Handle unsupported fs/writeBlock requests in exec-server (#49811)

## What changed

Register `fs/writeBlock` and require filesystem initialization before handling it. Validate the file handle ID, then return an invalid-request error stating that writable file streams are unsupported.

GitOrigin-RevId: 13440e497a88ca6505ee60d95bbb771d3cae06b8

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

@@ -42,6 +42,8 @@ use crate::protocol::FsRemoveParams;
 use crate::protocol::FsRemoveResponse;
 use crate::protocol::FsWalkParams;
 use crate::protocol::FsWalkResponse;
+use crate::protocol::FsWriteBlockParams;
+use crate::protocol::FsWriteBlockResponse;
 use crate::protocol::FsWriteFileParams;
 use crate::protocol::FsWriteFileResponse;
 use crate::rpc::internal_error;
@@ -158,6 +160,17 @@ impl FileSystemHandler {
         })
     }
 
+    pub(crate) async fn write_block(
+        &self,
+        params: FsWriteBlockParams,
+    ) -> Result<FsWriteBlockResponse, JSONRPCErrorError> {
+        validate_file_handle_id(&params.handle_id)?;
+        // TODO(anp): Implement positional writes before advertising writable file streams.
+        Err(invalid_request(
+            "exec-server does not support writable file streams".to_string(),
+        ))
+    }
+
     pub(crate) async fn close(
         &self,
         params: FsCloseParams,
@@ -371,6 +384,7 @@ mod tests {
     use super::*;
     use crate::FileSystemSandboxContext;
     use crate::protocol::FsReadFileParams;
+
     use crate::protocol::FsWriteFileParams;
 
     #[tokio::test]

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

@@ -50,6 +50,8 @@ use crate::protocol::FsRemoveParams;
 use crate::protocol::FsRemoveResponse;
 use crate::protocol::FsWalkParams;
 use crate::protocol::FsWalkResponse;
+use crate::protocol::FsWriteBlockParams;
+use crate::protocol::FsWriteBlockResponse;
 use crate::protocol::FsWriteFileParams;
 use crate::protocol::FsWriteFileResponse;
 use crate::protocol::HTTP_REQUEST_METHOD;
@@ -340,6 +342,14 @@ impl ExecServerHandler {
         self.file_system.read_block(params).await
     }
 
+    pub(crate) async fn fs_write_block(
+        &self,
+        params: FsWriteBlockParams,
+    ) -> Result<FsWriteBlockResponse, JSONRPCErrorError> {
+        self.require_initialized_for("filesystem")?;
+        self.file_system.write_block(params).await
+    }
+
     pub(crate) async fn fs_close(
         &self,
         params: FsCloseParams,

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

@@ -26,9 +26,11 @@ use crate::protocol::FS_READ_DIRECTORY_METHOD;
 use crate::protocol::FS_READ_FILE_METHOD;
 use crate::protocol::FS_REMOVE_METHOD;
 use crate::protocol::FS_WALK_METHOD;
+use crate::protocol::FS_WRITE_BLOCK_METHOD;
 use crate::protocol::FS_WRITE_FILE_METHOD;
 use crate::protocol::FsCloseParams;
 use crate::protocol::FsReadBlockParams;
+use crate::protocol::FsWriteBlockParams;
 use crate::protocol::HTTP_REQUEST_METHOD;
 use crate::protocol::HttpRequestParams;
 use crate::protocol::INITIALIZE_METHOD;
@@ -151,6 +153,12 @@ pub(crate) fn build_router() -> RpcRouter<ExecServerHandler> {
             handler.fs_read_block(params).await
         },
     );
+    router.request(
+        FS_WRITE_BLOCK_METHOD,
+        |handler: Arc<ExecServerHandler>, params: FsWriteBlockParams| async move {
+            handler.fs_write_block(params).await
+        },
+    );
     router.request(
         FS_CLOSE_METHOD,
         |handler: Arc<ExecServerHandler>, params: FsCloseParams| async move {