headius · Oct 15, 2026 · Oct 8, 2026 · Oct 8, 2026
diff --git a/core/src/main/java/org/jruby/RubyArgsFile.java b/core/src/main/java/org/jruby/RubyArgsFile.java
@@ -395,12 +395,18 @@ private static IRubyObject argf_getline(ThreadContext context, final int callInf
         while (true) {
             if (!data.next_argv(context)) return context.nil;
 
-            RubyIO currentFile = (RubyIO) data.currentFile;
-
             if (isGenericInput(context, data)) {
                 context.callInfo = callInfo; // restore callInfo for kwargs
-                line = data.currentFile.callMethod(context, "gets", args);
+                line = sites(context).gets.call(context, argsFile(context), data.currentFile, args);
+
+                // An IO updates the line number itself, but other objects do not
+                if (!line.isNil() && !(data.currentFile instanceof RubyIO)) {
+                    context.runtime.setCurrentLine(context.runtime.getCurrentLine() + 1);
+                }
             } else {
+                // $stdin may be any object with gets (e.g. StringIO), so only cast once we know it is not generic input
+                RubyIO currentFile = (RubyIO) data.currentFile;
+
                 if (args.length == 0 && context.runtime.getRecordSeparatorVar().get() == globalVariables(context).getDefaultSeparator()) {
                     line = (currentFile).gets(context);
                 } else {
@@ -443,6 +449,16 @@ public static IRubyObject gets(ThreadContext context, IRubyObject recv, IRubyObj
      */
     @JRubyMethod(name = "readline", optional = 2, keywords = true, checkArity = false, writes = LASTLINE)
     public static IRubyObject readline(ThreadContext context, IRubyObject recv, IRubyObject[] args) {
+        final int callInfo = ThreadContext.resetCallInfo(context);
+        ArgsFileData data = ArgsFileData.getArgsFileData(context.runtime);
+
+        // MRI: argf_readline forwards to $stdin.readline when $stdin is not an IO
+        if (data.next_argv(context) && !(data.currentFile instanceof RubyIO)) {
+            context.callInfo = callInfo; // restore callInfo for kwargs
+            return sites(context).readline.call(context, recv, data.currentFile, args);
+        }
+
+        context.callInfo = callInfo; // restore callInfo for kwargs
         IRubyObject line = gets(context, recv, args);
 
         if (line.isNil()) throw context.runtime.newEOFError();
@@ -461,7 +477,7 @@ public static IRubyObject readlines(ThreadContext context, IRubyObject recv, IRu
 
         if (!(data.currentFile instanceof RubyIO)) {
             context.callInfo = callInfo; // restore callInfo for kwargs
-            return data.currentFile.callMethod(context, "readlines", args);
+            return sites(context).readlines.call(context, recv, data.currentFile, args);
         }
 
         var ary = newArray(context);

diff --git a/core/src/main/java/org/jruby/runtime/JavaSites.java b/core/src/main/java/org/jruby/runtime/JavaSites.java
@@ -7,6 +7,7 @@
 import org.jruby.runtime.callsite.DivCallSite;
 import org.jruby.runtime.callsite.FunctionalCachingCallSite;
 import org.jruby.runtime.callsite.MulCallSite;
+import org.jruby.runtime.callsite.NormalCachingCallSite;
 import org.jruby.runtime.callsite.PlusCallSite;
 import org.jruby.runtime.callsite.RespondToCallSite;
 import org.jruby.runtime.callsite.SuperCallSite;
@@ -484,6 +485,11 @@
 
     public static class ArgfSites {
         public final CallSite each_codepoint = new FunctionalCachingCallSite("each_codepoint");
+        // MRI: forward_current calls only public methods on $stdin (rb_funcallv_public_kw). A functional call
+        // would reach the private Kernel#gets etc. on a plain object, which read from ARGF again and recurse.
+        public final CallSite gets = new NormalCachingCallSite("gets");
+        public final CallSite readline = new NormalCachingCallSite("readline");
+        public final CallSite readlines = new NormalCachingCallSite("readlines");
     }
 
