Module: ScoutSemaphore

Defined in:
lib/scout/semaphore.rb

Defined Under Namespace

Classes: SemaphoreInterrupted

Constant Summary collapse

SEM_MUTEX = 
Mutex.new
RETRIABLE_ERRNOS =

Errno numeric lists

[
  Errno::ENOENT,
  Errno::EIDRM,
  Errno::EAGAIN,
  Errno::EMFILE,
  Errno::ENFILE,
  Errno::EINTR
].map { |c| c.new.errno }
FATAL_ERRNOS = 
[
  Errno::EINVAL,
  Errno::EACCES
].map { |c| c.new.errno }

Class Method Summary collapse

Class Method Details

.create_semaphore(name, value, **opts) ⇒ Object

Safe wrappers that raise SystemCallError on final failure

Raises:

  • (SystemCallError) 


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# File 'lib/scout/semaphore.rb', line 163

def self.create_semaphore(name, value, **opts)
  ret = with_retry(**opts) { ScoutSemaphore.create_semaphore_c(name, value) }
  raise SystemCallError.new("Semaphore missing (#{name})") unless self.exists?(name)
  if ret < 0
    raise SystemCallError.new("create_semaphore(#{name}) failed", -ret)
  end
  ret
end

.delete_semaphore(name, **opts) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 172

def self.delete_semaphore(name, **opts)
  ret = with_retry(**opts) { ScoutSemaphore.delete_semaphore_c(name) }
  if ret < 0
    raise SystemCallError.new("delete_semaphore(#{name}) failed", -ret)
  end
  ret
end

.ensure_semaphore_name(file) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 107

def self.ensure_semaphore_name(file)
  # Ensure a valid POSIX named semaphore name: must start with '/'
  s = file.to_s.dup
  # strip leading slashes and replace other slashes with underscores, then prepend single '/'
  s.gsub!(%r{^/+}, '')
  s = '/' + s.gsub('/', '_')
  s
end

.exists?(name) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/scout/semaphore.rb', line 116

def self.exists?(name)
  file = File.join('/dev/shm', 'sem.' + name[1..-1])
  Open.exists? file
end

.fork_each_on_semaphore(elems, size, file = nil) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 260

def self.fork_each_on_semaphore(elems, size, file = nil)

  TSV.traverse elems, :cpus => size, :bar => "Fork each on semaphore: #{ Misc.fingerprint elems }", :into => Set.new do |elem|
    elems.annotate elem if elems.respond_to? :annotate
    begin
      yield elem
    rescue Interrupt
      Log.warn "Process #{Process.pid} was aborted"
    end
    nil
  end
  nil
end

.post_semaphore(name, **opts) ⇒ Object

Raises:

  • (SystemCallError) 


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# File 'lib/scout/semaphore.rb', line 194

def self.post_semaphore(name, **opts)
  raise SystemCallError.new("Semaphore missing (#{name})") unless self.exists?(name)
  ret = with_retry(**opts) { ScoutSemaphore.post_semaphore_c(name) }
  if ret < 0
    raise SystemCallError.new("post_semaphore(#{name}) failed", -ret)
  end
  ret
end

.synchronize(sem) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 203

def self.synchronize(sem)
  # Ensure name is normalized (caller should pass normalized name, but be safe)
  sem = ensure_semaphore_name(sem)

  # wait_semaphore returns 0 on success or -errno on error
  begin
    ScoutSemaphore.wait_semaphore(sem)
  rescue SemaphoreInterrupted
    raise
  rescue SystemCallError => e
    # bubble up for callers to handle
    raise
  end

  begin
    yield
  ensure
    begin
      ScoutSemaphore.post_semaphore(sem)
    rescue SystemCallError => e
      # Log but don't raise from ensure
      # Log.warn "post_semaphore(#{sem}) failed in ensure: #{e.message}"

