VoucherSupport.h

//===--- VoucherSupport.h - Support code for OS vouchers -----------*- C++ -*-//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// Support code for interfacing with OS voucher calls.
//
//===----------------------------------------------------------------------===//

#ifndef SWIFT_CONCURRENCY_VOUCHERSUPPORT_H
#define SWIFT_CONCURRENCY_VOUCHERSUPPORT_H

#include "llvm/ADT/Optional.h"
#include "swift/ABI/Task.h"
#include "swift/Runtime/VoucherShims.h"
#include "TaskPrivate.h"

namespace swift {

/// A class which manages voucher adoption for Job and Task objects.
class VoucherManager {
  /// The original voucher that was set on the thread before Swift started
  /// doing async work. This must be restored on the thread after we finish
  /// async work.
  llvm::Optional<voucher_t> OriginalVoucher;

public:
  VoucherManager() {
    SWIFT_TASK_DEBUG_LOG("[%p] Constructing VoucherManager", this);
  }

  /// Clean up after completing async work, restoring the original voucher on
  /// the current thread if necessary. This MUST be called before the
  /// VoucherManager object is destroyed. It may also be called in other
  /// places to restore the original voucher and reset the VoucherManager.
  void leave() {
    if (OriginalVoucher) {
      SWIFT_TASK_DEBUG_LOG("[%p] Restoring original voucher %p", this,
                           *OriginalVoucher);
      if (swift_voucher_needs_adopt(*OriginalVoucher)) {
        auto previous = voucher_adopt(*OriginalVoucher);
        swift_voucher_release(previous);
      } else {
        swift_voucher_release(*OriginalVoucher);
      }
      OriginalVoucher = llvm::None;
    } else
      SWIFT_TASK_DEBUG_LOG("[%p] Leaving empty VoucherManager", this);
  }

  ~VoucherManager() { assert(!OriginalVoucher); }

  /// Set up for a new Job by adopting its voucher on the current thread. This
  /// takes over ownership of the voucher from the Job object. For plain Jobs,
  /// this is permanent. For Tasks, the voucher must be restored using
  /// restoreVoucher if the task suspends.
  void swapToJob(Job *job) {
    SWIFT_TASK_DEBUG_LOG("[%p] Swapping jobs to %p", this, job);
    assert(job);
    assert(job->Voucher != SWIFT_DEAD_VOUCHER);

    voucher_t previous;
    if (swift_voucher_needs_adopt(job->Voucher)) {
      // If we need to adopt the voucher, do so, and grab the old one.
      SWIFT_TASK_DEBUG_LOG("[%p] Swapping jobs to %p, adopting voucher %p",
                           this, job, job->Voucher);
      previous = voucher_adopt(job->Voucher);
    } else {
      // If we don't need to adopt the voucher, take the voucher out of Job
      // directly.
      SWIFT_TASK_DEBUG_LOG(
          "[%p] Swapping jobs to to %p, voucher %p does not need adoption",
          this, job, job->Voucher);
      previous = job->Voucher;
    }

    // Either way, we've taken ownership of the job's voucher, so mark the job
    // as having a dead voucher.
    job->Voucher = SWIFT_DEAD_VOUCHER;
    if (!OriginalVoucher) {
      // If we don't yet have an original voucher, then save the one we grabbed
      // above to restore later.
      OriginalVoucher = previous;
      SWIFT_TASK_DEBUG_LOG("[%p] Saved original voucher %p", this, previous);
    } else {
      // We already have an original voucher. The one we grabbed above is not
      // needed. We own it, so destroy it here.
      swift_voucher_release(previous);
    }
  }

  // Take the current thread's adopted voucher and place it back into the task
  // that previously owned it, re-adopting the thread's original voucher.
  void restoreVoucher(AsyncTask *task) {
    SWIFT_TASK_DEBUG_LOG("[%p] Restoring %svoucher on task %p", this,
                         OriginalVoucher ? "" : "missing ", task);
    assert(OriginalVoucher);
    assert(task->Voucher == SWIFT_DEAD_VOUCHER);

    if (swift_voucher_needs_adopt(*OriginalVoucher)) {
      // Adopt the execution thread's original voucher. The task's voucher is
      // the one currently adopted, and is returned by voucher_adopt.
      task->Voucher = voucher_adopt(*OriginalVoucher);
    } else {
      // No need to adopt the voucher, so we can take the one out of
      // OriginalVoucher and return it to the task.
      task->Voucher = *OriginalVoucher;
    }

    // We've given up ownership of OriginalVoucher, clear it out.
    OriginalVoucher = llvm::None;
  }
};

} // end namespace swift

#endif