[erlang-questions] Reading a file before it has been completely written

David Mercer <>
Wed Mar 7 18:25:33 CET 2012


While this isn't an Erlang-specific question, the problem arises from my
using Richard Carlsson's file_monitor
(https://github.com/richcarl/eunit/blob/master/src/file_monitor.erl), which
sends messages when a file or directory is changed.  I have found that it is
not unusual to get a message about a new file before the file has been
completely written.

 

I had thought that by doing a file:open(Filepath, [read]) and making sure I
got back {ok, _} rather than {error, eacces} I could avoid those cases, but
that approach has failed for me: this morning, I got back {ok, _}, but the
file was not completely written yet.

 

Another approach I tried was to attempt to obtain an exclusive lock (I think
it was file:open(Filepath, [read, exclusive])), but in my testing I came
across the bizarre scenario where I would copy a file into the monitored
directory, the file_monitor would send the message, but the Erlang process
that does the file-open didn't see it, so created the file (the
documentation says it creates the file if it does not exist), and then I got
a message in my window where I was copying that the file already exists, do
I want to overwrite it.

 

Another approach I tried was renaming the file to itself.  All my tests
indicated that that approach would work, but all my tests also indicated
that just doing the file:open(Filepath, [read]) would work, too, so I chose
it, as it seemed cleaner.  I could revert to the rename approach, but I'm
not even sure now that that will work.

 

I imagine others among us have encountered this issue, and rather than
reinvent the wheel, what is the favored approach to handling this issue?

 

Cheers,

 

David Mercer

 

 

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