Add a flag to skip multihash mismatch errors

Any error returned by the link system other than ipld.ErrNotExists results into IPNI halting ingestion of such adchain. The new configuration option will allow boost to "skip" such advertisements instead without affecting the rest of the chain.

engine/linksystem.go

@@ -127,7 +127,11 @@ func (e *Engine) mkLinkSystem() ipld.LinkSystem {
 			}
 			if regeneratedLink == nil || !c.Equals(regeneratedLink.(cidlink.Link).Cid) {
 				log.Errorw("Regeneration of entries link from multihash iterator did not match the original link. Check that multihash iterator consistently returns the same entries for the same key.", "want", lnk, "got", regeneratedLink)
-				return nil, ErrEntriesLinkMismatch
+				if e.skipMultihashMismatchErros {
+					return nil, ipld.ErrNotExists{}
+				} else {
+					return nil, ErrEntriesLinkMismatch
+				}
 			}
 		} else {
 			log.Debugw("Found cache entry for CID", "cid", c)

engine/options.go

@@ -101,6 +101,8 @@ type (
 		purgeCache  bool
 		chunker     chunker.NewChunkerFunc
 
+		skipMultihashMismatchErros bool
+
 		syncPolicy *policy.Policy
 	}
 )
@@ -442,3 +444,13 @@ func WithExtraGossipData(extraData []byte) Option {
 		return nil
 	}
 }
+
+// WithSkipMultihashMismatchErros instructs the underlying link system to skip multihash mismatch errors
+// and return ipld.ErrNotExists if such occur. ipld.ErrNotExists is treated as content not found and will result to
+// 404 returned back to IPNI.
+func WithSkipMultihashMismatchErros(b bool) Option {
+	return func(o *options) error {
+		o.skipMultihashMismatchErros = b
+		return nil
+	}
+}