refactor: index, decouple 'Init' from 'Start'

So indexes can be initialized without spawning
the sync thread.

This makes asynchronous indexes startup
possible in the following commits.

src/index/base.cpp

@@ -81,6 +81,13 @@ BaseIndex::~BaseIndex()
 
 bool BaseIndex::Init()
 {
+    // m_chainstate member gives indexing code access to node internals. It is
+    // removed in followup https://github.com/bitcoin/bitcoin/pull/24230
+    m_chainstate = &m_chain->context()->chainman->ActiveChainstate();
+    // Register to validation interface before setting the 'm_synced' flag, so that
+    // callbacks are not missed once m_synced is true.
+    RegisterValidationInterface(this);
+
     CBlockLocator locator;
     if (!GetDB().ReadBestBlock(locator)) {
         locator.SetNull();
@@ -147,6 +154,7 @@ bool BaseIndex::Init()
     // datadir and an index enabled. If this is the case, indexation will happen solely
     // via `BlockConnected` signals until, possibly, the next restart.
     m_synced = synced;
+    m_init = true;
     return true;
 }
 
@@ -401,15 +409,9 @@ void BaseIndex::Interrupt()
     m_interrupt();
 }
 
-bool BaseIndex::Start()
+bool BaseIndex::StartBackgroundSync()
 {
-    // m_chainstate member gives indexing code access to node internals. It is
-    // removed in followup https://github.com/bitcoin/bitcoin/pull/24230
-    m_chainstate = &m_chain->context()->chainman->ActiveChainstate();
-    // Need to register this ValidationInterface before running Init(), so that
-    // callbacks are not missed if Init sets m_synced to true.
-    RegisterValidationInterface(this);
-    if (!Init()) return false;
+    if (!m_init) throw std::logic_error("Error: Cannot start a non-initialized index");
 
     m_thread_sync = std::thread(&util::TraceThread, GetName(), [this] { ThreadSync(); });
     return true;

src/index/base.h

@@ -54,6 +54,8 @@ protected:
     };
 
 private:
+    /// Whether the index has been initialized or not.
+    std::atomic<bool> m_init{false};
     /// Whether the index is in sync with the main chain. The flag is flipped
     /// from false to true once, after which point this starts processing
     /// ValidationInterface notifications to stay in sync.
@@ -69,9 +71,6 @@ private:
     std::thread m_thread_sync;
     CThreadInterrupt m_interrupt;
 
-    /// Read best block locator and check that data needed to sync has not been pruned.
-    bool Init();
-
     /// Sync the index with the block index starting from the current best block.
     /// Intended to be run in its own thread, m_thread_sync, and can be
     /// interrupted with m_interrupt. Once the index gets in sync, the m_synced
@@ -142,9 +141,12 @@ public:
 
     void Interrupt();
 
-    /// Start initializes the sync state and registers the instance as a
-    /// ValidationInterface so that it stays in sync with blockchain updates.
-    [[nodiscard]] bool Start();
+    /// Initializes the sync state and registers the instance to the
+    /// validation interface so that it stays in sync with blockchain updates.
+    [[nodiscard]] bool Init();
+
+    /// Starts the initial sync process.
+    [[nodiscard]] bool StartBackgroundSync();
 
     /// Stops the instance from staying in sync with blockchain updates.
     void Stop();