bindings: export rust process handling to Python

Summary:
Spawning processes turns out to be tricky.

Python 2:

- "fork & exec" in plain Python is potentially dangerous. See D22855986 (c35b808).
  Disabling GC might have solved it, but still seems fragile.
- "close_fds=True" works on Windows if there is no redirection.
- Does not work well with `disable_standard_handle_inheritability` from `hgmain`.
  We patched it. See `contrib/python2-winbuild/0002-windows-make-subprocess-work-with-non-inheritable-st.patch`.

Python 3:

- "subprocess" uses native code for "fork & exec". It's safer.
- (>= 3.8) "close_fds=True" works on Windows even with redirection.
- "subprocess" exposes options to tweak low-level details on Windows.

Rust:

- No "close_fds=True" support for both Windows and Unix.
- Does not have the `disable_standard_handle_inheritability` issue on Windows.
- Impossible to cleanly support "close_fds=True" on Windows with existing stdlib.
  rust-lang/rust#75551 attempts to add that to stdlib.
  D23124167 provides a short-term solution that can have corner cases.

Mercurial:

- `win32.spawndetached` uses raw Win32 APIs to spawn processes, bypassing
  the `subprocess` Python stdlib.
- Its use of `CreateProcessA` is undesirable. We probably want `CreateProcessW`
  (unless `CreateProcessA` speaks utf-8 natively).

We are still on Python 2 on Windows, and we'd need to spawn processes correctly
from Rust anyway, and D23124167 kind of fills the missing feature of `close_fds=True`
from Python. So let's expose the Rust APIs.

The binding APIs closely match the Rust API. So when we migrate from Python to
Rust, the translation is more straightforward.

Reviewed By: DurhamG

Differential Revision: D23124168

fbshipit-source-id: 94a404f19326e9b4cca7661da07a4b4c55bcc395

eden/scm/edenscmnative/bindings/Cargo.toml

@@ -17,11 +17,11 @@ incremental = true
 lto = false
 
 [dependencies]
-cpython = { version = "0.5", default-features = false }
 cpython-ext = { path = "../../lib/cpython-ext", default-features = false }
+cpython = { version = "0.5", default-features = false }
 pyblackbox = { path = "modules/pyblackbox" }
-pybytes = { path = "modules/pybytes" }
 pybookmarkstore = { path = "modules/pybookmarkstore" }
+pybytes = { path = "modules/pybytes" }
 pycliparser = { path = "modules/pycliparser" }
 pyconfigparser = { path = "modules/pyconfigparser" }
 pydag = { path = "modules/pydag" }
@@ -39,6 +39,7 @@ pymutationstore = { path = "modules/pymutationstore" }
 pynodemap = { path = "modules/pynodemap" }
 pypager = { path = "modules/pypager" }
 pypathmatcher = { path = "modules/pypathmatcher" }
+pyprocess = { path = "modules/pyprocess" }
 pyregex = { path = "modules/pyregex" }
 pyrenderdag = { path = "modules/pyrenderdag" }
 pyrevisionstore = { path = "modules/pyrevisionstore" }

eden/scm/edenscmnative/bindings/modules/pyprocess/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "pyprocess"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+cpython = { version = "0.5", default-features = false }
+cpython-ext = { path = "../../../../lib/cpython-ext", default-features = false }
+spawn-ext = { path = "../../../../lib/spawn-ext" }
+
+[features]
+python2 = ["cpython/python27-sys", "cpython-ext/python2"]
+python3 = ["cpython/python3-sys", "cpython-ext/python3"]

