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

[Tony Rogvall]

- Create and open a file with a temporary name.
-  Write the file content.
- Close the file.
- Rename the file to the name/place you want.

works ?
 
/Tony

On 7 mar 2012, at 18:25, David Mercer wrote:

> 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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