[CI] Read subprocess stdout on a background thread to avoid EOF deadlock (#36673)

python/sglang/test/ci/ci_utils.py

@@ -223,10 +223,18 @@ def run_unittest_files(
                     errors="ignore",  # Ignore non-UTF-8 bytes to prevent UnicodeDecodeError
                 )
                 output_lines = []
-                for line in process.stdout:
-                    logger.info(line.rstrip())
-                    output_lines.append(line)
+
+                def read_output():
+                    for line in process.stdout:
+                        logger.info(line.rstrip())
+                        output_lines.append(line)
+
+                # Read stdout on a background thread so the main thread won't block on EOF.
+                reader_thread = threading.Thread(target=read_output, daemon=True)
+                reader_thread.start()
                 process.wait()
+                # Bounded wait for the reader to finish.
+                reader_thread.join(timeout=60)
             else:
                 process = subprocess.Popen(cmd, stdout=None, stderr=None)
                 process.wait()