fix(harness): prevent pipe deadlock in LocalFilesystemWithShell.execute

Drain stdout/stderr on daemon threads concurrently with Process.waitFor.
Previously a child writing more than the OS pipe buffer (~4 KB on Windows,
64 KB on Linux) blocked in write() while the parent blocked in waitFor(),
deadlocking until the command was forcibly killed and misreported as a
timeout (exit 124).

Mirrors the fix already applied to ShellCommandTool in agentscope-core.
Adds a regression test that prints ~70 KB from the shell and asserts the
command completes with exit 0 and full output.

Fixes #2838

Author: zzz-ghost

agentscope-harness/src/main/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShell.java

@@ -21,7 +21,9 @@
 import io.agentscope.harness.agent.filesystem.sandbox.AbstractSandboxFilesystem;
 import io.agentscope.harness.agent.workspace.LocalFsMode;
 import io.agentscope.harness.agent.workspace.PathPolicy;
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
@@ -335,14 +337,27 @@ public ExecuteResponse execute(
 
             Process proc = pb.start();
 
+            // stdout/stderr must be drained concurrently with waitFor: if the child writes
+            // more than the OS pipe buffer (~4 KB on Windows, 64 KB default on Linux) while
+            // the parent blocks in waitFor, both sides deadlock and every such command is
+            // misreported as a timeout (exit 124).
+            ByteArrayOutputStream stdoutBuf = new ByteArrayOutputStream();
+            ByteArrayOutputStream stderrBuf = new ByteArrayOutputStream();
+            Thread stdoutDrainer = drainAsync(proc.getInputStream(), stdoutBuf);
+            Thread stderrDrainer = drainAsync(proc.getErrorStream(), stderrBuf);
+
             boolean finished = proc.waitFor(effectiveTimeout, TimeUnit.SECONDS);
+            if (!finished) {
+                proc.destroyForcibly();
+            }
+            joinQuietly(stdoutDrainer);
+            joinQuietly(stderrDrainer);
 
             Charset outputCharset = outputCharset(osName);
-            String stdout = new String(proc.getInputStream().readAllBytes(), outputCharset);
-            String stderr = new String(proc.getErrorStream().readAllBytes(), outputCharset);
+            String stdout = stdoutBuf.toString(outputCharset);
+            String stderr = stderrBuf.toString(outputCharset);
 
             if (!finished) {
-                proc.destroyForcibly();
                 String msg;
                 if (timeoutSeconds != null) {
                     msg =
@@ -432,6 +447,38 @@ private Path resolveExecuteCwd(RuntimeContext rc) {
         return namespaced;
     }
 
+    /**
+     * Continuously copies a subprocess stream into {@code buf} on a daemon thread so the child
+     * never blocks on a full OS pipe buffer. Read errors (e.g. the stream closing when the
+     * process is destroyed on timeout) end the drainer quietly.
+     */
+    private static Thread drainAsync(InputStream in, ByteArrayOutputStream buf) {
+        Thread t =
+                new Thread(
+                        () -> {
+                            byte[] chunk = new byte[8192];
+                            int n;
+                            try {
+                                while ((n = in.read(chunk)) != -1) {
+                                    buf.write(chunk, 0, n);
+                                }
+                            } catch (IOException ignored) {
+                                // Stream closed because the process was destroyed; nothing to do.
+                            }
+                        });
+        t.setDaemon(true);
+        t.start();
+        return t;
+    }
+
+    private static void joinQuietly(Thread t) {
+        try {
+            t.join(5000);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+
     static Charset outputCharset(String osName) {
         return outputCharset(osName, System.getProperty("native.encoding"));
     }

agentscope-harness/src/test/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShellTest.java

@@ -16,10 +16,14 @@
 package io.agentscope.harness.agent.filesystem.local;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 
+import io.agentscope.harness.agent.filesystem.model.ExecuteResponse;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.nio.file.Path;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 class LocalFilesystemWithShellTest {
 
@@ -41,4 +45,29 @@ void outputCharset_fallsBackToDefaultWhenWindowsNativeEncodingIsUnavailable() {
                 Charset.defaultCharset(),
                 LocalFilesystemWithShell.outputCharset("Windows 10", null));
     }
+
+    @Test
+    void execute_outputLargerThanOsPipeBufferCompletesWithoutDeadlock(@TempDir Path tempDir) {
+        // ~68-72 KB of stdout: beyond the OS pipe buffer (~4 KB on Windows, 64 KB on Linux),
+        // below the default maxOutputBytes cap. Before stdout/stderr were drained concurrently
+        // with waitFor, this deadlocked and was misreported as a timeout (exit 124).
+        int lines = 4000;
+        String payload = "0123456789abcdef"; // 16 chars per line
+        boolean windows = System.getProperty("os.name").toLowerCase().contains("win");
+        String command =
+                windows
+                        ? "for /l %i in (1,1," + lines + ") do @echo " + payload
+                        : "i=0; while [ \"$i\" -lt "
+                                + lines
+                                + " ]; do echo "
+                                + payload
+                                + "; i=$((i+1)); done";
+
+        LocalFilesystemWithShell fs = new LocalFilesystemWithShell(tempDir);
+        ExecuteResponse resp = fs.execute(null, command, 60);
+
+        assertEquals(0, resp.exitCode(), "unexpected exit code, output: " + resp.output());
+        assertFalse(resp.truncated());
+        assertEquals(lines, resp.output().split(payload, -1).length - 1);
+    }
 }