debugcommands: add debugserve command `hg serve --stdio` requires the exact command argument form `hg -R <path> serve --stdio` for security reasons. An upcoming commit will need to start an SSH protocol server process with custom settings. This commit creates a `hg debugserve` command for starting servers with custom options. There are no security restrictions and we can add options here that aren't appropriate for built-in commands. We currently only support starting an SSH protocol server using the process's stdio file descriptors. The server supports logging its I/O activity to a file descriptor number passed as a command argument. Differential Revision: https://phab.mercurial-scm.org/D2464

wireprotoserver: support logging SSH server I/O to a file descriptor We will soon introduce a debug command and tests for low-level I/O behavior of the SSH wire protocol. To facilitate this, we need to instrument the SSH server so it can log its I/O as events occur. We teach the SSH server to convert its stdout and stderr file objects into file object proxies. We configure these proxies to log to a file descriptor whose file number is specified via a config option. The idea is to have a future debug command start the SSH server process with access to an extra file descriptor that can be used by the server process to log I/O. Monitoring only the write I/O will be more robust than monitoring both writes and reads from the client process because read operations are not deterministic. This will matter for tests that capture raw I/O activity. Differential Revision: https://phab.mercurial-scm.org/D2463

util: enable observing of util.bufferedinputpipe Our file object proxy is useful. But it doesn't capture all I/O. The "os" module offers low-level interfaces to various system calls. For example, os.read() exposes read(2) to read from a file descriptor. bufferedinputpipe is special in a few ways. First, it acts as a proxy of sorts around our [potentially proxied] file object. In addition, it uses os.read() to satisfy all I/O. This means that our observer doesn't see notifications for reads on this type. This is preventing us from properly instrumenting reads on ssh peers. This commit teaches bufferedinputpipe to be aware of our observed file objects. We do this by introducing a class variation that notifies our observer of os.read() events. Since read() and readline() bypass os.read(), we also teach this instance to notify the observer for buffered variations of these reads as well. We don't report them as actual read() and readline() calls because these methods are never called on the actual file object but rather a buffered version of it. We introduce bufferedinputpipe.__new__ to swap in the new class if the passed file object is a fileobjectproxy. This makes hooking up the observer automatic. And it is a zero cost abstraction for I/O operations on non-proxied file objects. Differential Revision: https://phab.mercurial-scm.org/D2404

util: add a file object proxy that can notify observers There are various places in Mercurial where we may want to instrument low-level I/O. The use cases I can think of all involve development-type activities like monitoring the raw bytes passing through a file (for testing and debugging), counting the number of I/O function calls (for performance monitoring), and changing the behavior of I/O function calls (e.g. simulating a failure) (to facilitate testing). This commit invents a mechanism to wrap a file object so we can observe activity on it. We have similar functionality in badserverext.py. But that's a test-only extension and is pretty specific to the HTTP server. I would like a mechanism in core that is sufficiently generic so it can be used by multiple consumers, including `hg debug*` commands. The added code consists of a proxy type for file objects. It is bound to an "observer," which receives callbacks whenever I/O methods are called. We also add an implementation of an observer that logs specific I/O events. This observer will be used in an upcoming commit to record low-level wire protocol activity. A helper function to convert a file object into an observed file object has also been implemented. I don't anticipate any critical functionality in core using these types. So I don't think explicit test coverage is worth implementing. Differential Revision: https://phab.mercurial-scm.org/D2462

wireprotoserver: ability to run an SSH server until an event is set It seems useful to be able to start an SSH protocol server that won't run forever and won't call sys.exit() when it stops. This could be used to facilitate intra-process testing of the SSH protocol, for example. We teach the server function to loop until a threading.Event is set and invent a new API to run the server until an event is set. It also won't sys.exit() afterwards. There aren't many callers of serve_forever(). So we could refactor them relatively easily. But I was lazy. threading.Event might be a bit heavyweight. An alternative would be a list whose only elements is changed. We can't use a simple scalar value like a bool or int because those types are immutable. Events are what you use in systems programming for this use case, so the use of threading.Event seems justified. Differential Revision: https://phab.mercurial-scm.org/D2461

tests: fix run-tests environment cleanup on Python 3 Differential Revision: https://phab.mercurial-scm.org/D2521

templatekw: add compatlist() as a replacement for showlist() Just like compatdict(), this is mostly a copy of showlist(). showchildren() is ported to the new API as an example.

templatekw: add compatdict() as a replacement for showdict() This is mostly a copy of showdict(), which will be deprecated later. See the docstring for why it's called a "compat" dict. showenvvars() is ported to the new API as an example.

templatekw: pass templater to _showlist() by an explicit argument Prepares for switching to the (context, mapping) API.

hgweb: make templater mostly compatible with log templates Prepares for gradually switching templatekw.showsuccsandmarkers() to new API. This was a PoC showing how to reuse templatekw functions in hgweb. We could remove rev, node, author, etc. from the commonentry() table, but we'll have to carefully remove all corresponding symbols from webcommands.*(). Otherwise, we would face the issue5612. Still templatekw.keywords aren't exported. Otherwise some tests would fail because the atom template expects {files} to be empty in filelog, but templatekw.showfiles() provides the {files} implementation.