refers explicitly to
mp_stream_p_t but rather to an abstract "const void *protocol".  So there's
no longer any need to define mp_stream_p_t in obj.h and it can go with all
its associated definitions in stream.h.  Pretty much all users of this type
will already include the stream header.

The Wiznet5k series of chips support a MACRAW mode which allows the host to
send and receive Ethernet frames directly.  This can be hooked into the
lwIP stack to provide a full "socket" implementation using this Wiznet
Ethernet device.  This patch adds support for this feature.

To enable the feature one must add the following to mpconfigboard.mk, or
mpconfigport.mk:

    MICROPY_PY_WIZNET5K = 5500

and the following to mpconfigboard.h, or mpconfigport.h:

    #define MICROPY_PY_LWIP (1)

After wiring up the module (X5=CS, X4=RST), usage on a pyboard is:

    import time, network
    nic = network.WIZNET5K(pyb.SPI(1), pyb.Pin.board.X5, pyb.Pin.board.X4)
    nic.active(1)
    while not nic.isconnected():
        time.sleep_ms(50) # needed to poll the NIC
    print(nic.ifconfig())

Then use the socket module as usual.

Compared to using the built-in TCP/IP stack on the Wiznet module, some
performance is lost in MACRAW mode: with a lot of memory allocated to lwIP
buffers, lwIP gives Around 750,000 bytes/sec max TCP download, compared
with 1M/sec when using the TCP/IP stack on the Wiznet module.

All it needs is a lwIP netif to function.

mod_network_nic_type_t doesn't need to be an actual uPy type, it just needs
to be an object.

It should be up to the NIC itself to decide if the network interface is
removed upon soft reset.  Some NICs can keep the interface up over a soft
reset, which improves usability of the network.

If mbedtls_ctr_drbg_seed() is available in the mbedtls bulid then so should
be mbedtls_entropy_func().  Then it's up to the port to configure a valid
entropy source, eg via MBEDTLS_ENTROPY_HARDWARE_ALT.

Otherwise the "sock" member may have an undefined value if wrap_socket
fails with an exception and exits early, and then if the finaliser runs it
will try to close an invalid stream object.

Fixes issue #3828.

Fixes assertion failures and segmentation faults when making calls like:

    super(1, 1).x

Fixes assertion failures when the arguments to type() were not of valid
types, e.g., when making calls like:

    type("", (), 3)
    type("", 3, {})