Fix buf error on interrupt.

ext/zlib/zlib.c

@@ -1094,8 +1094,9 @@ zstream_run_func(struct zstream_run_args *args)
             break;
         }
 
-        if (err != Z_OK && err != Z_BUF_ERROR)
+        if (err != Z_OK && err != Z_BUF_ERROR) {
             break;
+        }
 
         if (z->stream.avail_out > 0) {
             z->flags |= ZSTREAM_FLAG_IN_STREAM;
@@ -1170,12 +1171,17 @@ zstream_run_try(VALUE value_arg)
     /* retry if no exception is thrown */
     if (err == Z_OK && args->interrupt) {
        args->interrupt = 0;
-       goto loop;
+
+       /* Retry only if both avail_in > 0 (more input to process) and avail_out > 0
+        * (output buffer has space). If avail_out == 0, the buffer is full and should
+        * be consumed by the caller first. If avail_in == 0, there's nothing more to process. */
+       if (z->stream.avail_in > 0 && z->stream.avail_out > 0) {
+          goto loop;
+       }
     }
 
-    if (flush != Z_FINISH && err == Z_BUF_ERROR
-	    && z->stream.avail_out > 0) {
-	z->flags |= ZSTREAM_FLAG_IN_STREAM;
+    if (flush != Z_FINISH && err == Z_BUF_ERROR && z->stream.avail_out > 0) {
+        z->flags |= ZSTREAM_FLAG_IN_STREAM;
     }
 
     zstream_reset_input(z);

test/zlib/test_zlib.rb

@@ -1266,6 +1266,36 @@ def test_gzfile_read_size_boundary
         end
       }
     end
+
+    # Test for signal interrupt bug: Z_BUF_ERROR with avail_out > 0
+    # This reproduces the issue where thread wakeup during GzipReader operations
+    # can cause Z_BUF_ERROR to be raised incorrectly
+    def test_thread_wakeup_interrupt
+      pend 'fails' if RUBY_ENGINE == 'truffleruby'
+      content = SecureRandom.base64(5000)
+      gzipped = Zlib.gzip(content)
+
+      1000.times do
+        thr = Thread.new do
+          loop do
+            Zlib::GzipReader.new(StringIO.new(gzipped)).read
+          end
+        end
+
+        # Wakeup the thread multiple times to trigger interrupts
+        10.times do
+          thr.wakeup
+          Thread.pass
+        end
+
+        # Give thread a moment to process
+        sleep 0.001
+
+        # Clean up
+        thr.kill
+        thr.join
+      end
+    end
   end
 
   class TestZlibGzipWriter < Test::Unit::TestCase
@@ -1534,5 +1564,36 @@ def test_gunzip_no_memory_leak
         10_000.times {Zlib.gunzip(d)}
       };
     end
+
+    # Test for signal interrupt bug: Z_BUF_ERROR with avail_out > 0
+    # This reproduces the issue where thread wakeup during GzipReader operations
+    # can cause Z_BUF_ERROR to be raised incorrectly
+    def test_thread_wakeup_interrupt
+      pend 'fails' if RUBY_ENGINE == 'truffleruby'
+
+      content = SecureRandom.base64(5000)
+      gzipped = Zlib.gzip(content)
+
+      1000.times do
+        thr = Thread.new do
+          loop do
+            Zlib::GzipReader.new(StringIO.new(gzipped)).read
+          end
+        end
+
+        # Wakeup the thread multiple times to trigger interrupts
+        10.times do
+          thr.wakeup
+          Thread.pass
+        end
+
+        # Give thread a moment to process
+        sleep 0.001
+
+        # Clean up
+        thr.kill
+        thr.join
+      end
+    end
   end
 end