4 files changed, 33 insertions, 3 deletions
diff --git a/spec/ruby/core/io/pread_spec.rb b/spec/ruby/core/io/pread_spec.rb
index 28afc80e5c..6d93b432c2 100644
--- a/spec/ruby/core/io/pread_spec.rb
+++ b/spec/ruby/core/io/pread_spec.rb
@@ -22,7 +22,7 @@ guard -> { platform_is_not :windows or ruby_version_is "3.3" } do
 
     it "accepts a length, an offset, and an output buffer" do
       buffer = +"foo"
-      @file.pread(3, 4, buffer)
+      @file.pread(3, 4, buffer).should.equal?(buffer)
       buffer.should == "567"
     end
 
@@ -38,6 +38,13 @@ guard -> { platform_is_not :windows or ruby_version_is "3.3" } do
       buffer.should == "12345"
     end
 
+    it "preserves the encoding of the given buffer" do
+      buffer = ''.encode(Encoding::ISO_8859_1)
+      @file.pread(10, 0, buffer)
+
+      buffer.encoding.should == Encoding::ISO_8859_1
+    end
+
     it "does not advance the file pointer" do
       @file.pread(4, 0).should == "1234"
       @file.read.should == "1234567890"
diff --git a/spec/ruby/core/io/read_spec.rb b/spec/ruby/core/io/read_spec.rb
index eb3652e692..8741d9f017 100644
--- a/spec/ruby/core/io/read_spec.rb
+++ b/spec/ruby/core/io/read_spec.rb
@@ -376,6 +376,21 @@ describe "IO#read" do
     buf.should == @contents[0..4]
   end
 
+  it "preserves the encoding of the given buffer" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    @io.read(10, buffer)
+
+    buffer.encoding.should == Encoding::ISO_8859_1
+  end
+
+  # https://bugs.ruby-lang.org/issues/20416
+  it "does not preserve the encoding of the given buffer when max length is not provided" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    @io.read(nil, buffer)
+
+    buffer.encoding.should_not == Encoding::ISO_8859_1
+  end
+
   it "returns the given buffer" do
     buf = +""
 
diff --git a/spec/ruby/core/io/readpartial_spec.rb b/spec/ruby/core/io/readpartial_spec.rb
index 0060beb545..0852f20b2d 100644
--- a/spec/ruby/core/io/readpartial_spec.rb
+++ b/spec/ruby/core/io/readpartial_spec.rb
@@ -62,7 +62,7 @@ describe "IO#readpartial" do
     buffer = +"existing content"
     @wr.write("hello world")
     @wr.close
-    @rd.readpartial(11, buffer)
+    @rd.readpartial(11, buffer).should.equal?(buffer)
     buffer.should == "hello world"
   end
 
@@ -106,6 +106,7 @@ describe "IO#readpartial" do
     @wr.write("abc")
     @wr.close
     @rd.readpartial(10, buffer)
+
     buffer.encoding.should == Encoding::ISO_8859_1
   end
 end
diff --git a/spec/ruby/core/io/sysread_spec.rb b/spec/ruby/core/io/sysread_spec.rb
index 003bb9eb94..8851214283 100644
--- a/spec/ruby/core/io/sysread_spec.rb
+++ b/spec/ruby/core/io/sysread_spec.rb
@@ -97,7 +97,7 @@ describe "IO#sysread on a file" do
 
   it "discards the existing buffer content upon successful read" do
     buffer = +"existing content"
-    @file.sysread(11, buffer)
+    @file.sysread(11, buffer).should.equal?(buffer)
     buffer.should == "01234567890"
   end
 
@@ -107,6 +107,13 @@ describe "IO#sysread on a file" do
     -> { @file.sysread(1, buffer) }.should raise_error(EOFError)
     buffer.should be_empty
   end
+
+  it "preserves the encoding of the given buffer" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    string = @file.sysread(10, buffer)
+
+    buffer.encoding.should == Encoding::ISO_8859_1
+  end
 end
 
 describe "IO#sysread" do