Compute the DSM base hash at startup

communication/src/main/java/datadog/communication/ddagent/DDAgentFeaturesDiscovery.java

@@ -199,6 +199,10 @@ private void doDiscovery(State newState) {
           newState.evpProxyEndpoint,
           newState.telemetryProxyEndpoint);
     }
+    // even if /info failed or sent no container tags hash (e.g. the Lambda extension)
+    if (BaseHash.getBaseHashStr() == null) {
+      BaseHash.recalcBaseHash(null);
+    }
   }
 
   private String probeTracesEndpoint(State newState, List<String> endpoints) {

communication/src/test/java/datadog/communication/ddagent/DDAgentFeaturesDiscoveryBaseHashForkedTest.java

@@ -0,0 +1,63 @@
+package datadog.communication.ddagent;
+
+import static datadog.communication.http.OkHttpUtils.DATADOG_CONTAINER_TAGS_HASH;
+import static datadog.trace.api.ProtocolVersion.V0_5;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import datadog.metrics.api.Monitoring;
+import datadog.trace.api.BaseHash;
+import okhttp3.HttpUrl;
+import okhttp3.OkHttpClient;
+import okhttp3.mockwebserver.Dispatcher;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import okhttp3.mockwebserver.RecordedRequest;
+import org.junit.jupiter.api.Test;
+
+// forked: the base hash is global, and this needs it unset
+class DDAgentFeaturesDiscoveryBaseHashForkedTest {
+
+  @Test
+  void setsBaseHashWithoutContainerTagsHashUntilOneArrives() throws Exception {
+    HttpUrl agentNotUp;
+    try (MockWebServer agent = new MockWebServer()) {
+      agentNotUp = agent.url("/");
+    }
+    assertNull(BaseHash.getBaseHashStr());
+    discover(agentNotUp);
+    assertEquals(BaseHash.calc(null), BaseHash.getBaseHash());
+
+    try (MockWebServer agent = agent(null)) {
+      discover(agent.url("/"));
+    }
+    assertEquals(BaseHash.calc(null), BaseHash.getBaseHash());
+
+    try (MockWebServer agent = agent("test-hash")) {
+      discover(agent.url("/"));
+    }
+    assertEquals(BaseHash.calc("test-hash"), BaseHash.getBaseHash());
+  }
+
+  private static MockWebServer agent(String containerTagsHash) {
+    MockWebServer agent = new MockWebServer();
+    agent.setDispatcher(
+        new Dispatcher() {
+          @Override
+          public MockResponse dispatch(RecordedRequest request) {
+            MockResponse response = new MockResponse().setBody("{}");
+            return containerTagsHash == null
+                ? response
+                : response.setHeader(DATADOG_CONTAINER_TAGS_HASH, containerTagsHash);
+          }
+        });
+    return agent;
+  }
+
+  // a new instance each time: discover() skips a run in the same millisecond as the last one
+  private static void discover(HttpUrl agentUrl) {
+    new DDAgentFeaturesDiscovery(
+            new OkHttpClient(), Monitoring.DISABLED, agentUrl, V0_5, true, false)
+        .discover();
+  }
+}