From b2b1e7b0409289162c3de9398f4361bac816a0f2 Mon Sep 17 00:00:00 2001
From: Charles Oliver Nutter <headius@headius.com>
Date: Mon, 1 Mar 2021 21:57:28 -0600
Subject: [PATCH] Be sure to get write IO for fast writes

---
 .../java/org/jruby/util/IOOutputStream.java   | 26 ++++++++++++++++---
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/core/src/main/java/org/jruby/util/IOOutputStream.java b/core/src/main/java/org/jruby/util/IOOutputStream.java
index bf85d2c2e0f..15d8d9ebfb1 100644
--- a/core/src/main/java/org/jruby/util/IOOutputStream.java
+++ b/core/src/main/java/org/jruby/util/IOOutputStream.java
@@ -67,9 +67,22 @@ public class IOOutputStream extends OutputStream {
     public IOOutputStream(final IRubyObject io, Encoding encoding, boolean checkAppend, boolean verifyCanWrite) {
         this.io = io;
         this.runtime = io.getRuntime();
-        this.realIO = ( io instanceof RubyIO && !((RubyIO) io).isClosed() &&
-                ((RubyIO) io).isBuiltin("write") ) ?
-                    ((RubyIO) io) : null;
+
+        // If we can get a real IO from the object, we can do fast writes
+        RubyIO realIO = null;
+        if (io instanceof RubyIO) {
+            RubyIO tmpIO = (RubyIO) io;
+            if (fastWritable(tmpIO)) {
+                tmpIO = tmpIO.GetWriteIO();
+
+                // recheck write IO for IOness
+                if (tmpIO == io || fastWritable(tmpIO)) {
+                    realIO = tmpIO;
+                }
+            }
+        }
+        this.realIO = realIO;
+
         if (realIO == null || verifyCanWrite) {
             final String site;
             if (io.respondsTo("write")) {
@@ -89,7 +102,12 @@ public IOOutputStream(final IRubyObject io, Encoding encoding, boolean checkAppe
         }
         this.encoding = encoding;
     }
-    
+
+    protected boolean fastWritable(RubyIO io) {
+        return !io.isClosed() &&
+                io.isBuiltin("write");
+    }
+
     public IOOutputStream(final IRubyObject io, boolean checkAppend, boolean verifyCanWrite) {
         this(io, ASCIIEncoding.INSTANCE, checkAppend, verifyCanWrite);
     }