Duplicated last line in CSV.foreach

[GabrielNagy]

I found a bug that only reproduces with a very specific set of prerequisites (all of the following must be true):

• file with CRLF endings
• file with no EOL/trailling newline (removed using a hex editor since vim always adds them back)
• file larger than 32768 bytes
• CSV.foreach
• option strip: true
• option skip_lines: /\A,+\n?\z/

The following example (where original.csv is a file containing the line AAAA1234567890 ~2500 times):

CSV.foreach('original.csv', strip: true, skip_lines: /\A,+\n?\z/) do |data|
  puts data[0]
end

will print the last line duplicated:

AAAA1234567890
AAAA1234567890
AAAA1234567890
AAAA1234567890
AAAA1234567890AAAA1234567890
...

As a workaround I used CSV.parse(File.read, ...) with the same options, but I still wanted to flag this issue.

[tmatsuura1]

@kou I could reproduce this bug at hand.

[kou]

Thanks for your report. It may be a bug of internal chunk based stream parser: https://github.com/ruby/csv/blob/22e62bc6e07a83f2aef11158138e046191473731/lib/csv/parser.rb#L299-L308

@abcdefg-1234567 Great! Do you want to work on fixing this problem? Could you share a script that produces a CSV that reproduces this problem as the first step?

[tmatsuura1]

@kou Yes, I would like to. I used a CSV file made by a tool for spreadsheet and IDE to reproduce this problem. So, I will make script that produces a CSV.

[tmatsuura1]

@kou I came up with the following code that creates a csv.

CSV.open('test.csv', 'w') do |csv|
  2500.times do
    csv << ['AAAA1234567890']
  end
end

However, I do not know how to make the following state by code.

• file with CRLF endings
• file with no EOL/trailling newline

I have confirmed that the bug is reproduced when the following conditions are set manually using the IDE. Could you please give me any ideas?

[GabrielNagy]

hey @abcdefg-1234567 you can use the following:

File.open('test.csv', 'w') do |f|
  2499.times do
    f.print("AAAA1234567890\r\n")
  end
  f.print("AAAA1234567890")
end

[tmatsuura1]

@GabrielNagy Thank you!

[kou]

OK. Let's reduce the reproducible CSV size as much as possible as the next step for easy to debug. If it's difficult, we can start debugging with the reproducible CSV. https://github.com/ruby/csv/issues/279#issuecomment-1547004607 may help you.

[tmatsuura1]

I have confirmed that the bug will not reproduce if the csv is less than 2048 rows.

[tmatsuura1]

I have confirmed the following.

The result of "value = parse_column_value (line 1030 of parser.rb)" when @ lineno=2048 is "AAAA1234567890AAAAA1234567890".

I am also wondering if changes are needed around the adjust_last_keep method. @kou Could you please explain the role of this method?

[kou]

Sure.

#adjust_last_keep was introduced for fixing https://bugs.ruby-lang.org/issues/18245 .

InputsScanner acts as logically one StringScanner with multiple inputs. (StringScanner can't work with multiple strings.)

CSV::Parser may want to push back read data. For example, if skip_lines is specified, CSV::Parser may push back read data. CSV::Parser reads a line from its scanner (CSV::Parser::Scanner or CSV::Parser::InputsScanner) to check whether the line should be skipped. If the read line isn't skip target, CSV::Parser pushes back the read line and parses the line as a CSV line. keep_start/keep_drop/keep_back/keep_end are for it.

adjust_last_keep is related to these keep_* methods.InputsScanner processes multiple inputs. So the target data (for example, one line for skip_lines) may exist in multiple inputs. For example, "# a", "bc" and "\n" are one line but they are 3 inputs. adjust_last_keep is for the situation. If we need to concatenate data from multiple inputs, adjust_last_keep does it.

I hope that this explanation helps you.

[tmatsuura1]

Thank you for your detailed explanation! I will refer to this and continue the investigation.

[kou]

Including line number in line contents will helpful:

File.open('/tmp/test.csv', 'w') do |f|
  lines = 2500.times.collect do |i|
    "A%013d" % i
  end
  f.print(lines.join("\r\n"))
end

Output with the test file:

...
A0000000002497
A0000000002498
A0000000002499A0000000002499

It seems that the last line was used twice.

[kou]

I cloud reproduce this with the script:

ENV["CSV_PARSER_SCANNER_TEST"] = "yes"

require "csv"

csv = CSV.new("a\r\nb", row_sep: "\r\n", strip: true, skip_lines: /\A *\z/)
csv.each do |row|
  pp row
end

["a"]
["bb"]

[GabrielNagy]

Thanks for fixing this!