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wireproto: implement batching on peer executor interface
This is a bit more complicated than non-batch requests because we
need to buffer sends until the last request arrives *and* we need
to support resolving futures as data arrives from the remote.
In a classical concurrent.futures executor model, the future
"starts" as soon as it is submitted. However, we have nothing to
start until the last command is submitted.
If we did nothing, calling result() would deadlock, since the future
hasn't "started." So in the case where we queue the command, we return
a special future type whose result() will trigger sendcommands().
This eliminates the deadlock potential. It also serves as a check
against callers who may be calling result() prematurely, as it will
prevent any subsequent callcommands() from working. This behavior
is slightly annoying and a bit restrictive. But it's the world
that half duplex connections forces on us.
In order to support streaming responses, we were previously using
a generator. But with a futures-based API, we're using futures
and not generators. So in order to get streaming, we need a
background thread to read data from the server.
The approach taken in this patch is to leverage the ThreadPoolExecutor
from concurrent.futures for managing a background thread. We create
an executor and future that resolves when all response data is
processed (or an error occurs). When exiting the context manager,
we wait on that background reading before returning.
I was hoping we could manually spin up a threading.Thread and this
would be simple. But I ran into a few deadlocks when implementing.
After looking at the source code to concurrent.futures, I figured
it would just be easier to use a ThreadPoolExecutor than implement
all the code needed to manually manage a thread.
To prove this works, a use of the batch API in discovery has been
updated.
Differential Revision: https://phab.mercurial-scm.org/D3269
author | Gregory Szorc <gregory.szorc@gmail.com> |
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date | Fri, 13 Apr 2018 11:02:34 -0700 |
parents | 964212780daf |
children | 8a3b045d9086 |
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=================== Mercurial Rust Code =================== This directory contains various Rust code for the Mercurial project. The top-level ``Cargo.toml`` file defines a workspace containing all primary Mercurial crates. Building ======== To build the Rust components:: $ cargo build If you prefer a non-debug / release configuration:: $ cargo build --release Features -------- The following Cargo features are available: localdev (default) Produce files that work with an in-source-tree build. In this mode, the build finds and uses a ``python2.7`` binary from ``PATH``. The ``hg`` binary assumes it runs from ``rust/target/<target>hg`` and it finds Mercurial files at ``dirname($0)/../../../``. Build Mechanism --------------- The produced ``hg`` binary is *bound* to a CPython installation. The binary links against and loads a CPython library that is discovered at build time (by a ``build.rs`` Cargo build script). The Python standard library defined by this CPython installation is also used. Finding the appropriate CPython installation to use is done by the ``python27-sys`` crate's ``build.rs``. Its search order is:: 1. ``PYTHON_SYS_EXECUTABLE`` environment variable. 2. ``python`` executable on ``PATH`` 3. ``python2`` executable on ``PATH`` 4. ``python2.7`` executable on ``PATH`` Additional verification of the found Python will be performed by our ``build.rs`` to ensure it meets Mercurial's requirements. Details about the build-time configured Python are built into the produced ``hg`` binary. This means that a built ``hg`` binary is only suitable for a specific, well-defined role. These roles are controlled by Cargo features (see above). Running ======= The ``hgcli`` crate produces an ``hg`` binary. You can run this binary via ``cargo run``:: $ cargo run --manifest-path hgcli/Cargo.toml Or directly:: $ target/debug/hg $ target/release/hg You can also run the test harness with this binary:: $ ./run-tests.py --with-hg ../rust/target/debug/hg .. note:: Integration with the test harness is still preliminary. Remember to ``cargo build`` after changes because the test harness doesn't yet automatically build Rust code.