Truncating file that has STDOUT being redirected to it - node.js

Question

I have a node application that is having it's STDOUT being redirected to a log file:
node app.js > /var/log/app.out
From inside my node program I want to reset the log in order to keep it from growing too large. I've attempted fs.truncate and fs.open('/var/log/app.out', 'w') in order to reset it's length, but when I try to verify the file size using ls or stat, the file size seems to quickly reset to 0, then reset to it's previous size plus new messages (I currently am forcing a lot of output).
root#ubuntu-1304-mike:/var/log# stat app.out
File: ‘app.out’
Size: 5885622 Blocks: 16 IO Block: 4096 regular file
Device: fc00h/64512d Inode: 270560 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2013-09-26 17:20:06.784564421 -0600
Modify: 2013-09-26 17:20:49.088565625 -0600
Change: 2013-09-26 17:20:49.088565625 -0600
Birth: -
root#ubuntu-1304-mike:/var/log# stat app.out
File: ‘app.out’
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: fc00h/64512d Inode: 270560 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2013-09-26 17:20:06.784564421 -0600
Modify: 2013-09-26 17:20:51.772565702 -0600
Change: 2013-09-26 17:20:51.772565702 -0600
Birth: -
root#ubuntu-1304-mike:/var/log# stat app.out
File: ‘app.out’
Size: 6038458 Blocks: 16 IO Block: 4096 regular file
Device: fc00h/64512d Inode: 270560 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2013-09-26 17:20:06.784564421 -0600
Modify: 2013-09-26 17:20:53.640565755 -0600
Change: 2013-09-26 17:20:53.640565755 -0600
Birth: -
It seems that STDOUT is re-writing it's buffer to the file (regardless of how large the buffer is).
How can I truncate a file that STDOUT is actively being redirected to, without breaking out of my node app?

Answer 1

It seems that STDOUT is re-writing it's buffer to the file (regardless of how large the buffer is).
Nobody is re-writing a buffer, it's already written on the disk. When application finishes writing something, it remembers position where it finished, and it starts with this position later. It's not a node issue, and there's nothing you can do about it, it's how unix pipes work (unless there's some ugly hack to rewind a pipe I don't know about).
How can I truncate a file that STDOUT is actively being redirected to, without breaking out of my node app?
Don't use stdout redirection and write to a file instead. If you can't modify your app, write another app that uses more clever approach and redirect stdout to it like that: node app.js | node write-stdin-to-app.out.js

Answer 2

I just ran into this issue, as well. Using the append operator is the best way to solve it:
node app.js >> /var/log/app.out
Now you can truncate the file with fs.truncate or even:
echo -n > /var/log/app.out