eden/scm/edenscmnative/bindings/modules/pyprocess/src/lib.rs

@@ -0,0 +1,241 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+use cpython::*;
+use cpython_ext::PyNone;
+use cpython_ext::PyPath;
+use cpython_ext::ResultPyErrExt;
+use spawn_ext::CommandExt;
+use std::cell::RefCell;
+use std::fs;
+use std::io;
+use std::process::Child as RustChild;
+use std::process::Command as RustCommand;
+use std::process::ExitStatus as RustExitStatus;
+use std::process::Stdio as RustStdio;
+
+py_class!(class Command |py| {
+    data inner: RefCell<RustCommand>;
+
+    /// Constructs a new Command for launching the program at path program, with
+    /// the following default configuration:
+    /// - No arguments to the program
+    /// - Inherit the current process's environment
+    /// - Inherit the current process's working directory
+    /// - Inherit stdin/stdout/stderr for spawn or status, but create pipes for output
+    @staticmethod
+    def new(program: String) -> PyResult<Self> {
+        let command = RustCommand::new(program);
+        Self::create_instance(py, RefCell::new(command))
+    }
+
+    /// Adds an argument to pass to the program.
+    def arg(&self, arg: &str) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.arg(arg))
+    }
+
+    /// Adds multiple arguments to pass to the program.
+    def args(&self, args: Vec<String>) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.args(args))
+    }
+
+    /// Inserts or updates an environment variable mapping.
+    def env(&self, key: &str, val: &str) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.env(key, val))
+    }
+
+    /// Adds or updates multiple environment variable mappings.
+    def envs(&self, items: Vec<(String, String)>) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.envs(items))
+    }
+
+    /// Clears the entire environment map for the child process.
+    def envclear(&self) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.env_clear())
+    }
+
+    /// Sets the working directory for the child process.
+    def currentdir(&self, dir: &PyPath) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.current_dir(dir))
+    }
+
+    /// Configuration for the child process's standard input (stdin) handle.
+    def stdin(&self, cfg: Stdio) -> PyResult<Self> {
+        let f = cfg.to_rust(py).map_pyerr(py)?;
+        self.mutate_then_clone(py, |c| c.stdin(f))
+    }
+
+    /// Configuration for the child process's standard output (stdout) handle.
+    def stdout(&self, cfg: Stdio) -> PyResult<Self> {
+        let f = cfg.to_rust(py).map_pyerr(py)?;
+        self.mutate_then_clone(py, |c| c.stdout(f))
+    }
+
+    /// Configuration for the child process's standard error (stderr) handle.
+    def stderr(&self, cfg: Stdio) -> PyResult<Self> {
+        let f = cfg.to_rust(py).map_pyerr(py)?;
+        self.mutate_then_clone(py, |c| c.stderr(f))
+    }
+
+    /// Attempt to avoid inheriting file handles.
+    /// Call this before setting up redirections.
+    def avoidinherithandles(&self) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.avoid_inherit_handles())
+    }
+
+    /// Use new session or process group.
+    /// Call this after avoidinherithandles.
+    def newsession(&self) -> PyResult<Self> {
+        self.mutate_then_clone(py, |c| c.new_session())
+    }
+
+    /// Executes the command as a child process, returning a handle to it.
+    def spawn(&self) -> PyResult<Child> {
+        // This is safer than `os.fork()` in Python because Python cannot
+        // interrupt between `fork()` and `exec()` due to Rust holding GIL.
+        let mut inner = self.inner(py).borrow_mut();
+        let child = inner.spawn().map_pyerr(py)?;
+        Child::from_rust(py, child)
+    }
+
+    /// Spawn the process then forget about it.
+    /// File handles are not inherited. stdio will be redirected to /dev/null.
+    def spawndetached(&self) -> PyResult<Child> {
+        // This is safer than `os.fork()` in Python because Python cannot
+        // interrupt between `fork()` and `exec()` due to Rust holding GIL.
+        let mut inner = self.inner(py).borrow_mut();
+        let child = inner.spawn_detached().map_pyerr(py)?;
+        Child::from_rust(py, child)
+    }
+
+});
+
+impl Command {
+    /// Make changes to `inner`, then clone self.
+    fn mutate_then_clone(
+        &self,
+        py: Python,
+        func: impl FnOnce(&mut RustCommand) -> &mut RustCommand,
+    ) -> PyResult<Self> {
