[ruby-core:83083] [Ruby trunk Bug#13965] Surprising behavior when using Tempfile.open

Issue #13965 has been updated by davemyron (Dave Myron).


To workaround the unexpected behavior, either use `IO#sync = true` or `IO#flush` after writing to the file.

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Bug #13965: Surprising behavior when using Tempfile.open
https://bugs.ruby-lang.org/issues/13965#change-67032

* Author: davemyron (Dave Myron)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.4.1p111
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN
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I was trying to write the contents of an array to a Tempfile using the #open() + block syntax: Tempfile.open { |f| my_array.each { |item| f.puts item }}

However, I was finding that only *most* of the items were being written to the file which, in my case, was Very Bad.

Witness (with Ruby 2.4.1p111; same behavior in 2.4.3):

```
>> Tempfile.open() { |f| (1..10000).each { |i| f.puts i }; `tail -n1 #{f.path}` }
=> "8416"
>> Tempfile.open() { |f| (1..100000).each { |i| f.puts i }; `tail -n1 #{f.path}` }
=> "98835"
>> Tempfile.open() { |f| f.puts "Test\n"; `tail -n1 #{f.path}` }
=> ""
```

All very surprising results! When using block syntax, I expect to be able to interact with the Tempfile inside the block and then forget about it. 

Storing the path of the Tempfile and accessing it after the block is closed produces the expected results:

```
>> path = ""; Tempfile.open() { |f| path = f.path; f.puts "Test\n" }; `tail -n1 #{path}`
=> "Test\n"
>> path = ""; Tempfile.open() { |f| path = f.path; (1..10000).each { |i| f.puts i }}; `tail -n1 #{path}`
=> "10000\n"
```

I suspect that it's due to some buffering. Nonetheless, it's unexpected to have to store the path to the Tempfile when using the block syntax and to interact with after closing the block. 



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