//! Job management on Windows for bootstrapping
//!
//! Most of the time when you're running a build system (e.g., make) you expect
//! Ctrl-C or abnormal termination to actually terminate the entire tree of
//! process in play, not just the one at the top. This currently works "by
//! default" on Unix platforms because Ctrl-C actually sends a signal to the
//! *process group* rather than the parent process, so everything will get torn
//! down. On Windows, however, this does not happen and Ctrl-C just kills the
//! parent process.
//!
//! To achieve the same semantics on Windows we use Job Objects to ensure that
//! all processes die at the same time. Job objects have a mode of operation
//! where when all handles to the object are closed it causes all child
//! processes associated with the object to be terminated immediately.
//! Conveniently whenever a process in the job object spawns a new process the
//! child will be associated with the job object as well. This means if we add
//! ourselves to the job object we create then everything will get torn down!
//!
//! Unfortunately most of the time the build system is actually called from a
//! python wrapper (which manages things like building the build system) so this
//! all doesn't quite cut it so far. To go the last mile we duplicate the job
//! object handle into our parent process (a python process probably) and then
//! close our own handle. This means that the only handle to the job object
//! resides in the parent python process, so when python dies the whole build
//! system dies (as one would probably expect!).
//!
//! Note that this module has a #[cfg(windows)] above it as none of this logic
//! is required on Unix.

use crate::Build;
use std::env;
use std::ffi::c_void;
use std::io;
use std::mem;

use windows::{
    core::PCWSTR,
    Win32::Foundation::{CloseHandle, DuplicateHandle, DUPLICATE_SAME_ACCESS, HANDLE},
    Win32::System::Diagnostics::Debug::{SetErrorMode, SEM_NOGPFAULTERRORBOX, THREAD_ERROR_MODE},
    Win32::System::JobObjects::{
        AssignProcessToJobObject, CreateJobObjectW, JobObjectExtendedLimitInformation,
        SetInformationJobObject, JOBOBJECT_EXTENDED_LIMIT_INFORMATION,
        JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE, JOB_OBJECT_LIMIT_PRIORITY_CLASS,
    },
    Win32::System::Threading::{
        GetCurrentProcess, OpenProcess, BELOW_NORMAL_PRIORITY_CLASS, PROCESS_DUP_HANDLE,
    },
};

pub unsafe fn setup(build: &mut Build) {
    // Enable the Windows Error Reporting dialog which msys disables,
    // so we can JIT debug rustc
    let mode = SetErrorMode(THREAD_ERROR_MODE::default());
    let mode = THREAD_ERROR_MODE(mode);
    SetErrorMode(mode & !SEM_NOGPFAULTERRORBOX);

    // Create a new job object for us to use
    let job = CreateJobObjectW(None, PCWSTR::null()).unwrap();

    // Indicate that when all handles to the job object are gone that all
    // process in the object should be killed. Note that this includes our
    // entire process tree by default because we've added ourselves and our
    // children will reside in the job by default.
    let mut info = JOBOBJECT_EXTENDED_LIMIT_INFORMATION::default();
    info.BasicLimitInformation.LimitFlags = JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE;
    if build.config.low_priority {
        info.BasicLimitInformation.LimitFlags |= JOB_OBJECT_LIMIT_PRIORITY_CLASS;
        info.BasicLimitInformation.PriorityClass = BELOW_NORMAL_PRIORITY_CLASS.0;
    }
    let r = SetInformationJobObject(
        job,
        JobObjectExtendedLimitInformation,
        &info as *const _ as *const c_void,
        mem::size_of_val(&info) as u32,
    )
    .ok();
    assert!(r.is_ok(), "{}", io::Error::last_os_error());

    // Assign our process to this job object. Note that if this fails, one very
    // likely reason is that we are ourselves already in a job object! This can
    // happen on the build bots that we've got for Windows, or if just anyone
    // else is instrumenting the build. In this case we just bail out
    // immediately and assume that they take care of it.
    //
    // Also note that nested jobs (why this might fail) are supported in recent
    // versions of Windows, but the version of Windows that our bots are running
    // at least don't support nested job objects.
    let r = AssignProcessToJobObject(job, GetCurrentProcess()).ok();
    if r.is_err() {
        CloseHandle(job);
        return;
    }

    // If we've got a parent process (e.g., the python script that called us)
    // then move ownership of this job object up to them. That way if the python
    // script is killed (e.g., via ctrl-c) then we'll all be torn down.
    //
    // If we don't have a parent (e.g., this was run directly) then we
    // intentionally leak the job object handle. When our process exits
    // (normally or abnormally) it will close the handle implicitly, causing all
    // processes in the job to be cleaned up.
    let pid = match env::var("BOOTSTRAP_PARENT_ID") {
        Ok(s) => s,
        Err(..) => return,
    };

    let parent = match OpenProcess(PROCESS_DUP_HANDLE, false, pid.parse().unwrap()).ok() {
        Some(parent) => parent,
        _ => {
            // If we get a null parent pointer here, it is possible that either
            // we have an invalid pid or the parent process has been closed.
            // Since the first case rarely happens
            // (only when wrongly setting the environmental variable),
            // it might be better to improve the experience of the second case
            // when users have interrupted the parent process and we haven't finish
            // duplicating the handle yet. We just need close the job object if that occurs.
            CloseHandle(job);
            return;
        }
    };

    let mut parent_handle = HANDLE::default();
    let r = DuplicateHandle(
        GetCurrentProcess(),
        job,
        parent,
        &mut parent_handle,
        0,
        false,
        DUPLICATE_SAME_ACCESS,
    )
    .ok();

    // If this failed, well at least we tried! An example of DuplicateHandle
    // failing in the past has been when the wrong python2 package spawned this
    // build system (e.g., the `python2` package in MSYS instead of
    // `mingw-w64-x86_64-python2`). Not sure why it failed, but the "failure
    // mode" here is that we only clean everything up when the build system
    // dies, not when the python parent does, so not too bad.
    if r.is_err() {
        CloseHandle(job);
    }
}