Calling python script from TCL and creating a file atomically
I wrote a python script that takes care of creating a file atomically as described in this other question. The file that gets created by the script is somewhat big (about ~1.5MB) as seen below:
$ ./my_script.py --output_file some_directory/my_file.txt
$ ls -l --block-size=K some_directory/my_file.txt
-rw------- 1 foouser foogroup 1477K Aug 7 17:39 some_directory/my_file.txt
Unfortunately, due to some legacy infrastructure and the way things are set up, this script needs to be called from within a TCL script. Here is the way in which I am currently doing that:
#!/usr/bin/env tclsh
set PY_SCRIPT my_script.py
puts [exec $PY_SCRIPT --output some_directory/my_file.txt]
The problem that I am seeing is that the generated file does not seem to be created atomically anymore (atomicity is a requirement for my application). The TCL puts command documentation says the following:
Tcl buffers output internally, so characters written with puts may not appear immediately on the output file or device; Tcl will normally delay output until the buffer is full or the channel is closed.
I believe this explains why my file is not being atomically created anymore. I'm not an expert in TCL, so I am not sure how to accomplish atomic file creation in the TCL script.
Any suggestions?
A direct translation of the Python code that you link to would be something like this (untested code):
package require Tclx
set f [open $tmpFile w]
puts -nonewline $f $text
sync $f ; # flush is performed implicitly
close $f
file rename -force $tmpFile $myFile
The TclX package is needed to get the sync command. (This will probably not work on Windows.)