feat(execd): extract stream buffer (sbuf) package for ring buffer queue

Author: Pangjiping

.github/workflows/execd-test.yml

@@ -101,6 +101,7 @@ jobs:
 
           sleep 5
           python3 tests/smoke_api.py
+          python3 tests/command_resume_test.py
       - name: Show logs
         if: always()
         run: |

components/execd/tests/command_resume_test.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright 2025 Alibaba Group Holding Ltd.
+# Copyright 2026 Alibaba Group Holding Ltd.
 #
 # Manual local test: POST /command (streaming output) -> disconnect -> GET /resume (catch-up + live tail),
 # repeat at least 3 disconnect/resume rounds, then read until execution_complete on the last connection.

specs/execd-api.yaml

@@ -502,6 +502,56 @@ paths:
         "500":
           $ref: "#/components/responses/InternalServerError"
 
+  /command/{id}/resume:
+    get:
+      summary: Resume command SSE stream (replay and optional live tail)
+      description: |
+        Replays stdout/stderr events from the server-side ring buffer for events with
+        `eid` strictly greater than `after_eid`, then—if the command is still running and
+        no other client holds the primary SSE slot—continues streaming live events until
+        completion or client disconnect. Event shape matches `POST /command` (`ServerStreamEvent`).
+
+        This endpoint is mutually exclusive with the primary `POST /command` SSE: if that
+        connection is still active, the server responds with 409 Conflict.
+      operationId: resumeCommandStream
+      tags:
+        - Command
+      parameters:
+        - name: id
+          in: path
+          required: true
+          description: Command ID returned by RunCommand
+          schema:
+            type: string
+          example: cmd-abc123
+        - name: after_eid
+          in: query
+          required: false
+          description: |
+            Only events with `eid` greater than this value are replayed from the buffer first
+            (then optional live tail). Omit or use `0` to replay from the oldest buffered events.
+          schema:
+            type: integer
+            format: int64
+            minimum: 0
+            default: 0
+          example: 42
+      responses:
+        "200":
+          description: Stream of command execution events (replay then optional live continuation)
+          content:
+            text/event-stream:
+              schema:
+                $ref: "#/components/schemas/ServerStreamEvent"
+        "400":
+          $ref: "#/components/responses/BadRequest"
+        "404":
+          $ref: "#/components/responses/NotFound"
+        "409":
+          $ref: "#/components/responses/Conflict"
+        "500":
+          $ref: "#/components/responses/InternalServerError"
+
   /command/{id}/logs:
     get:
       summary: Get background command stdout/stderr (non-streamed)
@@ -1396,6 +1446,16 @@ components:
             code: FILE_NOT_FOUND
             message: "file not found"
 
+    Conflict:
+      description: Request conflicts with current server state (e.g. resource in use)
+      content:
+        application/json:
+          schema:
+            $ref: "#/components/schemas/ErrorResponse"
+          example:
+            code: INVALID_REQUEST_BODY
+            message: "primary SSE stream is still active; disconnect it before resuming"
+
     InternalServerError:
       description: Runtime server error during operation
       content: