EventMachine read and write files in chunks - ruby

Question

I'm using EventMachine and EM-Synchrony in a REST API server. When a receive a POST request with a large binary file in body I receive it in chunks, writing that chunks to a Tempfile, not blocking the reactor.
Then, at some point, I need to read this file in chunks and write that chunks to a definitive file. This is working, but it blocks the reactor as expected, and cant find a way to make it work without blocking.
I call this function at some time, passing to it the tempfile and new file name:
def self.save(tmp_file, new_file)
tmp = File.open(tmp_file, "rb")
newf = File.open(new_file, "wb")
md5 = Digest::MD5.new
each_chunk(tmp, CHUNKSIZE) do |chunk|
newf << chunk
md5.update chunk
end
md5.hexdigest
end
def self.each_chunk(file, chunk_size=1024)
yield file.read(chunk_size) until file.eof?
end
I've been reading all other similar questions here at StackOverflow, trying to use EM#next_tick, which is perhaps the solution (not so much EM experience) but cant get it to work, perhaps I'm placing it in the wrong places.
Also, I've tried EM#defer, but I need the function to wait for the read/write process to complete before it returns the md5, as in my main file, after call this function I do a database update with the return value.
If someone can help me on this I would be grateful.
EDIT 1
I need that the save function only returns after complete the files read/write, as in the caller function I'm waiting for the final md5 value, something like this:
def copy_and_update(...)
checksum = SomeModule.save(temp_file, new_file)
do_database_update({:checksum => checksum}) # only with the final md5 value
end

Answer 1

You need to inject something in there to break it up:
def self.each_chunk(file, chunk_size=1024)
chunk_handler = lambda {
unless (file.eof?)
yield file.read(chunk_size)
EM.next_tick(&chunk_handler)
end
}
EM.next_tick(&chunk_handler)
end
It's kind of messy to do it this way, but such is asynchronous programming.