Update BaseHash when process tags are updated

internal-api/src/main/java/datadog/trace/api/BaseHash.java

@@ -5,12 +5,17 @@
 public final class BaseHash {
   private static volatile long baseHash;
   private static volatile String baseHashStr;
+  private static volatile String lastContainerTagsHash;
 
   private BaseHash() {}
 
   public static void recalcBaseHash(String containerTagsHash) {
-    long hash = calc(containerTagsHash);
-    updateBaseHash(hash);
+    lastContainerTagsHash = containerTagsHash;
+    updateBaseHash(calc(containerTagsHash));
+  }
+
+  static void recalcBaseHash() {
+    updateBaseHash(calc(lastContainerTagsHash));
   }
 
   public static void updateBaseHash(long hash) {

internal-api/src/main/java/datadog/trace/api/ProcessTags.java

@@ -182,6 +182,7 @@ public static void addTag(String key, String value) {
         Lazy.stringListForm = null;
         Lazy.utf8ListForm = null;
       }
+      BaseHash.recalcBaseHash();
     }
   }
 
@@ -242,6 +243,7 @@ public static void reset(Config config) {
       empty();
       enabled = config.isExperimentalPropagateProcessTagsEnabled();
       Lazy.TAGS.putAll(Lazy.loadTags(config));
+      BaseHash.recalcBaseHash();
     }
   }
 }

internal-api/src/test/groovy/datadog/trace/api/BaseHashTest.groovy

@@ -1,14 +1,26 @@
 package datadog.trace.api
 
-import datadog.trace.test.util.DDSpecification
-
 import static datadog.trace.api.config.GeneralConfig.ENV
 import static datadog.trace.api.config.GeneralConfig.EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED
 import static datadog.trace.api.config.GeneralConfig.PRIMARY_TAG
 import static datadog.trace.api.config.GeneralConfig.SERVICE_NAME
 
+import datadog.trace.test.util.DDSpecification
+
 class BaseHashTest extends DDSpecification {
 
+
+  def setup() {
+    // start with fresh process tags
+    ProcessTags.reset()
+  }
+
+  def cleanup() {
+    // restore the default enablement
+    injectSysConfig(EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED, "true")
+    ProcessTags.reset()
+  }
+
   def "Service used in hash calculation"() {
     when:
     def firstBaseHash = BaseHash.calc(null)
@@ -46,17 +58,54 @@ class BaseHashTest extends DDSpecification {
     when:
     def firstBaseHash = BaseHash.calc(null)
 
-    injectSysConfig(EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED, "true")
-    ProcessTags.reset()
     ProcessTags.addTag("000", "first")
     def secondBaseHash = BaseHash.calc(null)
 
     then:
     firstBaseHash != secondBaseHash
     assert ProcessTags.getTagsForSerialization().startsWithAny("000:first,")
-    cleanup:
+  }
+
+  def "addTag triggers BaseHash recalculation"() {
+    given:
+    BaseHash.recalcBaseHash("container-hash-1")
+    def hashBefore = BaseHash.getBaseHash()
+
+    when:
+    ProcessTags.addTag("cluster.name", "new-cluster")
+
+    then:
+    BaseHash.getBaseHash() != hashBefore
+  }
+
+  def "recalcBaseHash preserves last containerTagsHash across ProcessTags changes"() {
+    given:
+    def containerHash = "my-container-hash"
+    BaseHash.recalcBaseHash(containerHash)
+    def hashWithContainerTag = BaseHash.getBaseHash()
+
+    when: "a process tag is added"
+    ProcessTags.addTag("cluster.name", "new-cluster")
+
+    then: "hash differs from before the tag was added"
+    BaseHash.getBaseHash() != hashWithContainerTag
+
+    and: "hash equals a fresh calc with the same container hash"
+    BaseHash.getBaseHash() == BaseHash.calc(containerHash)
+  }
+
+  def "addTag does not recalculate BaseHash when ProcessTags disabled"() {
+    setup:
     injectSysConfig(EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED, "false")
     ProcessTags.reset()
+    BaseHash.recalcBaseHash(null)
+    def hashBefore = BaseHash.getBaseHash()
+
+    when:
+    ProcessTags.addTag("cluster.name", "ignored")
+
+    then:
+    BaseHash.getBaseHash() == hashBefore
   }
 
   def "ContainerTagsHash used in hash calculation when provided"() {
@@ -76,10 +125,6 @@ class BaseHashTest extends DDSpecification {
       assert secondBaseHash == firstBaseHash
     }
 
-    cleanup:
-    injectSysConfig(EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED, "false")
-    ProcessTags.reset()
-
     where:
     propagateTagsEnabled << [true, false]
   }