From 53e838c61d4892b91252f9f2da5e28eef441b34a Mon Sep 17 00:00:00 2001
From: fiatjaf <fiatjaf@gmail.com>
Date: Tue, 14 Oct 2025 00:38:51 +0000
Subject: [PATCH] mmm: freeranges computed on startup and kept only in memory.

---
 eventstore/mmm/fix.go        |  2 +-
 eventstore/mmm/freeranges.go | 61 ++++++++++++++++++++++++++++--------
 eventstore/mmm/mmmm.go       | 22 ++-----------
 eventstore/mmm/purge.go      |  2 +-
 eventstore/mmm/save.go       |  4 ---
 5 files changed, 52 insertions(+), 39 deletions(-)

diff --git a/eventstore/mmm/fix.go b/eventstore/mmm/fix.go
index 14572ae..08f9252 100644
--- a/eventstore/mmm/fix.go
+++ b/eventstore/mmm/fix.go
@@ -131,7 +131,7 @@ func (b *MultiMmapManager) Rescan() error {
 			}
 		}
 
-		return nil
+		return b.GatherFreeRanges(mmmtxn)
 	})
 }
 
diff --git a/eventstore/mmm/freeranges.go b/eventstore/mmm/freeranges.go
index fc3a734..63a9f30 100644
--- a/eventstore/mmm/freeranges.go
+++ b/eventstore/mmm/freeranges.go
@@ -1,12 +1,60 @@
 package mmm
 
 import (
+	"cmp"
 	"fmt"
 	"slices"
 
 	"github.com/PowerDNS/lmdb-go/lmdb"
 )
 
+func (b *MultiMmapManager) GatherFreeRanges(txn *lmdb.Txn) error {
+	cursor, err := txn.OpenCursor(b.indexId)
+	if err != nil {
+		return fmt.Errorf("failed to open cursor on indexId: %w", err)
+	}
+	defer cursor.Close()
+
+	usedPositions := make([]position, 0, 256)
+	for key, val, err := cursor.Get(nil, nil, lmdb.First); err == nil; key, val, err = cursor.Get(key, val, lmdb.Next) {
+		pos := positionFromBytes(val[0:12])
+		usedPositions = append(usedPositions, pos)
+	}
+
+	// sort used positions by start
+	slices.SortFunc(usedPositions, func(a, b position) int { return cmp.Compare(a.start, b.start) })
+
+	// calculate free ranges as gaps between used positions
+	b.freeRanges = make([]position, 0, len(usedPositions)/2)
+	var currentStart uint64 = 0
+	for _, pos := range usedPositions {
+		if pos.start > currentStart {
+			// gap from currentStart to pos.start
+			freeSize := pos.start - currentStart
+			if freeSize > 0 {
+				b.freeRanges = append(b.freeRanges, position{
+					start: currentStart,
+					size:  uint32(freeSize),
+				})
+			}
+		}
+		currentStart = pos.start + uint64(pos.size)
+	}
+
+	// sort free ranges by size (smallest first, as before)
+	slices.SortFunc(b.freeRanges, func(a, b position) int { return cmp.Compare(a.size, b.size) })
+
+	logOp := b.Logger.Debug()
+	for _, pos := range b.freeRanges {
+		if pos.size > 20 {
+			logOp = logOp.Uint32(fmt.Sprintf("%d", pos.start), pos.size)
+		}
+	}
+	logOp.Msg("calculated free ranges from index scan")
+
+	return nil
+}
+
 func (b *MultiMmapManager) mergeNewFreeRange(pos position) (isAtEnd bool) {
 	// before adding check if we can merge this with some other range
 	// (to merge means to delete the previous and add a new one)
@@ -53,16 +101,3 @@ func (b *MultiMmapManager) addNewFreeRange(pos position) {
 	})
 	b.freeRanges = slices.Insert(b.freeRanges, idx, pos)
 }
-
-func (b *MultiMmapManager) saveFreeRanges(txn *lmdb.Txn) error {
-	// save to database
-	valReserve, err := txn.PutReserve(b.stuff, FREERANGES_KEY, len(b.freeRanges)*12, 0)
-	if err != nil {
-		return fmt.Errorf("on put freeranges: %w", err)
-	}
-	for f, fr := range b.freeRanges {
-		bytesFromPosition(valReserve[f*12:], fr)
-	}
-
-	return nil
-}
diff --git a/eventstore/mmm/mmmm.go b/eventstore/mmm/mmmm.go
index 0ce3780..fd3c34d 100644
--- a/eventstore/mmm/mmmm.go
+++ b/eventstore/mmm/mmmm.go
@@ -50,8 +50,6 @@ const (
 	maxuint32          = 4294967295
 )
 
-var FREERANGES_KEY = []byte{'F'}
-
 func (b *MultiMmapManager) Init() error {
 	// create directory if it doesn't exist
 	dbpath := filepath.Join(b.Dir, "mmmm")
@@ -119,24 +117,8 @@ func (b *MultiMmapManager) Init() error {
 			b.indexId = dbi
 		}
 
-		// load all free ranges into memory
-		{
-			data, err := txn.Get(b.stuff, FREERANGES_KEY)
-			if err != nil && !lmdb.IsNotFound(err) {
-				return fmt.Errorf("on freeranges: %w", err)
-			}
-			b.freeRanges = make([]position, len(data)/12)
-			logOp := b.Logger.Debug()
-			for f := range b.freeRanges {
-				pos := positionFromBytes(data[f*12 : (f+1)*12])
-				b.freeRanges[f] = pos
-				if pos.size > 20 {
-					logOp = logOp.Uint32(fmt.Sprintf("%d", pos.start), pos.size)
-				}
-			}
-			slices.SortFunc(b.freeRanges, func(a, b position) int { return int(a.size - b.size) })
-			logOp.Msg("loaded free ranges")
-		}
+		// scan index table to calculate free ranges from used positions
+		b.GatherFreeRanges(txn)
 
 		return nil
 	}); err != nil {
diff --git a/eventstore/mmm/purge.go b/eventstore/mmm/purge.go
index 8ce73ff..82daa7d 100644
--- a/eventstore/mmm/purge.go
+++ b/eventstore/mmm/purge.go
@@ -32,5 +32,5 @@ func (b *MultiMmapManager) purge(txn *lmdb.Txn, idPrefix8 []byte, pos position)
 		copy(b.mmapf[pos.start:], bytes.Repeat([]byte{'!'}, int(pos.size)))
 	}
 
-	return b.saveFreeRanges(txn)
+	return nil
 }
diff --git a/eventstore/mmm/save.go b/eventstore/mmm/save.go
index 23974ac..fa17557 100644
--- a/eventstore/mmm/save.go
+++ b/eventstore/mmm/save.go
@@ -114,10 +114,6 @@ func (b *MultiMmapManager) storeOn(
 				})
 			}
 
-			if err := b.saveFreeRanges(mmmtxn); err != nil {
-				return false, fmt.Errorf("failed to save modified free ranges: %w", err)
-			}
-
 			break
 		}
 	}