From: "mame (Yusuke Endoh)" <noreply@...>
Date: 2022-06-09T06:51:50+00:00
Subject: [ruby-core:108816] [Ruby master Bug#16143] BOM UTF-8 is not removed after rewind

Issue #16143 has been updated by mame (Yusuke Endoh).

Status changed from Open to Feedback

I think this issue can be easily worked around by using `IO#set_encoding_by_bom` which was introduced by #15210.

```
csv = CSV.open('bom_test.csv', 'r:BOM|UTF-8', headers: true)
p csv.shift  #=> #<CSV::Row "Name":"John Doe" "City":"New York">

# workaround
csv.rewind
csv.binmode
csv.to_io.set_encoding_by_bom

p csv.shift  #=> #<CSV::Row "Name":"John Doe" "City":"New York">
```

Do we really need any change? It is surprising to me if `IO#pos` is non-zero after `IO#rewind`. `IO#rewind(bom: true)` or something, which @akr proposes, may be less surprising. But IMHO, `IO#set_encoding_by_bom` is enough.

----------------------------------------
Bug #16143: BOM UTF-8 is not removed after rewind
https://bugs.ruby-lang.org/issues/16143#change-97893

* Author: Dirk (Dirk Meier-Eickhoff)
* Status: Feedback
* Priority: Normal
* ruby -v: ruby 2.6.2p47 (2019-03-13 revision 67232) [x86_64-darwin17]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
I have a CSV file with "forced quotes" and UTF-8 BOM (\xEF\xBB\xBF) which CSV can not read after a `rewind`. I get "CSV::MalformedCSVError: Illegal quoting in line 1."

My UTF-8 CSV file with BOM:
``` ruby
File.open('bom_test.csv', 'w') do |io|
  io.write("\xEF\xBB\xBF\"Name\",\"City\"\n\"John Doe\",\"New York\"")
end
```

Reproduce error:


``` ruby
# Case 1
csv = CSV.open('bom_test.csv', 'r:BOM|UTF-8', {headers: true})
csv.shift
# => #<CSV::Row "Name":"John Doe" "City":"New York">
csv.rewind
csv.shift
# => CSV::MalformedCSVError (Illegal quoting in line 1.)

# Case 2
csv = CSV.open('bom_test.csv', 'r:BOM|UTF-8', {headers: true})
csv.readline
# => #<CSV::Row "Name":"John Doe" "City":"New York">
csv.rewind
csv.readline
# => CSV::MalformedCSVError (Illegal quoting in line 1.)
```

Sutou Kouhei has posted other reproducable code to my first issue at CSV gem: https://github.com/ruby/csv/issues/103
``` ruby
File.open("/tmp/a.txt", "w") do |x|
  x.puts("\xEF\xBB\xBFa,b,c")
end
File.open("/tmp/a.txt", "r:BOM|UTF-8") do |x|
  p x.gets.unpack("U*") # => [97, 44, 98, 44, 99, 10]
  x.rewind
  p x.gets.unpack("U*") # => [65279, 97, 44, 98, 44, 99, 10]
end
```

He said: "This [CSV] library rely on Ruby's BOM processing. It seems that Ruby's BOM processing doesn't support rewind."

My expectation is that reading a file with BOM always return the same content, regardless of first reading or after a rewind.



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