validation: pruning for multiple chainstates

Introduces ChainstateManager::GetPruneRange().

The prune budget is split evenly between the number of chainstates,
however the prune budget may be exceeded if the resulting shares are
beneath `MIN_DISK_SPACE_FOR_BLOCK_FILES`.

1 files changed, 33 insertions, 2 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 00da4a1c9e..ad135994ec 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -69,6 +69,7 @@
 #include <optional>
 #include <string>
 #include <utility>
+#include <tuple>
 
 using kernel::CCoinsStats;
 using kernel::CoinStatsHashType;
@@ -2552,11 +2553,14 @@ bool Chainstate::FlushStateToDisk(
             if (nManualPruneHeight > 0) {
                 LOG_TIME_MILLIS_WITH_CATEGORY("find files to prune (manual)", BCLog::BENCH);
 
-                m_blockman.FindFilesToPruneManual(setFilesToPrune, std::min(last_prune, nManualPruneHeight), m_chain.Height());
+                m_blockman.FindFilesToPruneManual(
+                    setFilesToPrune,
+                    std::min(last_prune, nManualPruneHeight),
+                    *this, m_chainman);
             } else {
                 LOG_TIME_MILLIS_WITH_CATEGORY("find files to prune", BCLog::BENCH);
 
-                m_blockman.FindFilesToPrune(setFilesToPrune, m_chainman.GetParams().PruneAfterHeight(), m_chain.Height(), last_prune, m_chainman.IsInitialBlockDownload());
+                m_blockman.FindFilesToPrune(setFilesToPrune, last_prune, *this, m_chainman);
                 m_blockman.m_check_for_pruning = false;
             }
             if (!setFilesToPrune.empty()) {
@@ -5939,3 +5943,30 @@ Chainstate& ChainstateManager::GetChainstateForIndexing()
     // indexed.
     return (this->GetAll().size() > 1) ? *m_ibd_chainstate : *m_active_chainstate;
 }
+
+std::pair<int, int> ChainstateManager::GetPruneRange(const Chainstate& chainstate, int last_height_can_prune)
+{
+    if (chainstate.m_chain.Height() <= 0) {
+        return {0, 0};
+    }
+    int prune_start{0};
+
+    if (this->GetAll().size() > 1 && m_snapshot_chainstate.get() == &chainstate) {
+        // Leave the blocks in the background IBD chain alone if we're pruning
+        // the snapshot chain.
+        prune_start = *Assert(GetSnapshotBaseHeight()) + 1;
+    }
+
+    int max_prune = std::max<int>(
+        0, chainstate.m_chain.Height() - static_cast<int>(MIN_BLOCKS_TO_KEEP));
+
+    // last block to prune is the lesser of (caller-specified height, MIN_BLOCKS_TO_KEEP from the tip)
+    //
+    // While you might be tempted to prune the background chainstate more
+    // aggressively (i.e. fewer MIN_BLOCKS_TO_KEEP), this won't work with index
+    // building - specifically blockfilterindex requires undo data, and if
+    // we don't maintain this trailing window, we hit indexing failures.
+    int prune_end = std::min(last_height_can_prune, max_prune);
+
+    return {prune_start, prune_end};
+}