Fix a really nontrivial RemoveRange bug

_, rm := split(rm) never changes `rm` itself, so `rm != b` wasn't triggered...

But split was still working 99% of the time! How?

The answer is that `changed = true` also wasn't triggered so ascendChange()
thought that Btree didn't change and fell through to the next item, and MOST
of the time Btree returned `rm` again because an item was just inserted before
it which changed the iterator, after which it was removed just fine as fully
contained by the removed range. HOWEVER, as Btree is not guaranteed to resume
iteration correctly after changes, and SOMETIMES it was actually skipping our
item, and it wasn't removed from the list.

Reproducing it in tests is rather hard, but I suppose you'd have to fill the
Btree to some extent.

Author: vitalif

internal/buffer_list.go

@@ -375,7 +375,7 @@ func (l *BufferList) delete(b *FileBuffer) (allocated int64) {
 // Remove buffers in range (offset..size)
 func (l *BufferList) RemoveRange(removeOffset, removeSize uint64, filter func(b *FileBuffer) bool) (allocated int64) {
 	endOffset := removeOffset + removeSize
-	ascendChange(&l.at, removeOffset, func(end uint64, b *FileBuffer) (cont bool, changed bool) {
+	ascendChange(&l.at, removeOffset+1, func(end uint64, b *FileBuffer) (cont bool, changed bool) {
 		if b.offset >= endOffset {
 			return false, false
 		}
@@ -387,16 +387,17 @@ func (l *BufferList) RemoveRange(removeOffset, removeSize uint64, filter func(b
 				changed = true
 			} else {
 				rm := b
+				bufStart := b.offset
 				// split-delete is simpler than cut regarding the dirty part reference count
 				if endOffset < bufEnd {
 					// remove beginning
 					rm, _ = l.split(rm, endOffset)
 				}
-				if removeOffset > rm.offset {
+				if removeOffset > bufStart {
 					// remove end
 					_, rm = l.split(rm, removeOffset)
 				}
-				if rm != b {
+				if removeOffset > bufStart || endOffset < bufEnd {
 					allocated += l.delete(rm)
 					changed = true
 				}
@@ -636,6 +637,16 @@ func (l *BufferList) split(b *FileBuffer, offset uint64) (left, right *FileBuffe
 }
 
 // Left here for the ease of debugging
+func (l *BufferList) Dump(offset, size uint64) {
+	l.at.Ascend(offset+1, func(end uint64, b *FileBuffer) bool {
+		if b.offset >= offset+size {
+			return false
+		}
+		fmt.Printf("%x-%x s%v l%v z%v d%v\n", b.offset, b.offset+b.length, b.state, b.loading, b.zero, b.dirtyID)
+		return true
+	})
+}
+
 func (l *BufferList) DebugCheckHoles(s string) {
 	var eof uint64
 	l.at.Descend(0xFFFFFFFFFFFFFFFF, func(end uint64, b *FileBuffer) bool {
@@ -645,10 +656,7 @@ func (l *BufferList) DebugCheckHoles(s string) {
 	h, _, _ := l.GetHoles(0, eof)
 	if len(h) > 0 {
 		fmt.Printf("Debug: holes detected%s: %#v\n", s, h)
-		l.at.Ascend(0, func(end uint64, b *FileBuffer) bool {
-			fmt.Printf("%x-%x s%v z%v d%v\n", b.offset, b.offset+b.length, b.state, b.zero, b.dirtyID)
-			return true
-		})
+		l.Dump(0, 0xFFFFFFFFFFFFFFFF)
 		panic("holes detected")
 	}
 }