fix(test): seed the mock collector before concurrent same-service reports

The mock collector's first-ever insert for a service name is a
check-then-act race (SegmentItems.addSegmentItem): when the fork test's
parent and child post their first segments concurrently under the same
service name, one segment is silently dropped while both reporters get
HTTP 200 — reproduced 15/200 with barrier-synchronized first POSTs and
0/200 once the key exists. This was the remaining sw_fork_support CI
flake (either side's segment could vanish, transport-independent).

Seed the service name via a parent-only /ping warm-up (which also serves
as the readiness probe) before /users triggers the concurrent pair; the
expected data now validates all three segments. The real fix belongs in
skywalking-agent-test-tool (computeIfAbsent) and an image repin.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: wu-sheng

tests/plugin/web/sw_fork_support/expected.data.yml

@@ -17,8 +17,27 @@
 
 segmentItems:
   - serviceName: provider
-    segmentSize: 2
+    segmentSize: 3
     segments:
+      - segmentId: not null
+        spans:
+          - operationName: /ping
+            parentSpanId: -1
+            spanId: 0
+            spanLayer: Http
+            tags:
+              - key: http.method
+                value: GET
+              - key: http.url
+                value: http://0.0.0.0:9090/ping
+              - key: http.status_code
+                value: '200'
+            startTime: gt 0
+            endTime: gt 0
+            componentId: 7001
+            spanType: Entry
+            peer: not null
+            skipAnalysis: false
       - segmentId: not null
         spans:
           - operationName: /users

tests/plugin/web/sw_fork_support/services/app.py

@@ -50,6 +50,14 @@ def users():
 
         frontend = Flask('frontend')
 
+        # readiness/warm-up endpoint: its segment arrives alone and seeds the service name
+        # in the mock collector, whose first-insert for a service is not concurrency-safe
+        # (SegmentItems.addSegmentItem check-then-act) — the parent and child later report
+        # their /users segments concurrently under the same service name
+        @frontend.route('/ping', methods=['GET'])
+        def ping():
+            return jsonify({'song': 'Despacito'})
+
         @frontend.route('/users', methods=['GET'])
         def call_backend():
             res = requests.get('http://127.0.0.1:9091/users', timeout=5)

tests/plugin/web/sw_fork_support/test_fork_support.py

@@ -25,7 +25,10 @@
 @pytest.fixture
 def prepare():
     # type: () -> Callable
-    return lambda *_: requests.get('http://0.0.0.0:9090/users', timeout=5).raise_for_status()
+    # /ping (parent-only) also seeds the service name in the mock collector before the
+    # parent and child post their /users segments concurrently: the collector's very first
+    # insert for a service name is not concurrency-safe and can silently drop one segment
+    return lambda *_: requests.get('http://0.0.0.0:9090/ping', timeout=5).raise_for_status()
 
 
 class TestPlugin(TestPluginBase):
@@ -40,6 +43,9 @@ class TestPlugin(TestPluginBase):
 
     @pytest.mark.parametrize('version', ['grpcio>=1.83'])
     def test_plugin(self, docker_compose, version):
+        response = requests.get('http://0.0.0.0:9090/users', timeout=5)
+        assert response.status_code == 200
+
         self.validate()
 
         stdout, stderr = docker_compose.get_logs()