#14731: refactor email policy framework.

This patch primarily does two things: (1) it adds some internal-interface
methods to Policy that allow for Policy to control the parsing and folding of
headers in such a way that we can construct a backward compatibility policy
that is 100% compatible with the 3.2 API, while allowing a new policy to
implement the email6 API.  (2) it adds that backward compatibility policy and
refactors the test suite so that the only differences between the 3.2
test_email.py file and the 3.3 test_email.py file is some small changes in
test framework and the addition of tests for bugs fixed that apply to the 3.2
API.

There are some additional teaks, such as moving just the code needed for the
compatibility policy into _policybase, so that the library code can import
only _policybase.  That way the new code that will be added for email6
will only get imported when a non-compatibility policy is imported.

1 files changed, 216 insertions, 0 deletions

diff --git a/Lib/email/architecture.rst b/Lib/email/architecture.rst
new file mode 100644
index 0000000..80d24fe
--- /dev/null
+++ b/Lib/email/architecture.rst
@@ -0,0 +1,216 @@
+:mod:`email` Package Architecture
+=================================
+
+Overview
+--------
+
+The email package consists of three major components:
+
+    Model
+        An object structure that represents an email message, and provides an
+        API for creating, querying, and modifying a message.
+
+    Parser
+        Takes a sequence of characters or bytes and produces a model of the
+        email message represented by those characters or bytes.
+
+    Generator
+        Takes a model and turns it into a sequence of characters or bytes.  The
+        sequence can either be intended for human consumption (a printable
+        unicode string) or bytes suitable for transmission over the wire.  In
+        the latter case all data is properly encoded using the content transfer
+        encodings specified by the relevant RFCs.
+
+Conceptually the package is organized around the model.  The model provides both
+"external" APIs intended for use by application programs using the library,
+and "internal" APIs intended for use by the Parser and Generator components.
+This division is intentionally a bit fuzy; the API described by this documentation
+is all a public, stable API.  This allows for an application with special needs
+to implement its own parser and/or generator.
+
+In addition to the three major functional components, there is a third key
+component to the architecture:
+
+    Policy
+        An object that specifies various behavioral settings and carries
+        implementations of various behavior-controlling methods.
+
+The Policy framework provides a simple and convenient way to control the
+behavior of the library, making it possible for the library to be used in a
+very flexible fashion while leveraging the common code required to parse,
+represent, and generate message-like objects.  For example, in addition to the
+default :rfc:`5322` email message policy, we also have a policy that manages
+HTTP headers in a fashion compliant with :rfc:`2616`.  Individual policy
+controls, such as the maximum line length produced by the generator, can also
+be controlled individually to meet specialized application requirements.
+
+
+The Model
+---------
+
+The message model is implemented by the :class:`~email.message.Message` class.
+The model divides a message into the two fundamental parts discussed by the
+RFC: the header section and the body.  The `Message` object acts as a
+pseudo-dictionary of named headers.  Its dictionary interface provides
+convenient access to individual headers by name.  However, all headers are kept
+internally in an ordered list, so that the information about the order of the
+headers in the original message is preserved.
+
+The `Message` object also has a `payload` that holds the body.  A `payload` can
+be one of two things: data, or a list of `Message` objects.  The latter is used
+to represent a multipart MIME message.  Lists can be nested arbitrarily deeply
+in order to represent the message, with all terminal leaves having non-list
+data payloads.
+
+
+Message Lifecycle
+-----------------
+
+The general lifecyle of a message is:
+
+    Creation
+        A `Message` object can be created by a Parser, or it can be
+        instantiated as an empty message by an application.
+
+    Manipulation
+        The application may examine one or more headers, and/or the
+        payload, and it may modify one or more headers and/or
+        the payload.  This may be done on the top level `Message`
+        object, or on any sub-object.
+
+    Finalization
+        The Model is converted into a unicode or binary stream,
+        or the model is discarded.
+
+
+
+Header Policy Control During Lifecycle
+--------------------------------------
+
+One of the major controls exerted by the Policy is the management of headers
+during the `Message` lifecycle.  Most applications don't need to be aware of
+this.
+
+A header enters the model in one of two ways: via a Parser, or by being set to
+a specific value by an application program after the Model already exists.
+Similarly, a header exits the model in one of two ways: by being serialized by
+a Generator, or by being retrieved from a Model by an application program.  The
+Policy object provides hooks for all four of these pathways.
+
