[2/8] Support piped stdin in exec process API (#18086)

## Summary
- Add an explicit stdin mode to process/start.
- Keep normal non-interactive exec stdin closed while allowing
pipe-backed processes.

## Stack
```text
o  #18027 [8/8] Fail exec client operations after disconnect
│
o  #18025 [7/8] Cover MCP stdio tests with executor placement
│
o  #18089 [6/8] Wire remote MCP stdio through executor
│
o  #18088 [5/8] Add executor process transport for MCP stdio
│
o  #18087 [4/8] Abstract MCP stdio server launching
│
o  #18020 [3/8] Add pushed exec process events
│
@  #18086 [2/8] Support piped stdin in exec process API
│
o  #18085 [1/8] Add MCP server environment config
│
o  main
```

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/environment.rs

@@ -346,6 +346,7 @@ mod tests {
                 env_policy: None,
                 env: Default::default(),
                 tty: false,
+                pipe_stdin: false,
                 arg0: None,
             })
             .await

codex-rs/exec-server/src/local_process.rs

@@ -59,6 +59,7 @@ struct RetainedOutputChunk {
 struct RunningProcess {
     session: ExecCommandSession,
     tty: bool,
+    pipe_stdin: bool,
     output: VecDeque<RetainedOutputChunk>,
     retained_bytes: usize,
     next_seq: u64,
@@ -165,6 +166,15 @@ impl LocalProcess {
                 TerminalSize::default(),
             )
             .await
+        } else if params.pipe_stdin {
+            codex_utils_pty::spawn_pipe_process(
+                program,
+                args,
+                params.cwd.as_path(),
+                &env,
+                &params.arg0,
+            )
+            .await
         } else {
             codex_utils_pty::spawn_pipe_process_no_stdin(
                 program,
@@ -195,6 +205,7 @@ impl LocalProcess {
                 ProcessEntry::Running(Box::new(RunningProcess {
                     session: spawned.session,
                     tty: params.tty,
+                    pipe_stdin: params.pipe_stdin,
                     output: VecDeque::new(),
                     retained_bytes: 0,
                     next_seq: 1,
@@ -339,7 +350,7 @@ impl LocalProcess {
                     status: WriteStatus::Starting,
                 });
             };
-            if !process.tty {
+            if !process.tty && !process.pipe_stdin {
                 return Ok(WriteResponse {
                     status: WriteStatus::StdinClosed,
                 });
@@ -667,6 +678,7 @@ mod tests {
             env_policy: None,
             env,
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         }
     }

codex-rs/exec-server/src/protocol.rs

@@ -69,6 +69,9 @@ pub struct ExecParams {
     pub env_policy: Option<ExecEnvPolicy>,
     pub env: HashMap<String, String>,
     pub tty: bool,
+    /// Keep non-tty stdin writable through `process/write`.
+    #[serde(default)]
+    pub pipe_stdin: bool,
     pub arg0: Option<String>,
 }
 

codex-rs/exec-server/src/server/handler/tests.rs

@@ -30,6 +30,7 @@ fn exec_params_with_argv(process_id: &str, argv: Vec<String>) -> ExecParams {
         env_policy: None,
         env: inherited_path_env(),
         tty: false,
+        pipe_stdin: false,
         arg0: None,
     }
 }

codex-rs/exec-server/src/server/processor.rs

@@ -393,6 +393,7 @@ mod tests {
             env_policy: None,
             env,
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         }
     }