Issue #15600 has been reported by chi (Chi Leung).

----------------------------------------
Bug #15600: CSV underlying IO object is not getting rewound after parsing
https://bugs.ruby-lang.org/issues/15600

* Author: chi (Chi Leung)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin16]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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Hello,

Sorry if this bug has already been reported but I couldn't find any reference to it.

For the following CSV test.csv file:
```
a,b,c,d
1,2,3,4
5,6,7,8
9,10,11,12
```

and the following code snippet:
``` ruby
require 'csv'
csv = ::CSV.open('./test.csv', {headers: true, col_sep: ','})
p csv.eof?
csv.readline
p csv.eof?
csv.readline
p csv.eof?
csv.readline
p csv.eof?

```

I got the following results with 2.6.1:
```
false
true
true
true
```

but using `ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin16]` I got:
```
false
false
false
true
```

It might be an intentional behaviour but in that case, I can't see anything in the documentation explicitly saying that we shouldn't rely on the underlying IO object to know the state of the current CSV object (as it was possible in the previous minor version) and couldn't find the related changelog.

Thanks for your work.



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