+The model storage for headers is a list of (name, value) tuples.
+
+The Parser identifies headers during parsing, and passes them to the
+:meth:`~email.policy.Policy.header_source_parse` method of the Policy.  The
+result of that method is the (name, value) tuple to be stored in the model.
+
+When an application program supplies a header value (for example, through the
+`Message` object `__setitem__` interface), the name and the value are passed to
+the :meth:`~email.policy.Policy.header_store_parse` method of the Policy, which
+returns the (name, value) tuple to be stored in the model.
+
+When an application program retrieves a header (through any of the dict or list
+interfaces of `Message`), the name and value are passed to the
+:meth:`~email.policy.Policy.header_fetch_parse` method of the Policy to
+obtain the value returned to the application.
+
+When a Generator requests a header during serialization, the name and value are
+passed to the :meth:`~email.policy.Policy.fold` method of the Policy, which
+returns a string containing line breaks in the appropriate places.  The
+:meth:`~email.policy.Policy.cte_type` Policy control determines whether or
+not Content Transfer Encoding is performed on the data in the header.  There is
+also a :meth:`~email.policy.Policy.binary_fold` method for use by generators
+that produce binary output, which returns the folded header as binary data,
+possibly folded at different places than the corresponding string would be.
+
+
+Handling Binary Data
+--------------------
+
+In an ideal world all message data would conform to the RFCs, meaning that the
+parser could decode the message into the idealized unicode message that the
+sender originally wrote.  In the real world, the email package must also be
+able to deal with badly formatted messages, including messages containing
+non-ASCII characters that either have no indicated character set or are not
+valid characters in the indicated character set.
+
+Since email messages are *primarily* text data, and operations on message data
+are primarily text operations (except for binary payloads of course), the model
+stores all text data as unicode strings.  Un-decodable binary inside text
+data is handled by using the `surrogateescape` error handler of the ASCII
+codec.  As with the binary filenames the error handler was introduced to
+handle, this allows the email package to "carry" the binary data received
+during parsing along until the output stage, at which time it is regenerated
+in its original form.
+
+This carried binary data is almost entirely an implementation detail.  The one
+place where it is visible in the API is in the "internal" API.  A Parser must
+do the `surrogateescape` encoding of binary input data, and pass that data to
+the appropriate Policy method.  The "internal" interface used by the Generator
+to access header values preserves the `surrogateescaped` bytes.  All other
+interfaces convert the binary data either back into bytes or into a safe form
+(losing information in some cases).
+
+
+Backward Compatibility
+----------------------
+
+The :class:`~email.policy.Policy.Compat32` Policy provides backward
+compatibility with version 5.1 of the email package.  It does this via the
+following implementation of the four+1 Policy methods described above:
+
+header_source_parse
+    Splits the first line on the colon to obtain the name, discards any spaces
+    after the colon, and joins the remainder of the line with all of the
+    remaining lines, preserving the linesep characters to obtain the value.
+    Trailing carriage return and/or linefeed characters are stripped from the
+    resulting value string.
+
+header_store_parse
+    Returns the name and value exactly as received from the application.
+
+header_fetch_parse
+    If the value contains any `surrogateescaped` binary data, return the value
+    as a :class:`~email.header.Header` object, using the character set
+    `unknown-8bit`.  Otherwise just returns the value.
+
+fold
+    Uses :class:`~email.header.Header`'s folding to fold headers in the
+    same way the email5.1 generator did.
+
+binary_fold
+    Same as fold, but encodes to 'ascii'.
+
+
+New Algorithm
+-------------
+
+header_source_parse
+    Same as legacy behavior.
+
+header_store_parse
+    Same as legacy behavior.
+
+header_fetch_parse
+    If the value is already a header object, returns it.  Otherwise, parses the
+    value using the new parser, and returns the resulting object as the value.
+    `surrogateescaped` bytes get turned into unicode unknown character code
+    points.
+
+fold
+    Uses the new header folding algorithm, respecting the policy settings.
+    surrogateescaped bytes are encoded using the ``unknown-8bit`` charset for
+    ``cte_type=7bit`` or ``8bit``.  Returns a string.
+
+    At some point there will also be a ``cte_type=unicode``, and for that
+    policy fold will serialize the idealized unicode message with RFC-like
+    folding, converting any surrogateescaped bytes into the unicode
+    unknown character glyph.
+
+binary_fold
+    Uses the new header folding algorithm, respecting the policy settings.
+    surrogateescaped bytes are encoded using the `unknown-8bit` charset for
+    ``cte_type=7bit``, and get turned back into bytes for ``cte_type=8bit``.
+    Returns bytes.
+
+    At some point there will also be a ``cte_type=unicode``, and for that
+    policy binary_fold will serialize the message according to :rfc:``5335``.