      # Actually, do raise
      raise e
    end
  end
end

.thread_each_on_semaphore(elems, size) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 274

def self.thread_each_on_semaphore(elems, size)
  mutex = Mutex.new
  count = 0
  cv = ConditionVariable.new
  wait_mutex = Mutex.new

  begin

    threads = []
    wait_mutex.synchronize do
      threads = elems.collect do |elem|
        Thread.new(elem) do |elem|

          continue = false
          mutex.synchronize do
            while not continue do
              if count < size
                continue = true
                count += 1
              end
              # wait briefly to avoid busy loop; ConditionVariable could be used here properly
              mutex.sleep 1 unless continue
            end
          end

          begin
            yield elem
          rescue Interrupt
            Log.error "Thread was aborted while processing: #{Misc.fingerprint elem}"
            raise $!
          ensure
            mutex.synchronize do
              count -= 1
              cv.signal if mutex.locked?
            end
          end
        end
      end
    end

    threads.each do |thread|
      thread.join
    end
  rescue Exception
    Log.exception $!
    Log.info "Ensuring threads are dead: #{threads.length}"
    threads.each do |thread| thread.kill end
  end
end

.wait_semaphore(name, **opts) ⇒ Object

Raises:

  • (SystemCallError) 


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# File 'lib/scout/semaphore.rb', line 180

def self.wait_semaphore(name, **opts)
  raise SystemCallError.new("Semaphore missing (#{name})") unless self.exists?(name)
  ret = with_retry(**opts) { ScoutSemaphore.wait_semaphore_c(name) }
  if ret < 0
    err = -ret
    if err == Errno::EINTR.new.errno
      raise SemaphoreInterrupted
    else
      raise SystemCallError.new("wait_semaphore(#{name}) failed", err)
    end
  end
  ret
end

.with_retry(max_attempts: 6, base_delay: 0.01, max_delay: 1.0, jitter: 0.5, retriable: RETRIABLE_ERRNOS) ⇒ Object

Generic retry wrapper with exponential backoff + jitter



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# File 'lib/scout/semaphore.rb', line 137

def self.with_retry(max_attempts: 6, base_delay: 0.01, max_delay: 1.0, jitter: 0.5, retriable: RETRIABLE_ERRNOS)
  attempts = 0
  while true
    attempts += 1
    ret = yield
    # caller expects 0 on success, negative errno on failure
    return ret if ret >= 0

    err = -ret
    # don't retry if it's clearly fatal or not in retriable list
    if FATAL_ERRNOS.include?(err) || attempts >= max_attempts || !retriable.include?(err)
      return ret
    end

    # exponential backoff with jitter
    base = base_delay * (2 ** (attempts - 1))
    sleep_time = [base, max_delay].min
    # add jitter in range [0, jitter * sleep_time)
    sleep_time += rand * jitter * sleep_time

    Log.warn "Semaphore operation failed (errno=#{err}), retrying in #{'%.3f' % sleep_time}s (attempt #{attempts}/#{max_attempts})"
    sleep(sleep_time)
  end
end

.with_semaphore(size, file = nil) ⇒ Object 



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# File 'lib/scout/semaphore.rb', line 232

def self.with_semaphore(size, file = nil)
  if file.nil?
    file = "/scout-" + Misc.digest(rand(100000000000).to_s)[0..10]
  else
    # ensure valid POSIX name
    file = ensure_semaphore_name(file)
  end

  begin
    Log.debug "Creating semaphore (#{ size }): #{file}"
    begin
      ScoutSemaphore.create_semaphore(file, size)
    rescue SystemCallError => e
      Log.error "Failed to create semaphore #{file}: #{e.message}"
      raise
    end

    yield file
  ensure
    Log.debug "Removing semaphore #{ file }"
    begin
      ScoutSemaphore.delete_semaphore(file)
    rescue SystemCallError => e
      Log.warn "delete_semaphore(#{file}) failed: #{e.message}"
    end
  end
end