headerdownload: fix OOM due to inifinitely growing children in link (#…

…12404)

relates to #11387,
#11473,
#10734

tried to simulate the OOM using
#11799

What I found was infinitely growing alloc of headers when receiving new
header messages in sentry's `blockHeaders66` handler (check screenshot
below).

It looks like this is happening because in the case of a bad child
header: we delete it from the `links` map, however its parent link still
holds a reference to it so the deleted link & header never get gc-ed.
Furthermore if new similar bad hashes arrive after deletion they get
appended to their parent header's link and the children of that link can
grow indefinitely
([here](https://github.com/erigontech/erigon/blob/main/turbo/stages/headerdownload/header_algos.go#L1085-L1086)).
Ie confirmed with debug logs (note link at 13450415 has 140124
children):
```
DBUG[10-21|18:18:05.003] [downloader] InsertHeader: Rejected header parent info hash=0xb488d67deaf4103880fa972fd72a7a9be552e3bc653f124f1ad9cb45f36bcd07 height=13450415 children=140124
```

<br/>

The solution for this is to remove the bad link from its parent child
list
[here](https://github.com/erigontech/erigon/blob/main/turbo/stages/headerdownload/header_algos.go#L544)
so that 1) it gets gc-ed and 2) the children list does not grow
indefinitely.

<br/>


![oom-heap-profile2](https://github.com/user-attachments/assets/518fa658-c199-48b6-aa2d-110673264144)

turbo/stages/headerdownload/header_algos.go

@@ -177,6 +177,11 @@ func (hd *HeaderDownload) removeUpwards(link *Link) {
 	if link == nil {
 		return
 	}
+	if link.header != nil {
+		if parentLink, ok := hd.links[link.header.ParentHash]; ok {
+			parentLink.RemoveChild(link)
+		}
+	}
 	var toRemove = []*Link{link}
 	for len(toRemove) > 0 {
 		removal := toRemove[len(toRemove)-1]
@@ -535,8 +540,6 @@ func (hd *HeaderDownload) InsertHeader(hf FeedHeaderFunc, terminalTotalDifficult
 		}
 		if bad {
 			// If the link or its parent is marked bad, throw it out
-			hd.moveLinkToQueue(link, NoQueue)
-			delete(hd.links, link.hash)
 			hd.removeUpwards(link)
 			dataflow.HeaderDownloadStates.AddChange(link.blockHeight, dataflow.HeaderBad)
 			hd.stats.RejectedBadHeaders++
@@ -553,12 +556,7 @@ func (hd *HeaderDownload) InsertHeader(hf FeedHeaderFunc, terminalTotalDifficult
 					return false, false, 0, lastTime, nil // prevent removal of the link from the hd.linkQueue
 				} else {
 					hd.logger.Debug("[downloader] Verification failed for header", "hash", link.hash, "height", link.blockHeight, "err", err)
-					hd.moveLinkToQueue(link, NoQueue)
-					delete(hd.links, link.hash)
 					hd.removeUpwards(link)
-					if parentLink, ok := hd.links[link.header.ParentHash]; ok {
-						parentLink.RemoveChild(link)
-					}
 					dataflow.HeaderDownloadStates.AddChange(link.blockHeight, dataflow.HeaderEvicted)
 					hd.stats.InvalidHeaders++
 					return true, false, 0, lastTime, nil