Support sampling/createMessage per MCP specification
## Motivation and Context The MCP specification defines `sampling/createMessage` to request LLM completions from the client. The Ruby SDK did not yet support server-to-client requests, so the server had no way to ask the client to sample an LLM. This caused the `tools-call-sampling` conformance test to fail. This adds `Server#create_sampling_message` to send a `sampling/createMessage` request to the client via the transport layer. The method validates that the client declared the `sampling` capability during initialization, and optionally checks the `sampling.tools` capability when tools or tool_choice parameters are provided. On the transport side, `Transport#send_request` is introduced as an abstract method, with implementations in both `StdioTransport` and `StreamableHTTPTransport`. Each transport assigns a unique request ID, sends the JSON-RPC request to the client, and blocks until the client responds. Client capabilities are stored per session via `ServerSession`, so validation is scoped to the originating client. `ServerSession#create_sampling_message` uses `build_sampling_params` for capability validation and sends the request directly through the transport with the session ID, following the same pattern as `notify_progress`. `Server#create_sampling_message` is the public API for single-client transports (e.g., StdioTransport). For multi-client transports (e.g., StreamableHTTPTransport), `ServerSession#create_sampling_message` must be used to route requests to the correct client. Tools using `server_context.create_sampling_message` automatically route to the correct session regardless of transport. `StreamableHTTPTransport#send_request` requires `session_id` to prevent broadcasting sampling requests to all connected clients. Missing or invalid session IDs raise explicit errors instead of silently failing. The `include_context` parameter is soft-deprecated in the MCP specification but is included for compatibility with existing clients that declare the `sampling.context` capability. Python and TypeScript SDKs also retain this parameter. Ref: https://modelcontextprotocol.io/specification/latest/server/utilities/sampling ## How Has This Been Tested? Added comprehensive tests in `test/mcp/server_sampling_test.rb` covering required and optional parameters, capability validation, error handling, nil-param omission, per-session capability isolation, and HTTP init capability scoping. Added transport tests in `test/mcp/server/transports/stdio_transport_test.rb` and `test/mcp/server/transports/streamable_http_transport_test.rb` for `send_request` round-trip behavior, error responses, stateless mode rejection, missing session_id, invalid session_id, and no-active-stream error. Conformance: `tools-call-sampling` is removed from expected failures. The conformance server's `TestSampling` tool now calls `server_context.create_sampling_message` instead of returning a stub error. This aligns the Ruby SDK with the MCP specification and other SDK implementations. ## Breaking Changes None for end users. `Transport#send_request` is a new abstract method. `ServerSession#create_sampling_message` is a new method.
This commit is contained in:
Koichi ITO
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1 changed files with 1449 additions and 13 deletions
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@ -151,6 +151,247 @@ module MCP
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$stdout = original_stdout
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end
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end
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test "send_request sends request to stdout and waits for response" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Send response from client in a thread.
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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response = {
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jsonrpc: "2.0",
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id: request[:id],
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result: { content: "test response" },
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}
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writer.puts(response.to_json)
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writer.flush
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end
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result = @transport.send_request("test/method", { param: "value" })
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assert_equal({ content: "test response" }, result)
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# Verify request was sent.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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assert_equal("2.0", request[:jsonrpc])
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assert_equal("test/method", request[:method])
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assert_equal({ param: "value" }, request[:params])
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assert(request[:id])
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request raises on error response from client" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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error_response = {
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jsonrpc: "2.0",
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id: request[:id],
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error: { code: -1, message: "User rejected sampling request" },
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}
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writer.puts(error_response.to_json)
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writer.flush
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end
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error = assert_raises(StandardError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_equal("Client returned an error for sampling/createMessage request (code: -1): User rejected sampling request", error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request does not double-report intentional raises via exception_reporter" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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reported_errors = []
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original_reporter = MCP.configuration.exception_reporter
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begin
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MCP.configuration.exception_reporter = ->(e, ctx) { reported_errors << [e, ctx] }
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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error_response = {
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jsonrpc: "2.0",
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id: request[:id],
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error: { code: -1, message: "rejected" },
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}
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writer.puts(error_response.to_json)
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writer.flush
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end
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assert_raises(StandardError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_empty(reported_errors)
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ensure
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MCP.configuration.exception_reporter = original_reporter
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request processes interleaved requests via session" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Initialize a session so @session is set.
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session = MCP::ServerSession.new(server: @server, transport: @transport)
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@transport.instance_variable_set(:@session, session)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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# Send an interleaved ping request before the response.
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ping = { jsonrpc: "2.0", method: "ping", id: "ping-1" }
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writer.puts(ping.to_json)
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writer.flush
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sleep(0.05) # Wait for the ping to be processed.
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# Then send the actual response.
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response = {
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jsonrpc: "2.0",
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id: request[:id],
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result: { content: "done" },
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}
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writer.puts(response.to_json)
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writer.flush
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end
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result = @transport.send_request("test/method", { param: "value" })
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assert_equal({ content: "done" }, result)
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# Verify the interleaved ping was handled (response sent to output).
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lines = output.string.lines
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ping_response = lines.find { |l| l.include?("ping-1") }
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assert(ping_response, "Interleaved ping request should have been handled")
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request raises when transport is closed while waiting" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Close transport while waiting for response.
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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@transport.instance_variable_set(:@open, false)
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writer.close
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end
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error = assert_raises(RuntimeError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_equal("Transport closed while waiting for response to sampling/createMessage request.", error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue IOError
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nil
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end
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begin
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reader.close
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rescue IOError
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nil
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end
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end
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end
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end
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end
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end
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