https://git.ruby-lang.org/ruby.git/commit/?id=fdf3996349

From fdf39963490cf2cf95b30d91bb9b35964c2c2350 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Wed, 22 Dec 2021 15:33:12 +0900
Subject: Empty and return the buffer if zero size is given [Bug #18421]

In `IO#readpartial` and `IO#read_nonblock`, as well as `IO#read`.
---
 io.c                 |  8 ++++++--
 test/ruby/test_io.rb | 21 +++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/io.c b/io.c
index dc883f36b43..8ac29083aee 100644
--- a/io.c
+++ b/io.c
@@ -3161,8 +3161,10 @@ io_getpartial(int argc, VALUE *argv, VALUE io, int no_exception, int nonblock) https://github.com/ruby/ruby/blob/trunk/io.c#L3161
     GetOpenFile(io, fptr);
     rb_io_check_byte_readable(fptr);
 
-    if (len == 0)
+    if (len == 0) {
+	io_set_read_length(str, 0, shrinkable);
 	return str;
+    }
 
     if (!nonblock)
         READ_CHECK(fptr);
@@ -3305,8 +3307,10 @@ io_read_nonblock(rb_execution_context_t *ec, VALUE io, VALUE length, VALUE str, https://github.com/ruby/ruby/blob/trunk/io.c#L3307
     GetOpenFile(io, fptr);
     rb_io_check_byte_readable(fptr);
 
-    if (len == 0)
+    if (len == 0) {
+	io_set_read_length(str, 0, shrinkable);
 	return str;
+    }
 
     n = read_buffered_data(RSTRING_PTR(str), len, fptr);
     if (n <= 0) {
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index b46777df5ff..4f54052d3ba 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1494,6 +1494,13 @@ class TestIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io.rb#L1494
     end)
   end
 
+  def test_readpartial_zero_size
+    File.open(IO::NULL) do |r|
+      assert_empty(r.readpartial(0, s = "01234567"))
+      assert_empty(s)
+    end
+  end
+
   def test_readpartial_buffer_error
     with_pipe do |r, w|
       s = ""
@@ -1539,6 +1546,13 @@ class TestIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io.rb#L1546
     end)
   end
 
+  def test_read_zero_size
+    File.open(IO::NULL) do |r|
+      assert_empty(r.read(0, s = "01234567"))
+      assert_empty(s)
+    end
+  end
+
   def test_read_buffer_error
     with_pipe do |r, w|
       s = ""
@@ -1576,6 +1590,13 @@ class TestIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io.rb#L1590
     }
   end
 
+  def test_read_nonblock_zero_size
+    File.open(IO::NULL) do |r|
+      assert_empty(r.read_nonblock(0, s = "01234567"))
+      assert_empty(s)
+    end
+  end
+
   def test_write_nonblock_simple_no_exceptions
     pipe(proc do |w|
       w.write_nonblock('1', exception: false)
-- 
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