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共享/独占锁，也称为读/写锁。

en.wikipedia.org/wiki/Readers%E2%80%93writer_lock

方法
E
N
S
Y
包含的模块

类公共方法

new()

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 50
def initialize
  super()

  @cv = new_cond

  @sharing = Hash.new(0)
  @waiting = {}
  @sleeping = {}
  @exclusive_thread = nil
  @exclusive_depth = 0
end

实例公共方法

exclusive(purpose: nil, compatible: [], after_compatible: [], no_wait: false)

在持有独占锁时执行提供的代码块。如果设置了 no_wait 且锁不可立即使用，则返回 nil 而不会生成。否则，返回代码块的结果。

有关其他选项，请参见 start_exclusive。

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 148
def exclusive(purpose: nil, compatible: [], after_compatible: [], no_wait: false)
  if start_exclusive(purpose: purpose, compatible: compatible, no_wait: no_wait)
    begin
      yield
    ensure
      stop_exclusive(compatible: after_compatible)
    end
  end
end

sharing()

在持有共享锁时执行提供的代码块。

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 159
def sharing
  start_sharing
  begin
    yield
  ensure
    stop_sharing
  end
end

start_exclusive(purpose: nil, compatible: [], no_wait: false)

如果设置了 no_wait 且锁不可立即使用，则返回 false。否则，在获取锁后返回 true。

purpose 和 compatible 协同工作；当此线程正在等待独占锁时，它会将其共享（如果有）让渡给任何其他尝试，其 purpose 出现于此尝试的 compatible 列表中。这允许“松散”升级，由于较不严格，因此可以防止某些类别的死锁。

对于许多资源，松散升级就足够了：如果一个线程正在等待锁，则它不会运行任何其他代码。通过匹配 purpose，可以仅让渡给其他线程，其活动不会干扰。

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 76
def start_exclusive(purpose: nil, compatible: [], no_wait: false)
  synchronize do
    unless @exclusive_thread == Thread.current
      if busy_for_exclusive?(purpose)
        return false if no_wait

        yield_shares(purpose: purpose, compatible: compatible, block_share: true) do
          wait_for(:start_exclusive) { busy_for_exclusive?(purpose) }
        end
      end
      @exclusive_thread = Thread.current
    end
    @exclusive_depth += 1

    true
  end
end

start_sharing()

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 114
def start_sharing
  synchronize do
    if @sharing[Thread.current] > 0 || @exclusive_thread == Thread.current
      # We already hold a lock; nothing to wait for
    elsif @waiting[Thread.current]
      # We're nested inside a +yield_shares+ call: we'll resume as
      # soon as there isn't an exclusive lock in our way
      wait_for(:start_sharing) { @exclusive_thread }
    else
      # This is an initial / outermost share call: any outstanding
      # requests for an exclusive lock get to go first
      wait_for(:start_sharing) { busy_for_sharing?(false) }
    end
    @sharing[Thread.current] += 1
  end
end

stop_exclusive(compatible: [])

放弃独占锁。只能由调用 start_exclusive 的线程调用（且当前持有锁）。

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 96
def stop_exclusive(compatible: [])
  synchronize do
    raise "invalid unlock" if @exclusive_thread != Thread.current

    @exclusive_depth -= 1
    if @exclusive_depth == 0
      @exclusive_thread = nil

      if eligible_waiters?(compatible)
        yield_shares(compatible: compatible, block_share: true) do
          wait_for(:stop_exclusive) { @exclusive_thread || eligible_waiters?(compatible) }
        end
      end
      @cv.broadcast
    end
  end
end

stop_sharing()

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 131
def stop_sharing
  synchronize do
    if @sharing[Thread.current] > 1
      @sharing[Thread.current] -= 1
    else
      @sharing.delete Thread.current
      @cv.broadcast
    end
  end
end

yield_shares(purpose: nil, compatible: [], block_share: false)

在执行所提供的代码块时暂时放弃所有持有的 Share 锁，允许任何 compatible 独占锁请求继续进行。

来源：显示 | 在 GitHub 上 

# File activesupport/lib/active_support/concurrency/share_lock.rb, line 171
def yield_shares(purpose: nil, compatible: [], block_share: false)
  loose_shares = previous_wait = nil
  synchronize do
    if loose_shares = @sharing.delete(Thread.current)
      if previous_wait = @waiting[Thread.current]
        purpose = nil unless purpose == previous_wait[0]
        compatible &= previous_wait[1]
      end
      compatible |= [false] unless block_share
      @waiting[Thread.current] = [purpose, compatible]
    end

    @cv.broadcast
  end

  begin
    yield
  ensure
    synchronize do
      wait_for(:yield_shares) { @exclusive_thread && @exclusive_thread != Thread.current }

      if previous_wait
        @waiting[Thread.current] = previous_wait
      else
        @waiting.delete Thread.current
      end
      @sharing[Thread.current] = loose_shares if loose_shares
    end
  end
end