     public static class TracePointSites {

diff --git a/spec/ruby/core/argf/gets_spec.rb b/spec/ruby/core/argf/gets_spec.rb
@@ -47,3 +47,26 @@
   end
 
 end
+
+describe "ARGF.gets when reading $stdin and $stdin is not an IO" do
+  before :each do
+    @stdin = $stdin
+    $stdin = StringIO.new("a\nb\n")
+    @argf_stdin = ARGF.class.new
+  end
+
+  after :each do
+    $stdin = @stdin
+  end
+
+  it "increments lineno for each line read" do
+    @argf_stdin.gets
+    @argf_stdin.gets
+    @argf_stdin.lineno.should == 2
+  end
+
+  it "raises NoMethodError when $stdin has no public gets" do
+    $stdin = Object.new
+    -> { ARGF.class.new.gets }.should raise_error(NoMethodError)
+  end
+end
diff --git a/spec/ruby/core/argf/readline_spec.rb b/spec/ruby/core/argf/readline_spec.rb
@@ -21,3 +21,33 @@
     end
   end
 end
+
+describe "ARGF.readline when reading $stdin and $stdin is not an IO" do
+  before :each do
+    @stdin = $stdin
+  end
+
+  after :each do
+    $stdin = @stdin
+  end
+
+  it "calls readline on $stdin" do
+    $stdin = mock("stdin")
+    $stdin.should_receive(:readline).with("\n", 1, chomp: true).and_return("line")
+    ARGF.class.new.readline("\n", 1, chomp: true).should == "line"
+  end
+
+  it "raises an EOFError when reaching end of $stdin" do
+    $stdin = StringIO.new("a\n")
+    argf = ARGF.class.new
+    argf.readline.should == "a\n"
+    -> { argf.readline }.should raise_error(EOFError)
+  end
+
+  it "raises NoMethodError when $stdin has no public readline" do
+    stdin = Object.new
+    def stdin.gets(*) = "line\n"
+    $stdin = stdin
+    -> { ARGF.class.new.readline }.should raise_error(NoMethodError)
+  end
+end
diff --git a/spec/ruby/core/argf/readlines_spec.rb b/spec/ruby/core/argf/readlines_spec.rb
@@ -4,3 +4,18 @@
 describe "ARGF.readlines" do
   it_behaves_like :argf_readlines, :readlines
 end
+
+describe "ARGF.readlines when reading $stdin and $stdin is not an IO" do
+  before :each do
+    @stdin = $stdin
+  end
+
+  after :each do
+    $stdin = @stdin
+  end
+
+  it "raises NoMethodError when $stdin has no public readlines" do
+    $stdin = Object.new
+    -> { ARGF.class.new.readlines }.should raise_error(NoMethodError)
+  end
+end
diff --git a/spec/ruby/core/argf/shared/gets.rb b/spec/ruby/core/argf/shared/gets.rb
@@ -1,3 +1,5 @@
+require 'stringio'
+
 describe :argf_gets, shared: true do
   before :each do
     @file1_name = fixture __FILE__, "file1.txt"
@@ -33,6 +35,27 @@
     end
   end
 
+  describe "when reading $stdin and $stdin is not an IO" do
+    before :each do
+      @stdin = $stdin
+      $stdin = StringIO.new("a\nb\n")
+      @stdin_argf = ARGF.class.new
+    end
+
+    after :each do
+      $stdin = @stdin
+    end
+
+    it "reads each line of $stdin" do
+      @stdin_argf.send(@method).should == "a\n"
+      @stdin_argf.send(@method).should == "b\n"
+    end
+
+    it "passes keywords on to $stdin" do
+      @stdin_argf.send(@method, chomp: true).should == "a"
+    end
+  end
+
   it "reads all lines of a file" do
     argf [@file1_name] do
       lines = []