[net] Add documentation for feature process & supported platforms

net/docs/life-of-a-feature.md

+# Life of a Feature
+
+In the years since the Chromium browser was first open-sourced, the `//net`
+directory has expanded from being the basis of loading web content in the
+Chromium browser to accommodating a wide variety of networking needs,
+both in the Chromium browser and in other Google and third-party products
+and projects.
+
+This brings with it many new opportunities, such as the ability to
+introduce new protocols rapidly or push Web security forward, as well as
+new challenges, such as how to balance the needs of various `//net`
+consumers effectively.
+
+To make it easier to contribute new features or to change existing
+behaviors in `//net`, this document tries to capture the life of a
+feature in `//net`, from initial design to the eventual possibility of
+deprecation and removal.
+
+## Supported Projects
+
+When considering the introduction of a new `//net` feature or changing
+a `//net` behavior, it's first necessary to understand where `//net`
+is used, how it is used, and what the various constraints and limits are.
+
+To understand a more comprehensive matrix of the supported platforms and
+constraints, see [Supported Projects](supported-projects.md). When
+examining a new feature request, or a change in behavior, it's necessary
+to consider dimensions such as:
+
+  * Does this feature apply to all supported projects, or is this something
+    like a Browser-only feature?
+  * Does this feature apply to all supported platforms, or is this something
+    specific to a particular subset?
+  * Is the feature a basic networking library feature, or is it specific to
+    something in the Web Platform?
+  * Will some projects wish to strip the feature in order to meet targets
+    such as memory usage (RAM) or binary size?
+  * Does it depend on Google services or Google-specific behaviors or
+    features?
+  * How will this feature be tested / experimented with? For example,
+    __Field Trials (Finch)__ and __User Metrics (UMA)__ may not be available
+    on a number of platforms.
+  * How risky is the feature towards compatibility/stability? How will it
+    be undone if there is a bug?
+  * Are the power/memory/CPU/bandwidth requirements appropriate for the
+    targeted projects and/or platforms? 
+
+## Design and Layering
+
+Once the supported platforms and constraints are identified, it's necessary
+to determine how to actually design the feature to meet those constraints,
+in hopefully the easiest way possible both for implementation and consumption.
+
+### Designing for multiple platforms
+
+In general, `//net` features try to support all platforms with a common
+interface, and generally eschew OS-specific interfaces from being exposed as
+part of `//net`.
+
+Cross-platform code is generally done via declaring an interface named
+`foo.h`, which is common for all platforms, and then using the build-system to
+do compile-time switching between implementations in `foo_win.cc`, `foo_mac.cc`,
+`foo_android.cc`, etc.
+
+The goal is to ensure that consumers generally don't have to think about
+OS-specific considerations, and can instead code to the interface.
+
+### Designing for multiple products
+
+While premature abstraction can significantly harm readability, if it is
+anticipated that different products will have different implementation needs,
+or may wish to selectively disable the feature, it's often necessary to
+abstract that interface sufficiently in `//net` to allow for dependency
+injection.
+
+This is true whether discussing concrete classes and interfaces or something
+as simple a boolean configuration flag that different consumers wish to set
+differently.
+
+The two most common approaches in `//net` are injection and delegation.
+
+#### Injection
+
+Injection refers to the pattern of defining the interface or concrete
+configuration parameter (such as a boolean), along with the concrete
+implementation, but requiring the `//net` embedder to supply an instance
+of the interface or the configuration parameters (perhaps optionally).
+
+Examples of this pattern include things such as the `ProxyConfigService`,
+which has concrete implementations in `//net` for a variety of platforms'
+configuration of proxies, but which requires it be supplied as part of the
+`URLRequestContextGetter` building, if proxies are going to be supported.
+
+An example of injecting configuration flags can be seen in the
+`HttpNetworkSession::Params` structure, which is used to provide much of
+the initialization parameters for the HTTP layer.
+
+The ideal form of injection is to pass ownership of the injected object,
+such as via a `std::unique_ptr<Foo>`. While this is not consistently
+applied in `//net`, as there are a number of places in which ownership is
+either shared or left to the embedder, with the injected object passed
+around as a naked/raw pointer, this is generally seen as an anti-pattern
+and not to be mirrored for new features.
+
+#### Delegation
+
+Delegation refers to forcing the `//net` embedder to respond to specific
+delegated calls via a Delegate interface that it implements. In general,
+when using the delegate pattern, ownership of the delegate should be
+transferred, so that the lifetime and threading semantics are clear and
+unambiguous.
+
+That is, for a given class `Foo`, which has a `Foo::Delegate` interface
+defined to allow embedders to alter behavior, prefer a constructor that
+is
+```
+explicit Foo(std::unique_ptr<Delegate> delegate);
+```
+so that it is clear that the lifetime of `delegate` is determined by
+`Foo`.
