require 'thread' require 'monitor' module ActiveSupport module Concurrency # A share/exclusive lock, otherwise known as a read/write lock. # # https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock class ShareLock include MonitorMixin # We track Thread objects, instead of just using counters, because # we need exclusive locks to be reentrant, and we need to be able # to upgrade share locks to exclusive. def initialize super() @cv = new_cond @sharing = Hash.new(0) @waiting = {} @exclusive_thread = nil @exclusive_depth = 0 end # 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. 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 @cv.wait_while { busy_for_exclusive?(purpose) } end end @exclusive_thread = Thread.current end @exclusive_depth += 1 true end end # Relinquish the exclusive lock. Must only be called by the thread # that called start_exclusive (and currently holds the lock). 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 @cv.wait_while { @exclusive_thread || eligible_waiters?(compatible) } end end @cv.broadcast end end end 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 @cv.wait_while { @exclusive_thread } else # This is an initial / outermost share call: any outstanding # requests for an exclusive lock get to go first @cv.wait_while { busy_for_sharing?(false) } end @sharing[Thread.current] += 1 end end def stop_sharing synchronize do if @sharing[Thread.current] > 1 @sharing[Thread.current] -= 1 else @sharing.delete Thread.current @cv.broadcast end end end # 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. 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 # Execute the supplied block while holding the Share lock. def sharing start_sharing begin yield ensure stop_sharing end end # Temporarily give up all held Share locks while executing the # supplied block, allowing any +compatible+ exclusive lock request # to proceed. 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 @cv.wait_while { @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 private # Must be called within synchronize def busy_for_exclusive?(purpose) busy_for_sharing?(purpose) || @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0) end def busy_for_sharing?(purpose) (@exclusive_thread && @exclusive_thread != Thread.current) || @waiting.any? { |t, (_, c)| t != Thread.current && !c.include?(purpose) } end def eligible_waiters?(compatible) @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } } end end end end