Bug #18767: IO.foreach hangs up when passes limit=0

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.0.3
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

[andrykonchin (Andrew Konchin)]

IO.foreach behaves in an unexpected way in a corner case when passes 0 as a limit parameter. It never stops and hangs up.

IO.foreach('file.txt', 0) { |s| p s }
""
""
""
""
""
""
""
""
""
""

Expected behavior - to raise ArgumentError "invalid limit: 0" like it does the IO.readlines method.

I observe this behavior on 2.6, 2.7, and 3.0. Didn't check on 3.1 and master though.

[jeremyevans0 (Jeremy Evans)]

I've submitted a pull request to fix this: https://github.com/ruby/ruby/pull/5954

[mame (Yusuke Endoh)]

• Status changed from Open to Rejected

I believe this is not a bug. I don't think the current behavior is very useful, but I don't think it is so problematic either. Considering the following example, the behavior is consistent in a sense.

irb(main):001:0> File.write("foo.txt", [*?A..?Z].join)
=> 26
irb(main):002:0> File.foreach("foo.txt", 4).take(4)
=> ["ABCD", "EFGH", "IJKL", "MNOP"]
irb(main):003:0> File.foreach("foo.txt", 3).take(4)
=> ["ABC", "DEF", "GHI", "JKL"]
irb(main):004:0> File.foreach("foo.txt", 2).take(4)
=> ["AB", "CD", "EF", "GH"]
irb(main):005:0> File.foreach("foo.txt", 1).take(4)
=> ["A", "B", "C", "D"]
irb(main):006:0> File.foreach("foo.txt", 0).take(4)
=> ["", "", "", ""]

@andrykonchin (Andrew Konchin) If you really want to change this behavior, please open a feature request with an explanation of a real-world use case.

[andrykonchin (Andrew Konchin)]

I agree that the current behaviour completely makes sense.

The point is that there are other IO methods (that read multiple lines) which don't accept 0 as a limit. So it's more a consistency issue.

For instance IO.readlines and IO#each_line:

File.open('Rakefile').each_line(0).to_a
# (irb):8:in `each_line': invalid limit: 0 for each_line (ArgumentError)

IO.readlines('Rakefile', 0)
# (irb):11:in `readlines': invalid limit: 0 for readlines (ArgumentError)

[andrykonchin (Andrew Konchin)]

@mame (Yusuke Endoh)

I still believe that it's a compatibility issue and not a feature request or so. Could the ticket be reopened?

cc @jeremyevans0 (Jeremy Evans)

[mame (Yusuke Endoh)]

• Status changed from Rejected to Open

Okay reopened. Sorry, I overlooked the fact that IO#each_line(0) raises an error.

IMHO, it is reasonable for IO#readlines(0) to raise an exception. If not, it just gets stuck until the memory is exhausted. I'm unsure about IO#each_line(0).

[mame (Yusuke Endoh)]

• Related to Bug #4024: IO#readlines(0) never return added

[andrykonchin (Andrew Konchin)]

Hi, just a kind reminder.

Looks like the issue is accepted but the PR was closed and the fix isn't merged.

cc @jeremyevans0 (Jeremy Evans), @mame (Yusuke Endoh)

[mame (Yusuke Endoh)]

I reopened this issue because there is reasonable doubt about the current situation, but I would like @matz (Yukihiro Matsumoto) 's judgment on whether and how it should actually be fixed.

[matz (Yukihiro Matsumoto)]

OK, let's make it raise exceptions for zero length.

Matz.

• Status changed from Open to Closed

Applied in changeset git|21cac42385e1a116d287e155e461453b830640d2.

---

Raise ArgumentError for IO.foreach with limit of 0

Makes behavior consistent with IO.readlines.

Fixes [Bug #18767]