Add a crash course to debugging using about:net-internals.

net/docs/crash-course-in-net-internals.md

+# A Crash Course in Debugging with about:net-internals
+
+This document is intended to help get people started debugging network errors
+with about:net-internals, with some commonly useful tips and tricks.  This
+document is aimed more at how to get started using some of its features to
+investigate bug reports, rather than as a feature overview.
+
+It would probably be useful to read [life-of-a-url-request.md](
+life-of-a-url-request.md) before this document.
+
+# What Data Net-Internals Contains
+
+about:net-internals provides a view of browser activity from net/'s perspective.
+For this reason, it lacks knowledge of tabs, navigation, frames, resource types,
+etc.
+
+The top level network stack object is the URLRequestContext.  The Events View
+has information for all Chrome URLRequestContexts that are hooked up to the
+single, global, ChromeNetLog object.  This includes both incognito and non-
+incognito profiles, among other things.  The Events view only shows events for
+the period that net-internals was open and running, and is incrementally updated
+as events occur.  The code attempts to add a top level event for URLRequests
+that were active when the tab was opened, to help debug hung requests, but
+that's best-effort only, and only includes requests for the current profile and
+the system URLRequestContext.
+
+The other views are all snapshots of the current state of the main
+URLRequestContext's components, and are updated on a 5 second timer.  These will
+show objects that were created before about:net-internals was opened.  Most
+debugging is done with the Events view (which will be all this document
+covers), but it's good to be aware of this distinction.
+
+# Events vs Sources
+
+The Event View shows events logged by the NetLog.  The NetLog model is that
+long-lived network stack objects, called sources, emit events over their
+lifetime.  Some events have a beginning and end point (during which other
+subevents may occur), and some only occur at a single point in time.  Generally
+only one event can be occuring for a source at a time.  If there can be multiple
+events doing completely independent thing, the code often uses new sources to
+represent the parallelism.
+
+"Sources" correspond to certain net objects, however, multiple layers of net/
+will often log to a single source.  Here are the main source types and what they
+include (Excluding HTTP2 [SPDY]/QUIC):
+
+* URL_REQUEST:  This corresponds to the URLRequest object.  It includes events
+from all the URLRequestJobs, HttpCache::Transactions, NetworkTransactions,
+HttpStreamFactoryImpl::Requests, HttpStream implementations, and
+HttpStreamParsers used to service a response.  If the URL_REQUEST follows HTTP
+redirects, it will include each redirect.  This is a lot of stuff, but generally
+only object is doing work at a time.  This event source includes the full URL
+and generally includes the request / response headers (Except when the cache
+handles the response).
+
+* HTTP_STREAM_JOB:  This corresponds to HttpStreamFactoryImpl::Job (Note that
+one Request can have multiple Jobs).  It also includes its proxy and DNS
+lookups.  HTTP_STREAM_JOB log events are separate from URL_REQUEST because
+two stream jobs may be created and races against each other, in some cases -
+one for one for QUIC, and one for HTTP.  One of the final events of this source
+indicates how an HttpStream was created (Reusing an existing SOCKET /
+HTTP2_SESSION / QUIC_SESSION, or creating a new one).
+
+* CONNECT_JOB:  This corresponds to the ConnectJob subclasses that each socket
+pool uses.  A successful CONNECT_JOB return a SOCKET.  The events here vary a
+lot by job type.  Their main event is generally either to create a socket, or
+request a socket from another socket pool (Which creates another CONNECT_JOB)
+and then do some extra work on top of that - like establish an SSL connection on
+top of a TCP connection.
+
+* SOCKET:  These correspond to TCPSockets, but may also have other classes
+layered on top of them (Like an SSLClientSocket).  This is a bit different from
+the other classes, where the name corresponds to the topmost class, instead of
+the bottommost one.  This is largely an artifact of the fact the socket is
+created first, and then SSL (Or a proxy connection) is layered on top of it.
+SOCKETs may be reused between multiple requests, and a request may end up
+getting a socket created for another request.
+
+* HOST_RESOLVER_IMPL_JOB:  These correspond to HostResolverImpl::Job.  The
+include information about how long the lookup was queued, each DNS request that
+was attempted (With the platform or built-in resolver) and all the other sources
+that are waiting on the job.
+
+When one source depends on another, the code generally logs an event with
+"source_dependency" value to both sources, which lets you jump between the two
+related events.
