fix: prevent pipe deadlock in shell command execution (#140)

Drain stdout and stderr concurrently with child.wait() using tokio::join
to prevent deadlocks when command output exceeds the OS pipe buffer
(64KB on Linux, 16KB on macOS).

Use AsyncReadExt::take() for memory-bounded reads and
tokio::io::copy to sink for draining excess output.

Add regression test that generates 128KB of output to verify the
fix prevents deadlocks.

src/tools/builtin/shell.rs

@@ -507,25 +507,47 @@ impl ShellTool {
             .spawn()
             .map_err(|e| ToolError::ExecutionFailed(format!("Failed to spawn command: {}", e)))?;
 
-        // Wait with timeout
+        // Drain stdout/stderr concurrently with wait() to prevent deadlocks.
+        // If we call wait() without draining the pipes and the child's output
+        // exceeds the OS pipe buffer (64KB Linux, 16KB macOS), the child blocks
+        // on write and wait() never returns.
+        let stdout_handle = child.stdout.take();
+        let stderr_handle = child.stderr.take();
+
         let result = tokio::time::timeout(timeout, async {
-            let status = child.wait().await?;
+            let stdout_fut = async {
+                if let Some(mut out) = stdout_handle {
+                    let mut buf = Vec::new();
+                    (&mut out)
+                        .take(MAX_OUTPUT_SIZE as u64)
+                        .read_to_end(&mut buf)
+                        .await
+                        .ok();
+                    // Drain any remaining output so the child does not block
+                    tokio::io::copy(&mut out, &mut tokio::io::sink()).await.ok();
+                    String::from_utf8_lossy(&buf).to_string()
+                } else {
+                    String::new()
+                }
+            };
 
-            // Read stdout
-            let mut stdout = String::new();
-            if let Some(mut out) = child.stdout.take() {
-                let mut buf = vec![0u8; MAX_OUTPUT_SIZE];
-                let n = out.read(&mut buf).await.unwrap_or(0);
-                stdout = String::from_utf8_lossy(&buf[..n]).to_string();
-            }
+            let stderr_fut = async {
+                if let Some(mut err) = stderr_handle {
+                    let mut buf = Vec::new();
+                    (&mut err)
+                        .take(MAX_OUTPUT_SIZE as u64)
+                        .read_to_end(&mut buf)
+                        .await
+                        .ok();
+                    tokio::io::copy(&mut err, &mut tokio::io::sink()).await.ok();
+                    String::from_utf8_lossy(&buf).to_string()
+                } else {
+                    String::new()
+                }
+            };
 
-            // Read stderr
-            let mut stderr = String::new();
-            if let Some(mut err) = child.stderr.take() {
-                let mut buf = vec![0u8; MAX_OUTPUT_SIZE];
-                let n = err.read(&mut buf).await.unwrap_or(0);
-                stderr = String::from_utf8_lossy(&buf[..n]).to_string();
-            }
+            let (stdout, stderr, wait_result) = tokio::join!(stdout_fut, stderr_fut, child.wait());
+            let status = wait_result?;
 
             // Combine output
             let output = if stderr.is_empty() {
@@ -1184,6 +1206,26 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn test_large_output_command() {
+        let tool = ShellTool::new().with_timeout(Duration::from_secs(10));
+        let ctx = JobContext::default();
+
+        // Generate output larger than OS pipe buffer (64KB on Linux, 16KB on macOS).
+        // Without draining pipes before wait(), this would deadlock.
+        let result = tool
+            .execute(
+                serde_json::json!({"command": "python3 -c \"print('A' * 131072)\""}),
+                &ctx,
+            )
+            .await
+            .unwrap();
+
+        let output = result.result.get("output").unwrap().as_str().unwrap();
+        assert_eq!(output.len(), MAX_OUTPUT_SIZE);
+        assert_eq!(result.result.get("exit_code").unwrap().as_i64().unwrap(), 0);
+    }
+
     #[tokio::test]
     async fn test_netcat_blocked_at_execution() {
         let tool = ShellTool::new();