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/*
* Copyright 2012 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.bitcoinj.governance;
import org.bitcoinj.core.PeerAddress;
import org.bitcoinj.core.PeerGroup;
import org.bitcoinj.core.Sha256Hash;
import org.bitcoinj.utils.Threading;
import javax.annotation.Nullable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.locks.ReentrantLock;
import static com.google.common.base.Preconditions.checkNotNull;
/**
* Tracks GovernanceVotes that are being announced across the network. Typically one is created for you by a
* {@link PeerGroup} and then given to each Peer to update. The current purpose is to let Peers update the confidence
* (number of peers broadcasting). It helps address an attack scenario in which a malicious remote peer (or several)
* feeds you invalid GovernanceVotes, eg, ones that spend coins which don't exist. If you don't see most of the peers
* announce the GovernanceVote within a reasonable time, it may be that the TX is not valid. Alternatively, an attacker
* may control your entire internet connection: in this scenario counting broadcasting peers does not help you.
*
* It is not at this time directly equivalent to the Dash Core memory pool, which tracks
* all GovernanceVotes not currently included in the best chain - it's simply a cache.
*/
public class VoteConfidenceTable {
protected ReentrantLock lock = Threading.lock("txconfidencetable");
private static class WeakConfidenceReference extends WeakReference {
public Sha256Hash hash;
public WeakConfidenceReference(GovernanceVoteConfidence confidence, ReferenceQueue queue) {
super(confidence, queue);
hash = confidence.getTransactionHash();
}
}
private LinkedHashMap table;
// This ReferenceQueue gets entries added to it when they are only weakly reachable, ie, the TxConfidenceTable is the
// only thing that is tracking the confidence data anymore. We check it from time to time and delete table entries
// corresponding to expired transactions. In this way memory usage of the system is in line with however many
// GovernanceVotes you actually care to track the confidence of. We can still end up with lots of hashes being stored
// if our peers flood us with invs but the MAX_SIZE param caps this.
private ReferenceQueue referenceQueue;
/** The max size of a table created with the no-args constructor. */
public static final int MAX_SIZE = 1000;
/**
* Creates a table that will track at most the given number of transactions (allowing you to bound memory
* usage).
* @param size Max number of GovernanceVotes to track. The table will fill up to this size then stop growing.
*/
public VoteConfidenceTable(final int size) {
table = new LinkedHashMap() {
@Override
protected boolean removeEldestEntry(Map.Entry entry) {
// An arbitrary choice to stop the memory used by tracked GovernanceVotes getting too huge in the event
// of some kind of DoS attack.
return size() > size;
}
};
referenceQueue = new ReferenceQueue();
}
/**
* Creates a table that will track at most {@link VoteConfidenceTable#MAX_SIZE} entries. You should normally use
* this constructor.
*/
public VoteConfidenceTable() {
this(MAX_SIZE);
}
/**
* If any GovernanceVotes have expired due to being only weakly reachable through us, go ahead and delete their
* table entries - it means we downloaded the GovernanceVote and sent it to various event listeners, none of
* which bothered to keep a reference. Typically, this is because the GovernanceVote does not involve any keys that
* are relevant to any of our wallets.
*/
private void cleanTable() {
lock.lock();
try {
Reference ref;
while ((ref = referenceQueue.poll()) != null) {
// Find which GovernanceVote got deleted by the GC.
WeakConfidenceReference txRef = (WeakConfidenceReference) ref;
// And remove the associated map entry so the other bits of memory can also be reclaimed.
table.remove(txRef.hash);
}
} finally {
lock.unlock();
}
}
/**
* Returns the number of peers that have seen the given hash recently.
*/
public int numBroadcastPeers(Sha256Hash txHash) {
lock.lock();
try {
cleanTable();
WeakConfidenceReference entry = table.get(txHash);
if (entry == null) {
return 0; // No such TX known.
} else {
GovernanceVoteConfidence confidence = entry.get();
if (confidence == null) {
// Such a TX hash was seen, but nothing seemed to care so we ended up throwing away the data.
table.remove(txHash);
return 0;
} else {
return confidence.numBroadcastPeers();
}
}
} finally {
lock.unlock();
}
}
/**
* Called by peers when they see a GovernanceVote advertised in an "inv" message. It passes the data on to the relevant
* {@link GovernanceVoteConfidence} object, creating it if needed.
*
* @return the number of peers that have now announced this hash (including the caller)
*/
public GovernanceVoteConfidence seen(Sha256Hash hash, PeerAddress byPeer) {
GovernanceVoteConfidence confidence;
boolean fresh = false;
lock.lock();
{
cleanTable();
confidence = getOrCreate(hash);
fresh = confidence.markBroadcastBy(byPeer);
}
lock.unlock();
if (fresh)
confidence.queueListeners(GovernanceVoteConfidence.Listener.ChangeReason.SEEN_PEERS);
return confidence;
}
/**
* Returns the {@link GovernanceVoteConfidence} for the given hash if we have downloaded it, or null if that tx hash
* is unknown to the system at this time.
*/
public GovernanceVoteConfidence getOrCreate(Sha256Hash hash) {
checkNotNull(hash);
lock.lock();
try {
WeakConfidenceReference reference = table.get(hash);
if (reference != null) {
GovernanceVoteConfidence confidence = reference.get();
if (confidence != null)
return confidence;
}
GovernanceVoteConfidence newConfidence = new GovernanceVoteConfidence(hash);
table.put(hash, new WeakConfidenceReference(newConfidence, referenceQueue));
return newConfidence;
} finally {
lock.unlock();
}
}
/**
* Returns the {@link GovernanceVoteConfidence} for the given hash if we have downloaded it, or null if that tx hash
* is unknown to the system at this time.
*/
@Nullable
public GovernanceVoteConfidence get(Sha256Hash hash) {
lock.lock();
try {
WeakConfidenceReference ref = table.get(hash);
if (ref == null)
return null;
GovernanceVoteConfidence confidence = ref.get();
if (confidence != null)
return confidence;
else
return null;
} finally {
lock.unlock();
}
}
}