coins: compact chainstate in background

Full chainstate compaction can take minutes on large databases.
Move `CCoinsViewDB::CompactFull()` to a named `utxocompact` one-shot background thread so validation only schedules the work.

When validation selects compaction after a full flush, the chainstate was just written and another write is less likely to be needed immediately.
The coins view destructor waits for completion, and a mutex prevents compaction from using `m_db` while `ResizeCache()` replaces it.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>

src/test/coins_tests.cpp

@@ -1078,7 +1078,7 @@ BOOST_FIXTURE_TEST_CASE(coins_db_leveldb_layout, FlushTest)
     cache.Sync();
 
     BOOST_CHECK_EQUAL(level2_files(base), 0);
-    WITH_LOCK(::cs_main, base.CompactFull());
+    WITH_LOCK(::cs_main, return base.CompactFull()).wait();
     BOOST_CHECK_EQUAL(level2_files(base), 1);
 
     BOOST_CHECK(*Assert(base.GetCoin(outpoint)) == coin);

src/txdb.cpp

@@ -14,10 +14,14 @@
 #include <uint256.h>
 #include <util/byte_units.h>
 #include <util/log.h>
+#include <util/threadnames.h>
 #include <util/vector.h>
 
 #include <cassert>
+#include <chrono>
 #include <cstdlib>
+#include <exception>
+#include <future>
 #include <iterator>
 #include <utility>
 
@@ -56,11 +60,22 @@ CCoinsViewDB::CCoinsViewDB(DBParams db_params, CoinsViewOptions options) :
     m_options{std::move(options)},
     m_db{std::make_unique<CDBWrapper>(m_db_params)} { }
 
+CCoinsViewDB::~CCoinsViewDB()
+{
+    if (m_compaction.valid()) {
+        if (m_compaction.wait_for(std::chrono::seconds{0}) != std::future_status::ready) {
+            LogInfo("Waiting for background chainstate compaction of %s", fs::PathToString(m_db_params.path));
+        }
+        m_compaction.wait();
+    }
+}
+
 void CCoinsViewDB::ResizeCache(size_t new_cache_size)
 {
     // We can't do this operation with an in-memory DB since we'll lose all the coins upon
     // reset.
     if (!m_db_params.memory_only) {
+        LOCK(m_db_mutex);
         // Have to do a reset first to get the original `m_db` state to release its
         // filesystem lock.
         m_db.reset();
@@ -180,12 +195,23 @@ std::optional<std::string> CCoinsViewDB::GetDBProperty(const std::string& proper
     return m_db->GetProperty(property);
 }
 
-void CCoinsViewDB::CompactFull()
+std::shared_future<void> CCoinsViewDB::CompactFull()
 {
     AssertLockHeld(::cs_main);
-    LogDebug(BCLog::COINDB, "Starting chainstate compaction of %s", fs::PathToString(m_db_params.path));
-    m_db->CompactFull();
-    LogDebug(BCLog::COINDB, "Finished chainstate compaction of %s", fs::PathToString(m_db_params.path));
+    if (m_compaction.valid() && m_compaction.wait_for(std::chrono::seconds{0}) != std::future_status::ready) return m_compaction;
+    m_compaction = std::async(std::launch::async, [this] {
+        try {
+            util::ThreadRename("utxocompact");
+            LOCK(m_db_mutex);
+
+            LogDebug(BCLog::COINDB, "Starting chainstate compaction of %s", fs::PathToString(m_db_params.path));
+            m_db->CompactFull();
+            LogDebug(BCLog::COINDB, "Finished chainstate compaction of %s", fs::PathToString(m_db_params.path));
+        } catch (const std::exception& e) {
+            LogWarning("Failed chainstate compaction (%s)", e.what());
+        }
+    }).share();
+    return m_compaction;
 }
 
 /** Specialization of CCoinsViewCursor to iterate over a CCoinsViewDB */

src/txdb.h

@@ -15,6 +15,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <future>
 #include <memory>
 #include <optional>
 #include <string>
@@ -37,9 +38,13 @@ class CCoinsViewDB final : public CCoinsView
 protected:
     DBParams m_db_params;
     CoinsViewOptions m_options;
+    //! Prevents CompactFull() from using m_db while ResizeCache() replaces it.
+    Mutex m_db_mutex;
     std::unique_ptr<CDBWrapper> m_db;
+    std::shared_future<void> m_compaction;
 public:
     explicit CCoinsViewDB(DBParams db_params, CoinsViewOptions options);
+    ~CCoinsViewDB() override;
 
     std::optional<Coin> GetCoin(const COutPoint& outpoint) const override;
     std::optional<Coin> PeekCoin(const COutPoint& outpoint) const override;
@@ -54,10 +59,10 @@ public:
     size_t EstimateSize() const override;
 
     //! Dynamically alter the underlying leveldb cache size.
-    void ResizeCache(size_t new_cache_size) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    void ResizeCache(size_t new_cache_size) EXCLUSIVE_LOCKS_REQUIRED(cs_main, !m_db_mutex);
 
-    //! Perform a blocking full compaction of the underlying LevelDB.
-    void CompactFull() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    //! Perform a full compaction of the underlying LevelDB on a one-shot background thread.
+    std::shared_future<void> CompactFull() EXCLUSIVE_LOCKS_REQUIRED(cs_main, !m_db_mutex);
 
     //! Return an underlying LevelDB property value, if available.
     std::optional<std::string> GetDBProperty(const std::string& property);

src/validation.cpp

@@ -2838,9 +2838,11 @@ bool Chainstate::FlushStateToDisk(
             m_chainman.m_options.signals->ChainStateFlushed(this->GetRole(), GetLocator(m_chain.Tip()));
         }
 
-        if (!m_chainman.m_interrupt && m_chainman.m_chainstates.size() == 1) { // Skip AssumeUTXO
-            if (ShouldCompactChainstate(m_chainman.IsInitialBlockDownload())) {
+        if (!m_chainman.m_interrupt && ShouldCompactChainstate(m_chainman.IsInitialBlockDownload())) {
+            try {
                 CoinsDB().CompactFull();
+            } catch (const std::exception& e) {
+                LogWarning("Failed to start chainstate compaction (%s)", e.what());
             }
         }
     }