src/librustc_errors/lock.rs

//! Bindings to acquire a global named lock.
//!
//! This is intended to be used to synchronize multiple compiler processes to
//! ensure that we can output complete errors without interleaving on Windows.
//! Note that this is currently only needed for allowing only one 32-bit MSVC
//! linker to execute at once on MSVC hosts, so this is only implemented for
//! `cfg(windows)`. Also note that this may not always be used on Windows,
//! only when targeting 32-bit MSVC.
//!
//! For more information about why this is necessary, see where this is called.

use std::any::Any;

#[cfg(windows)]
#[allow(nonstandard_style)]
pub fn acquire_global_lock(name: &str) -> Box<dyn Any> {
    use std::ffi::CString;
    use std::io;

    type LPSECURITY_ATTRIBUTES = *mut u8;
    type BOOL = i32;
    type LPCSTR = *const u8;
    type HANDLE = *mut u8;
    type DWORD = u32;

    const INFINITE: DWORD = !0;
    const WAIT_OBJECT_0: DWORD = 0;
    const WAIT_ABANDONED: DWORD = 0x00000080;

    extern "system" {
        fn CreateMutexA(lpMutexAttributes: LPSECURITY_ATTRIBUTES,
                        bInitialOwner: BOOL,
                        lpName: LPCSTR)
                        -> HANDLE;
        fn WaitForSingleObject(hHandle: HANDLE, dwMilliseconds: DWORD) -> DWORD;
        fn ReleaseMutex(hMutex: HANDLE) -> BOOL;
        fn CloseHandle(hObject: HANDLE) -> BOOL;
    }

    struct Handle(HANDLE);

    impl Drop for Handle {
        fn drop(&mut self) {
            unsafe {
                CloseHandle(self.0);
            }
        }
    }

    struct Guard(Handle);

    impl Drop for Guard {
        fn drop(&mut self) {
            unsafe {
                ReleaseMutex((self.0).0);
            }
        }
    }

    let cname = CString::new(name).unwrap();
    unsafe {
        // Create a named mutex, with no security attributes and also not
        // acquired when we create it.
        //
        // This will silently create one if it doesn't already exist, or it'll
        // open up a handle to one if it already exists.
        let mutex = CreateMutexA(std::ptr::null_mut(), 0, cname.as_ptr() as *const u8);
        if mutex.is_null() {
            panic!("failed to create global mutex named `{}`: {}",
                   name,
                   io::Error::last_os_error());
        }
        let mutex = Handle(mutex);

        // Acquire the lock through `WaitForSingleObject`.
        //
        // A return value of `WAIT_OBJECT_0` means we successfully acquired it.
        //
        // A return value of `WAIT_ABANDONED` means that the previous holder of
        // the thread exited without calling `ReleaseMutex`. This can happen,
        // for example, when the compiler crashes or is interrupted via ctrl-c
        // or the like. In this case, however, we are still transferred
        // ownership of the lock so we continue.
        //
        // If an error happens.. well... that's surprising!
        match WaitForSingleObject(mutex.0, INFINITE) {
            WAIT_OBJECT_0 | WAIT_ABANDONED => {}
            code => {
                panic!("WaitForSingleObject failed on global mutex named \
                        `{}`: {} (ret={:x})",
                       name,
                       io::Error::last_os_error(),
                       code);
            }
        }

        // Return a guard which will call `ReleaseMutex` when dropped.
        Box::new(Guard(mutex))
    }
}

#[cfg(not(windows))]
pub fn acquire_global_lock(_name: &str) -> Box<dyn Any> {
    Box::new(())
}