Define exec-server protocol types for streamed file writes (#49778)

## What changed

- Add `read` and `replace` modes to `fs/open`, defaulting omitted modes to `read` for existing callers.
- Define `fs/writeBlock` request and response types for offset-based writes, with a maximum decoded chunk size of 1 MiB.
- Add the `fileWriteStreaming` capability, defaulting to `false`. Keep writable streams unsupported in the server and explicitly reject replacement opens.

## Testing

Add a protocol test verifying that legacy `fs/open` requests without a mode deserialize as read-only opens.

GitOrigin-RevId: 4d3997717e821cd1f62df12a03c8eec36332a66d

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

@@ -87,6 +87,7 @@ pub use codex_file_system::EnvironmentAccessKey;
 pub use codex_file_system::ExecutorFileSystem;
 pub use codex_file_system::ExecutorFileSystemFuture;
 pub use codex_file_system::FILE_READ_CHUNK_SIZE;
+pub use codex_file_system::FILE_WRITE_CHUNK_SIZE;
 pub use codex_file_system::FileMetadata;
 pub use codex_file_system::FileSystemEnvironmentAccessor;
 pub use codex_file_system::FileSystemReadStream;
@@ -184,6 +185,7 @@ pub use protocol::FsCreateDirectoryParams;
 pub use protocol::FsCreateDirectoryResponse;
 pub use protocol::FsGetMetadataParams;
 pub use protocol::FsGetMetadataResponse;
+pub use protocol::FsOpenMode;
 pub use protocol::FsOpenParams;
 pub use protocol::FsOpenResponse;
 pub use protocol::FsReadBlockParams;
@@ -197,6 +199,8 @@ pub use protocol::FsRemoveParams;
 pub use protocol::FsRemoveResponse;
 pub use protocol::FsWalkParams;
 pub use protocol::FsWalkResponse;
+pub use protocol::FsWriteBlockParams;
+pub use protocol::FsWriteBlockResponse;
 pub use protocol::FsWriteFileParams;
 pub use protocol::FsWriteFileResponse;
 pub use protocol::HttpHeader;

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

@@ -11,6 +11,7 @@ use crate::FileSystemResult;
 use crate::FileSystemSandboxContext;
 use crate::protocol::FS_READ_BLOCK_METHOD;
 use crate::protocol::FsCloseParams;
+use crate::protocol::FsOpenMode;
 use crate::protocol::FsOpenParams;
 use crate::protocol::FsReadBlockParams;
 
@@ -37,6 +38,7 @@ pub(super) async fn open(
         .fs_open(FsOpenParams {
             handle_id: registration.handle_id.clone(),
             path,
+            mode: FsOpenMode::Read,
             sandbox: sandbox.cloned(),
         })
         .await

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

@@ -28,6 +28,7 @@ use crate::protocol::FsCreateDirectoryParams;
 use crate::protocol::FsCreateDirectoryResponse;
 use crate::protocol::FsGetMetadataParams;
 use crate::protocol::FsGetMetadataResponse;
+use crate::protocol::FsOpenMode;
 use crate::protocol::FsOpenParams;
 use crate::protocol::FsOpenResponse;
 use crate::protocol::FsReadBlockParams;
@@ -122,6 +123,12 @@ impl FileSystemHandler {
         params: FsOpenParams,
     ) -> Result<FsOpenResponse, JSONRPCErrorError> {
         validate_file_handle_id(&params.handle_id)?;
+        // TODO(anp): Enable replacement opens when writable file streams are implemented.
+        if params.mode == FsOpenMode::Replace {
+            return Err(invalid_request(
+                "exec-server does not support writable file streams".to_string(),
+            ));
+        }
         let file = self
             .file_system
             .open_file_for_read(&params.path, params.sandbox.as_ref())

codex-rs/exec-server/tests/file_stream.rs

@@ -8,6 +8,7 @@ use codex_exec_server::ExecServerClient;
 use codex_exec_server::ExecServerError;
 use codex_exec_server::ExecutorFileSystem;
 use codex_exec_server::FsCloseParams;
+use codex_exec_server::FsOpenMode;
 use codex_exec_server::FsOpenParams;
 use codex_exec_server::FsReadBlockParams;
 use codex_exec_server::FsReadBlockResponse;
@@ -227,6 +228,7 @@ async fn read_block_supports_non_sequential_offsets_and_lengths() -> Result<()>
         .fs_open(FsOpenParams {
             handle_id: Uuid::new_v4().simple().to_string(),
             path: PathUri::from_host_native_path(path)?,
+            mode: FsOpenMode::Read,
             sandbox: None,
         })
         .await?;
@@ -293,6 +295,7 @@ async fn open_enforces_the_per_connection_limit_and_close_releases_capacity() ->
             .fs_open(FsOpenParams {
                 handle_id: Uuid::new_v4().simple().to_string(),
                 path: path.clone(),
+                mode: FsOpenMode::Read,
                 sandbox: None,
             })
             .await?;
@@ -303,6 +306,7 @@ async fn open_enforces_the_per_connection_limit_and_close_releases_capacity() ->
         .fs_open(FsOpenParams {
             handle_id: Uuid::new_v4().simple().to_string(),
             path: path.clone(),
+            mode: FsOpenMode::Read,
             sandbox: None,
         })
         .await
@@ -327,6 +331,7 @@ async fn open_enforces_the_per_connection_limit_and_close_releases_capacity() ->
         .fs_open(FsOpenParams {
             handle_id: Uuid::new_v4().simple().to_string(),
             path,
+            mode: FsOpenMode::Read,
             sandbox: None,
         })
         .await?;
@@ -352,6 +357,7 @@ async fn open_rejects_handle_ids_longer_than_32_bytes() -> Result<()> {
         .fs_open(FsOpenParams {
             handle_id: "x".repeat(33),
             path: PathUri::from_host_native_path(path)?,
+            mode: FsOpenMode::Read,
             sandbox: None,
         })
         .await