+
+# Debugging
+
+When you receive a report from the user, the first thing you'll generally want
+to do find the URL_REQUEST[s] that are misbehaving.  If the user gives an ERR_*
+code or the exact URL of the resource that won't load, you can just search for
+it.  If it's an upload, you can search for "post", or if it's a redirect issue,
+you can search for "redirect".  However, you often won't have much information
+about the actual problem.  There are two filters in net-internals that can help
+in a lot of cases:
+
+* "type:URL_REQUEST is:error" will restrict the list to URL_REQUEST object with
+an error of some sort (red background).  Cache errors are often non-fatal, so
+you should generally ignore those, and look for a more interesting one.
+
+* "type:URL_REQUEST sort:duration" will show the lonest-lived requests (as of
+when about:net-internals was opened) first.  This is often useful in finding
+hung or slow requests.
+
+For a list of other filter commands, you can mouse over the question mark on
+about:net-internals.
+
+Once you locate the problematic request, the next is to figure out where the
+problem is - it's often one of the last events, though it could also be related
+to response or request headers.  You can use "source_dependency" links to drill
+down into other related sources, or up from layers below URL_REQUEST.
+
+You can use the name of an event to search for the code responsible for that
+event, and try to deduce what went wrong before/after a particular event.  Note
+that the event names used in net-internals are not the entire string names, so
+you should not do an entire string match.
+
+Some things to look for while debugging:
+
+* CANCELLED events almost always come from outside the network stack.
+
+* Changing networks and entering / exiting suspend mode can have all sorts of
+fun and exciting effects on underway network activity.  Network changes log a
+top level NETWORK_CHANGED event with no source - the event itself is treated as
+its own source.  Suspend events are currently not logged.
+
+* URL_REQUEST_DELEGATE / DELEGATE_INFO events mean a URL_REQUEST is blocked on a
+URLRequest::Delegate or the NetworkDelegate, which are implemented outside the
+network stack.  A request will sometimes be CANCELED here for reasons known only
+to the delegate.  Or the delegate may cause a hang.  In general, to debug issues
+related to delegates, one needs to figure out which method of which object is
+causing the problem.  The object may be the a NetworkDelegate, a
+ResourceThrottle, a ResourceHandler, the ResourceLoader itself, or the
+ResourceDispatcherHost.
+
+* Sockets are often reused between requests.  If a request is on a stale
+(reused) socket, what was the previous request that used the socket, how long
+ago was it made?
+
+* SSL negotation is a process fraught with peril, particularly with broken
+proxies.  These will generally stall or fail in the SSL_CONNECT phase at the
+SOCKET layer.
+
+* Range requests have magic to handle them at the cache layer, and are often
+issued by the media and PDF code.
+
+* Late binding:  HTTP_STREAM_JOBs are not associated with any CONNECT_JOB until
+a CONNECT_JOB actually connects.  This is so the highest priority pending job
+gets the first available socket (Which may be a new socket, or an old one that's
+freed up).  For this reason, it can be a little tricky to relate hung
+HTTP_STREAM_JOBs to CONNECT_JOBs.
+
+* Each CONNECT_JOB belongs to a "group", which has a limit of 6 connections.  If
+all CONNECT_JOBs beling to a group (The CONNECT_JOB's description field) are
+stalled waiting on an available socket, the group probably has 6 sockets that
+that are hung - either hung trying to connect, or used by stalled requests and
+thus outside the socket pool's control.
+
+* There's a limit on number of DNS resolutions that can be started at once.  If
+everything is stalled while resolving DNS addresses, you've probably hit this
+limit, and the DNS lookups are also misbehaving in some fashion.
+
+# Miscellany
+
+These are just miscellaneous things you may notice when looking through the
+logs.
+
+* URLRequests that look to start twice for no obvious reason.  These are
+typically main frame requests, and the first request is AppCache.  Can just
+ignore it and move on with your life.
+
+* Some HTTP requests are not handled by URLRequestHttpJobs.  These include
+things like HSTS redirects (URLRequestRedirectJob), AppCache, ServiceWorker,
+etc.  These generally don't log as much information, so it can be tricky to
+figure out what's going on with these.
+
+* Non-HTTP requests also appear in the log, and also generally don't log much
+(blob URLs, chrome URLs, etc).
+
+* Preconnects create a "HTTP_STREAM_JOB" event that may create multiple
+CONNECT_JOBs (or none) and is then destroyed.  These can be identified by the
+"SOCKET_POOL_CONNECTING_N_SOCKETS" events.