Fix behaviour with multiple subscribers

Author: mrober

firebase-sessions/src/main/kotlin/com/google/firebase/sessions/FirebaseSessions.kt

@@ -78,6 +78,14 @@ internal constructor(
       "Registering Sessions SDK subscriber with name: ${subscriber.sessionSubscriberName}, " +
         "data collection enabled: ${subscriber.isDataCollectionEnabled}"
     )
+
+    // Immediately call the callback if Sessions generated a session before the
+    // subscriber subscribed, otherwise subscribers might miss the first session.
+    if (sessionGenerator.hasGenerateSession) {
+      subscriber.onSessionChanged(
+        SessionSubscriber.SessionDetails(sessionGenerator.currentSession.sessionId)
+      )
+    }
   }
 
   private fun initiateSessionStart() {

firebase-sessions/src/main/kotlin/com/google/firebase/sessions/SessionGenerator.kt

@@ -46,6 +46,10 @@ internal class SessionGenerator(
   lateinit var currentSession: SessionDetails
     private set
 
+  /** Returns if a session has been generated. */
+  val hasGenerateSession: Boolean
+    get() = ::currentSession.isInitialized
+
   /** Generates a new session. The first session's sessionId will match firstSessionId. */
   fun generateNewSession(): SessionDetails {
     sessionIndex++

firebase-sessions/src/test/kotlin/com/google/firebase/sessions/SessionGeneratorTest.kt

@@ -43,6 +43,22 @@ class SessionGeneratorTest {
     sessionGenerator.currentSession
   }
 
+  @Test
+  fun hasGenerateSession_beforeGenerate_returnsFalse() {
+    val sessionGenerator = SessionGenerator(collectEvents = false)
+
+    assertThat(sessionGenerator.hasGenerateSession).isFalse()
+  }
+
+  @Test
+  fun hasGenerateSession_afterGenerate_returnsTrue() {
+    val sessionGenerator = SessionGenerator(collectEvents = false)
+
+    sessionGenerator.generateNewSession()
+
+    assertThat(sessionGenerator.hasGenerateSession).isTrue()
+  }
+
   @Test
   fun generateNewSession_generatesValidSessionIds() {
     val sessionGenerator = SessionGenerator(collectEvents = true) // defaults to UUID::randomUUID