+
+While this may appear similar to Injection, the general difference
+between the two approaches is determining where the bulk of the
+implementation lies. With Injection, the interface describes a
+behavior contract that concrete implementations must adhere to; this
+allows for much more flexibility with behavior, but with the downside
+of significantly more work to implement or extend. Delegation attempts
+to keep the bulk of the implementation in `//net`, and the decision as
+to which implementation to use in `//net`, but allows `//net` to
+provide specific ways in which embedders can alter behaviors.
+
+The most notable example of the delegate pattern is `URLRequest::Delegate`,
+which keeps the vast majority of the loading logic within `URLRequest`,
+but allows the `URLRequest::Delegate` to participate during specific times
+in the request lifetime and alter specific behaviors as necessary. (Note:
+`URLRequest::Delegate`, like much of the original `//net` code, doesn't
+adhere to the recommended lifetime patterns of passing ownership of the
+Delegate. It is from the experience debugging and supporting these APIs
+that the `//net` team strongly encourages all new code pass explicit
+ownership, to reduce the complexity and risk of lifetime issues).
+
+While the use of a `base::Callback` can also be considered a form of
+delegation, the `//net` layer tries to eschew any callbacks that can be
+called more than once, and instead favors defining class interfaces
+with concrete behavioral requirements in order to ensure the correct
+lifetimes of objects and to adjust over time. When `//net` takes a
+callback (e.g. `net::CompletionCallback`), the intended pattern is to
+signal the asynchronous completion of a single method, invoking that
+callback at most once before deallocating it. For more discussion
+of these patterns, see [Code Patterns](code-patterns.md).
+
+### Understanding the Layering
+
+A significant challenge many feature proposals face is understanding the
+layering in `//net` and what different portions of `//net` are allowed to
+know. 
+
+#### Socket Pools
+
+The most common challenge feature proposals encounter is the awareness
+that the act of associating an actual request to make with a socket is
+done lazily, referred to as "late-binding".
+
+With late-bound sockets, a given `URLRequest` will not be assigned an actual
+transport connection until the request is ready to be sent. This allows for
+reprioritizing requests as they come in, to ensure that higher priority requests
+get preferential treatment, but it also means that features or data associated
+with a `URLRequest` generally don't participate in socket establishment or
+maintenance.
+
+For example, a feature that wants to associate the low-level network socket
+with a `URLRequest` during connection establishment is not something that the
+`//net` design supports, since the `URLRequest` is kept unaware of how sockets
+are established by virtue of the socket pools and late binding. This allows for
+more flexibility when working to improve performance, such as the ability to
+coalesce multiple logical 'sockets' over a single HTTP/2 or QUIC stream, which
+may only have a single physical network socket involved.
+
+#### Making Additional Requests
+
+From time to time, `//net` feature proposals will involve needing to load
+a secondary resource as part of processing. For example, SDCH involves loading
+additional dictionaries that are advertised in a header, and other feature
+proposals have involved fetching a `/.well-known/` URI or reporting errors to
+a particular URL.
+
+This is particularly challenging, because often, these features are implemented
+deeper in the network stack, such as [`//net/cert`](../cert), [`//net/http`](../http),
+or [`//net/filter`](../filter), which [`//net/url_request`](../url_request) depends
+on. Because `//net/url_request` depends on these low-level directories, it would
+be a circular dependency to have these directories depend on `//net/url_request`,
+and circular dependencies are forbidden.
+
+The recommended solution to address this is to adopt the delegation or injection
+patterns. The lower-level directory will define some interface that represents the
+"I need this URL" request, and then elsewhere, in a directory allowed to depend
+on `//net/url_request`, an implementation of that interface/delegate that uses
+`//net/url_request` is implemented.
+
+### Understanding the Lifetimes
+
+Understanding the object lifetime and dependency graph can be one of the largest
+challenges to contributing and maintaining `//net`. As a consequence, features
+which require introducing more complexity to the lifetimes of objects generally
+have a greater challenge to acceptance.
+
+The `//net` stack is designed heavily around a sync-or-async pattern, as
+documented in [Code Patterns](code-patterns.md), while also having a strong
+requirement that it be possible to cleanly shutdown the network stack. As a
+consequence, features should have precise, well-defined lifetime semantics
+and support graceful cleanup. Further, because much of the network stack can
+have web-observable side-effects, it is often required for tasks to have
+defined sequencing that cannot be reordered. To be ensure these requirements
+are met, features should attempt to model object lifetimes as a hierarchical
+DAG, using explicit ownership and avoiding the use of reference counting or
+weak pointers as part of any of the exposed API contracts (even for features
+only consumed in `//net`). Features that pay close attention to the lifetime
+semantics are more likely to be reviewed and accepted than those that leave
+it ambiguous.
