merge revision(s) fdf39963490cf2cf95b30d91bb9b35964c2c2350: [Backport #18421]

	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(-)

1 files changed, 21 insertions, 0 deletions

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 58a739be22..5b84fd6a4d 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1482,6 +1482,13 @@ class TestIO < Test::Unit::TestCase
     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 = ""
@@ -1527,6 +1534,13 @@ class TestIO < Test::Unit::TestCase
     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 = ""
@@ -1564,6 +1578,13 @@ class TestIO < Test::Unit::TestCase
     }
   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)