Issue #12467 has been updated by Nobuyoshi Nakada.

Description updated
Status changed from Open to Third Party's Issue

Windows does not provide append mode, but ruby emulates it.

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Bug #12467: Open mode 'a' does not work for Process.spawn on Windows 
https://bugs.ruby-lang.org/issues/12467#change-59684

* Author: Yutaka HARA
* Status: Third Party's Issue
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.3.0p0 (2015-12-25 revision 53290) [x64-mingw32]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
Example:

~~~ruby
spawn 'ruby -e "p Time.now"', [:out, :err] => ['log.txt', 'a']
sleep 1
spawn 'ruby -e "p Time.now"', [:out, :err] => ['log.txt', 'a']
sleep 1
p File.read('log.txt')
~~~

This program looks work correctly at first.

~~~
c:\>ruby spawn_bug.rb
"2016-06-07 18:47:25 +0900\n2016-06-07 18:28:17 +0900\n"
~~~

However, when I run it again, the log file is truncated like 'w' flag.

~~~
c:\>ruby spawn_bug.rb
"2016-06-07 18:47:38 +0900\n2016-06-07 18:28:17 +0900\n"
~~~

This is the same if I open the log file manually with File.open.

~~~ruby
f = File.open('log.txt', 'a')
spawn 'ruby -e "p Time.now"', [:out, :err] => f
sleep 1
spawn 'ruby -e "p Time.now"', [:out, :err] => f
sleep 1
p File.read('log.txt')
~~~

I tried this on Windows 10. (Also confirmed this does not happen on Mac OS X 10.11 + ruby 2.2)



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