+        let mut inner = self.inner(py).borrow_mut();
+        func(&mut inner);
+        Ok(self.clone_ref(py))
+    }
+}
+
+py_class!(class Stdio |py| {
+    data inner: Box<dyn Fn() -> io::Result<RustStdio> + Send + 'static> ;
+
+    /// A new pipe should be arranged to connect the parent and child processes.
+    @staticmethod
+    def piped() -> PyResult<Self> {
+        Self::create_instance(py, Box::new(|| Ok(RustStdio::piped())))
+    }
+
+    /// The child inherits from the corresponding parent descriptor.
+    @staticmethod
+    def inherit() -> PyResult<Self> {
+        Self::create_instance(py, Box::new(|| Ok(RustStdio::inherit())))
+    }
+
+    /// This stream will be ignored. This is the equivalent of attaching the
+    /// stream to /dev/null.
+    @staticmethod
+    def null() -> PyResult<Self> {
+        Self::create_instance(py, Box::new(|| Ok(RustStdio::null())))
+    }
+
+    /// Open a file as `Stdio`.
+    @staticmethod
+    def open(path: &PyPath, read: bool = false, write: bool = false, create: bool = false, append: bool = false) -> PyResult<Self> {
+        let path = path.to_path_buf();
+        Self::create_instance(py, Box::new(move || {
+            let mut opts = fs::OpenOptions::new();
+            let file = opts.write(write).read(read).create(create).append(append).open(&path)?;
+            Ok(file.into())
+        }))
+    }
+});
+
+impl Stdio {
+    fn to_rust(&self, py: Python) -> io::Result<RustStdio> {
+        self.inner(py)()
+    }
+}
+
+py_class!(class Child |py| {
+    data inner: RefCell<RustChild>;
+
+    /// Forces the child process to exit. If the child has already exited, an
+    /// InvalidInput error is returned.
+    def kill(&self) -> PyResult<PyNone> {
+        let mut inner = self.inner(py).borrow_mut();
+        inner.kill().map_pyerr(py)?;
+        Ok(PyNone)
+    }
+
+    /// Returns the OS-assigned process identifier associated with this child.
+    def id(&self) -> PyResult<u32> {
+        let inner = self.inner(py).borrow();
+        Ok(inner.id())
+    }
+
+    /// Waits for the child to exit completely, returning the status that it
+    /// exited with. This function will continue to have the same return value
+    /// after it has been called at least once.
+    def wait(&self) -> PyResult<ExitStatus> {
+        let mut inner = self.inner(py).borrow_mut();
+        let status = inner.wait().map_pyerr(py)?;
+        ExitStatus::from_rust(py, status)
+    }
+
+    /// Attempts to collect the exit status of the child if it has already exited.
+    def try_wait(&self) -> PyResult<Option<ExitStatus>> {
+        let mut inner = self.inner(py).borrow_mut();
+        match inner.try_wait().map_pyerr(py)? {
+            Some(s) => Ok(Some(ExitStatus::from_rust(py, s)?)),
+            None => Ok(None)
+        }
+    }
+});
+
+impl Child {
+    fn from_rust(py: Python, child: RustChild) -> PyResult<Self> {
+        Self::create_instance(py, RefCell::new(child))
+    }
+}
+
+py_class!(class ExitStatus |py| {
+    data inner: RustExitStatus;
+
+    /// Was termination successful? Signal termination is not considered a
+    /// success, and success is defined as a zero exit status.
+    def success(&self) -> PyResult<bool> {
+        Ok(self.inner(py).success())
+    }
+
+    /// Returns the exit code of the process, if any.
+    /// On Unix, this will return None if the process was terminated by a signal.
+    def code(&self) -> PyResult<Option<i32>> {
+        Ok(self.inner(py).code())
+    }
+});
+
+impl ExitStatus {
+    fn from_rust(py: Python, status: RustExitStatus) -> PyResult<Self> {
+        Self::create_instance(py, status)
+    }
+}
+
+pub fn init_module(py: Python, package: &str) -> PyResult<PyModule> {
+    let name = [package, "process"].join(".");
+    let m = PyModule::new(py, &name)?;
+    m.add_class::<Child>(py)?;
+    m.add_class::<Command>(py)?;
+    m.add_class::<Stdio>(py)?;
+    Ok(m)
+}

eden/scm/edenscmnative/bindings/src/lib.rs

@@ -43,6 +43,7 @@ pub fn populate_module(py: Python<'_>, module: &PyModule) -> PyResult<PyNone> {
     m.add(py, "nodemap", pynodemap::init_module(py, &name)?)?;
     m.add(py, "pager", pypager::init_module(py, &name)?)?;
     m.add(py, "pathmatcher", pypathmatcher::init_module(py, &name)?)?;
+    m.add(py, "process", pyprocess::init_module(py, &name)?)?;
     m.add(py, "regex", pyregex::init_module(py, &name)?)?;
     m.add(py, "renderdag", pyrenderdag::init_module(py, &name)?)?;
     m.add(