Prune more aggressively during IBD

Pruning forces a chainstate flush, which can defeat the dbcache and harm performance significantly.

During IBD we now prune based on the worst case size of the remaining blocks, but no further than
the minimum prune size of 550 MB.

Based on @luke-jr's bitcoin#11658.

src/validation.cpp

@@ -3569,6 +3569,9 @@ static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfte
 
     unsigned int nLastBlockWeCanPrune = chainActive.Tip()->nHeight - MIN_BLOCKS_TO_KEEP;
     uint64_t nCurrentUsage = CalculateCurrentUsage();
+    // Worst case remaining block space:
+    uint64_t nRemainingBlockSpace = (pindexBestHeader->nHeight - chainActive.Tip()->nHeight) * (uint64_t)MAX_BLOCK_SERIALIZED_SIZE;
+
     // We don't check to prune until after we've allocated new space for files
     // So we should leave a buffer under our target to account for another allocation
     // before the next pruning.
@@ -3577,13 +3580,30 @@ static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfte
     int count=0;
 
     if (nCurrentUsage + nBuffer >= nPruneTarget) {
+        if (IsInitialBlockDownload()) {
+            LogPrintf("Prune: worst case remaining block space=%d MB\n", nRemainingBlockSpace / (uint64_t)1024 / (uint64_t)1024);
+        }
         for (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
             nBytesToPrune = vinfoBlockFile[fileNumber].nSize + vinfoBlockFile[fileNumber].nUndoSize;
 
             if (vinfoBlockFile[fileNumber].nSize == 0)
                 continue;
 
-            if (nCurrentUsage + nBuffer < nPruneTarget)  // are we below our target?
+            // On a prune event, the chainstate DB is flushed. To avoid excessive
+            // prune events negating the benefit of high dbcache values, we should
+            // not prune too rapidly.
+            // During IDB use the worst case size of the remaining blocks, but
+            // prune no further than the minimum prune size of 550 MB.
+            uint64_t nPruneAdjustedTarget = nPruneTarget;
+            if (IsInitialBlockDownload()) {
+                if (nRemainingBlockSpace > nPruneTarget - MIN_DISK_SPACE_FOR_BLOCK_FILES) {
+                    nPruneAdjustedTarget = MIN_DISK_SPACE_FOR_BLOCK_FILES;
+                } else {
+                    nPruneAdjustedTarget = nPruneTarget - nRemainingBlockSpace;
+                }
+            }
+
+            if (nCurrentUsage + nBuffer < nPruneAdjustedTarget)  // are we below our target?
                 break;
 
             // don't prune files that could have a block within MIN_BLOCKS_TO_KEEP of the main chain's tip but keep scanning