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fix: prevent interactive commands from stalling MCP shell #21

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fix: prevent interactive commands from stalling MCP shell #21

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56 changes: 53 additions & 3 deletions shell/main.go
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Original file line number Diff line number Diff line change
Expand Up @@ -6,8 +6,10 @@ import (
"log"
"os"
"os/exec"
"os/signal"
"strconv"
"strings"
"syscall"
"time"

"github.com/modelcontextprotocol/go-sdk/mcp"
Expand All @@ -29,6 +31,24 @@ type ExecuteCommandOutput struct {
Error string `json:"error,omitempty" jsonschema:"error message if execution failed"`
}

// Interactive flags that commonly cause commands to hang
var interactiveFlags = []string{
"-i", "--interactive", "--tty", "-t",
"vim", "vi", "nano", "emacs", "less", "more", "top", "htop",
"ftp", "sftp", "ssh", "ping", "tail -f", "tail -F",
}

// isInteractiveCommand checks if the script contains interactive commands
func isInteractiveCommand(script string) bool {
scriptLower := strings.ToLower(script)
for _, flag := range interactiveFlags {
if strings.Contains(scriptLower, flag) {
return true
}
}
return false
}

// getShellCommand returns the shell command to use, defaulting to "sh" if not set
func getShellCommand() string {
shellCmd := os.Getenv("SHELL_CMD")
Expand Down Expand Up @@ -70,11 +90,17 @@ func ExecuteCommand(ctx context.Context, req *mcp.CallToolRequest, input Execute
timeout = getTimeout()
}

// Warn about interactive commands (but still attempt execution with proper safeguards)
warningMsg := ""
if isInteractiveCommand(input.Script) {
warningMsg = "Warning: Command appears to be interactive and may hang. "
}

// Create a context with timeout
cmdCtx, cancel := context.WithTimeout(ctx, time.Duration(timeout)*time.Second)
defer cancel()

// Get shell command from environment variable (default: "sh")
// Get shell command from environment variable (default: "sh -c")
shellCmd := getShellCommand()

// Parse shell command - support both single command and command with args
Expand All @@ -97,11 +123,24 @@ func ExecuteCommand(ctx context.Context, req *mcp.CallToolRequest, input Execute
cmd.Dir = workDir
}

// CRITICAL FIX: Set process group to enable killing entire process tree
cmd.SysProcAttr = &syscall.SysProcAttr{
Setpgid: true, // Create new process group
}

// Create buffers to capture stdout and stderr separately
var stdoutBuf, stderrBuf bytes.Buffer
cmd.Stdout = &stdoutBuf
cmd.Stderr = &stderrBuf

// Set environment variables to force non-interactive mode
env := os.Environ()
env = append(env, "CI=true") // Common flag for CI/CD to disable interactive mode
env = append(env, "TERM=dumb") // Force dumb terminal
env = append(env, "INPUT=/dev/null") // Redirect stdin from /dev/null
env = append(env, "NONINTERACTIVE=1") // Common flag for non-interactive mode
cmd.Env = env

// Execute command
err := cmd.Run()

Expand All @@ -119,12 +158,23 @@ func ExecuteCommand(ctx context.Context, req *mcp.CallToolRequest, input Execute
} else {
// Context timeout or other error
if cmdCtx.Err() == context.DeadlineExceeded {
errorMsg = "Command timed out"
errorMsg = "Command timed out after " + strconv.Itoa(timeout) + " seconds"

// CRITICAL: Kill the entire process group on timeout
if cmd.Process != nil && cmd.Process.Pid > 0 {
// Kill the process group (negative PID)
syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
}
}
exitCode = -1
}
}

// Add warning to stderr if interactive command detected
if warningMsg != "" {
stderrBuf.WriteString("\n" + warningMsg)
}

output := ExecuteCommandOutput{
Script: input.Script,
Stdout: stdoutBuf.String(),
Expand Down Expand Up @@ -161,7 +211,7 @@ func main() {
// Add tool for executing shell scripts
mcp.AddTool(server, &mcp.Tool{
Name: configurableName,
Description: "Execute a shell script and return the output, exit code, and any errors. The shell command can be configured via SHELL_CMD environment variable (default: 'sh -c'). The working directory can be set via SHELL_WORKING_DIR environment variable. The default timeout can be configured via SHELL_TIMEOUT environment variable (default: 30 seconds). An initialization script can be run before server startup via SHELL_INIT_SCRIPT environment variable.",
Description: "Execute a shell script and return the output, exit code, and any errors. The shell command can be configured via SHELL_CMD environment variable (default: 'sh -c'). The working directory can be set via SHELL_WORKING_DIR environment variable. The default timeout can be configured via SHELL_TIMEOUT environment variable (default: 30 seconds). An initialization script can be run before server startup via SHELL_INIT_SCRIPT environment variable. NOTE: Interactive commands are detected and force non-interactive mode with CI=true, TERM=dumb, and stdin redirected from /dev/null. Process group management ensures proper cleanup on timeout.",
}, ExecuteCommand)

// Run the server
Expand Down
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