A share/exclusive lock, otherwise known as a read/write lock.

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

Methods

Included Modules

  • MonitorMixin

Class Public methods

new()

📝 Source code 
# 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
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Instance Public methods

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

Execute the supplied block while holding the Exclusive lock. If no_wait is set and the lock is not immediately available, returns nil without yielding. Otherwise, returns the result of the block.

See start_exclusive for other options.

📝 Source code 
# 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
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sharing()

Execute the supplied block while holding the Share lock.

📝 Source code 
# File activesupport/lib/active_support/concurrency/share_lock.rb, line 159
      def sharing
        start_sharing
        begin
          yield
        ensure
          stop_sharing
        end
      end
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start_exclusive(purpose: nil, compatible: [], no_wait: false)

Returns false if no_wait is set and the lock is not immediately available. Otherwise, returns true after the lock has been acquired.

purpose and compatible work together; while this thread is waiting for the exclusive lock, it will yield its share (if any) to any other attempt whose purpose appears in this attempt's compatible list. This allows a “loose” upgrade, which, being less strict, prevents some classes of deadlocks.

For many resources, loose upgrades are sufficient: if a thread is awaiting a lock, it is not running any other code. With purpose matching, it is possible to yield only to other threads whose activity will not interfere.

📝 Source code 
# 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
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start_sharing()

📝 Source code 
# 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
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stop_exclusive(compatible: [])

Relinquish the exclusive lock. Must only be called by the thread that called start_exclusive (and currently holds the lock).

📝 Source code 
# 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
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stop_sharing()

📝 Source code 
# 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
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yield_shares(purpose: nil, compatible: [], block_share: false)

Temporarily give up all held Share locks while executing the supplied block, allowing any compatible exclusive lock request to proceed.

📝 Source code 
# 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
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