downloader: extract findAncestor search functions

This is a simple refactoring, extracting common ancestor
negotiation logic to named functions.

This commit addresses the concern at the following link:
ethereum#21063 (comment)

---

This is a combination of 6 commits.
This is the 1st commit message:

downloader: extract span search to function

This is a plain refactoring, extracting span search
logic to its own function.

An error 'errNoAncestor' is introduced to handle
the case when the function does not find a common
ancestor.

Signed-off-by: meows <b5c6@protonmail.com>

downloader: extract binary search to function

This is a simple refactoring extracting
the binary search for common ancestor logic
to its own function.

A tweak was made to the 'floor' variable(s) which
I'm still not happy with, but logic is functional --
passing tests -- for now.

Signed-off-by: meows <b5c6@protonmail.com>

downloader: fixup floor type and logic to minimize change

This minimizes the diff, keeping the logic as nearly
the same as possible, limiting the span and binary search
logic extraction to functions as cleanly as possible.

Signed-off-by: meows <b5c6@protonmail.com>

downloader: remove limiting ancestor to localheight

This is logic not existing at ethereum/go-ethereum master,
and thus is not pertinent to a just-refactor.

Signed-off-by: meows <b5c6@protonmail.com>

downloader: tweaks to get the diff as clean as possible

Signed-off-by: meows <b5c6@protonmail.com>

downloader: rename error noAncestor -> noAncestorFound

Signed-off-by: meows <b5c6@protonmail.com>

sq

eth/downloader/downloader.go

@@ -90,6 +90,7 @@ var (
 	errCanceled                = errors.New("syncing canceled (requested)")
 	errNoSyncActive            = errors.New("no sync active")
 	errTooOld                  = errors.New("peer's protocol version too old")
+	errNoAncestorFound         = errors.New("no common ancestor found")
 )
 
 type Downloader struct {
@@ -814,6 +815,26 @@ func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header)
 		}
 	}
 
+	ancestor, err := d.findAncestorSpanSearch(p, mode, remoteHeight, localHeight, floor)
+	if err == nil {
+		return ancestor, nil
+	}
+	// The returned error was not nil.
+	// If the error returned does not reflect that a common ancestor was not found, return it.
+	// If the error reflects that a common ancestor was not found, continue to binary search,
+	// where the error value will be reassigned.
+	if err != errNoAncestorFound {
+		return 0, err
+	}
+
+	ancestor, err = d.findAncestorBinarySearch(p, mode, remoteHeight, floor)
+	if err != nil {
+		return 0, err
+	}
+	return ancestor, nil
+}
+
+func (d *Downloader) findAncestorSpanSearch(p *peerConnection, mode SyncMode, remoteHeight, localHeight uint64, floor int64) (commonAncestor uint64, err error) {
 	from, count, skip, max := calculateRequestSpan(remoteHeight, localHeight)
 
 	p.log.Trace("Span searching for common ancestor", "count", count, "from", from, "skip", skip)
@@ -894,6 +915,12 @@ func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header)
 		p.log.Debug("Found common ancestor", "number", number, "hash", hash)
 		return number, nil
 	}
+	return 0, errNoAncestorFound
+}
+
+func (d *Downloader) findAncestorBinarySearch(p *peerConnection, mode SyncMode, remoteHeight uint64, floor int64) (commonAncestor uint64, err error) {
+	hash := common.Hash{}
+
 	// Ancestor not found, we need to binary search over our chain
 	start, end := uint64(0), remoteHeight
 	if floor > 0 {