doc: BaseIndex sync behavior with empty datadir

Make a note about a potentially confusing behavior with `BaseIndex::m_synced`;
if the user starts bitcoind with an empty datadir and an index enabled,
BaseIndex will consider itself synced (as a degenerate case). This affects
how indices are built during IBD (relying solely on BlockConnected signals vs.
using ThreadSync()).

1 files changed, 4 insertions, 0 deletions

diff --git a/src/index/base.h b/src/index/base.h
index a8f6a18c8d..6dfaa4361e 100644
--- a/src/index/base.h
+++ b/src/index/base.h
@@ -51,6 +51,10 @@ private:
     /// 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.
+    ///
+    /// Note that this will latch to true *immediately* upon startup if
+    /// `m_chainstate->m_chain` is empty, which will be the case upon startup
+    /// with an empty datadir if, e.g., `-txindex=1` is specified.
     std::atomic<bool> m_synced{false};
 
     /// The last block in the chain that the index is in sync with.