+
+In addition to preferring explicit lifetimes, such as through judicious use of
+`std::unique_ptr<>` to indicate ownership transfer of dependencies, many
+features in `//net` also expect that if a `base::Callback` is involved (which
+includes `net::CompletionCallback`), then it's possible that invoking the
+callback may result in the deletion of the current (calling) object. As
+further expanded upon in [Code Patterns](code-patterns.md), features and
+changes should be designed such that any callback invocation is the last
+bit of code executed, and that the callback is accessed via the stack (such
+as through the use of either `base::ResetAndReturn(callback_).Run()` or
+`std::move(callback_).Run()`.
+
+### Specs: What Are They Good For
+
+As `//net` is used as the basis for a number of browsers, it's an important part
+of the design philosophy to ensure behaviors are well-specified, and that the
+implementation conforms to those specifications. This may be seen as burdensome
+when it's unclear whether or not a feature will 'take off,' but it's equally
+critical to ensure that the Chromium projects do not fork the Web Platform.
+
+#### Incubation Is Required
+
+`//net` respects Chromium's overall position of [incubation first](https://groups.google.com/a/chromium.org/d/msg/blink-dev/PJ_E04kcFb8/baiLN3DTBgAJ) standards development.
+
+With an incubation first approach, before introducing any new features that
+might be exposed over the wire to servers, whether they are explicit behaviors,
+such as adding new headers, or implicit behaviors such as
+[Happy Eyeballs](https://tools.ietf.org/html/rfc6555), should have some form
+of specification written. That specification should at least be on an
+incubation track, and the expectation is that the specification should have a
+direct path to an appropriate standards track. Features which don't adhere to
+this pattern, or which are not making progress towards a standards track, will
+require high-level approvals, to ensure that the Platform doesn't fragment.
+
+#### Introducing New Headers
+
+A common form of feature request is the introduction of new headers, either via
+the `//net` implementation directly, or through consuming `//net` interfaces
+and modifying headers on the fly.
+
+The introduction of any additional headers SHOULD have an incubated spec attached,
+ideally with cross-vendor interest. Particularly, headers which only apply to
+Google or Google services are very likely to be rejected outright.
+
+#### Making Additional Requests
+
+While it's necessary to provide abstraction around `//net/url_request` for
+any lower-level components that may need to make additional requests, for most
+features, that's not all that is necessary. Because `//net/url_request` only
+provides a basic HTTP fetching mechanism, it's insufficient for any Web Platform
+feature, because it doesn't consider the broader platform concerns such as
+interactions with CORS, Service Workers, cookie and authentication policies, or
+even basic interactions with optional features like Extensions or SafeBrowsing.
+
+To account for all of these things, any resource fetching that is to support
+a feature of the Web Platform, whether because the resource will be directly
+exposed to web content (for example, an image load or prefetch) or because it
+is in response to invoking a Web Platform API (for example, invoking the
+credential management API), the feature's resource fetching should be
+explainable in terms of the  [Fetch Living Standard](https://fetch.spec.whatwg.org/).
+The Fetch standard defines a JavaScript API for fetching resources, but more
+importantly, defines a common set of infrastructure and terminology that
+tries to define how all resource loads in the Web Platform happen - whether
+it be through the JavaScript API, through `XMLHttpRequest`, or the `src`
+attribute in HTML tags, for example.
+
+This also includes any resource fetching that wishes to use the same socket
+pools or caches as the Web Platform, to ensure that every resource that is
+web exposed (directly or indirectly) is fetched in a consistent and
+well-documented way, thus minimizing platform fragmentation and security
+issues.
+
+There are exceptions to this, however, but they're generally few and far
+between. In general, any feature that needs to define an abstraction to
+allow it to "fetch resources," likely needs to also be "explained in terms
+of Fetch".
+
+## Implementation
+
+In general, prior to implementing, try to get a review on net-dev@chromium.org
+for the general feedback and design review.
+
+In addition to the net-dev@chromium.org early review, `//net` requires that any
+browser-exposed behavior should also adhere to the
+[Blink Process](https://www.chromium.org/blink#new-features), which includes an
+"Intent to Implement" message to blink-dev@chromium.org
+
+For features that are unclear about their future, such as experiments or trials,
+it's also expected that the design planning will also account for how features
+will be removed cleanly. For features that radically affect the architecture of
+`//net`, expect a high bar of justification, since reversing those changes if
+they fail to pan out can cause significant disruption to productivity and
+stability.
+
+## Deprecation
+
+Plan for obsolence, hope for success. Similar to implementation, features that
+are to be removed should also go through the
+[Blink Process](https://www.chromium.org/blink#TOC-Web-Platform-Changes:-Process)
+for removing features.
+
+Note that due to the diversity of [Supported Projects](supported-projects.md),
+removing a feature while minimizing disruption can be just as complex as adding
+a feature. For example, relying solely on __User Metrics (UMA)__ to signal the
+safety of removing a feature may not consider all projects, and relying on
+__Field Trials (Finch)__ to assess risk or restore the 'legacy' behavior may not
+work on all projects either.
+
+It's precisely because of these challenges that there's such a high bar for
+adding features, because they may represent multi-year commitments to support,
+even when the feature itself is deprecated or targeted for removal.