lock the cormorant store against client connection reads and serve history as a copy, and close the client socket however its handler exits

### src/main/java/com/sparrowwallet/sparrow/net/cormorant/electrum/RequestHandler.java
@@ -52,6 +52,12 @@ public void run() {
             }
         } catch(IOException e) {
             log.error("Could not communicate with client socket", e);
+        } finally {
+            try {
+                clientSocket.close();
+            } catch(IOException e) {
+                log.debug("Error closing client socket", e);
+            }
         }
     }
 

### src/main/java/com/sparrowwallet/sparrow/net/cormorant/index/Store.java
@@ -18,7 +18,7 @@ public class Store {
     private final Map<Integer, String> blockHeightHashes = new HashMap<>();
     private final Map<String, MempoolEntry> mempoolEntries = new HashMap<>();
 
-    public String addAddressTransaction(Address address, ListTransaction listTransaction) {
+    public synchronized String addAddressTransaction(Address address, ListTransaction listTransaction) {
         if(listTransaction.category() == Category.receive || listTransaction.category() == Category.immature || listTransaction.category() == Category.generate) {
             fundingAddresses.put(new HashIndex(Sha256Hash.wrap(listTransaction.txid()), listTransaction.vout()), address);
         }
@@ -49,7 +49,7 @@ public String addAddressTransaction(Address address, ListTransaction listTransac
         return null;
     }
 
-    public Set<String> updateMempoolTransactions() {
+    public synchronized Set<String> updateMempoolTransactions() {
         Set<String> updatedScriptHashes = new HashSet<>();
 
         for(Map.Entry<String, Set<TxEntry>> scriptHashEntry : scriptHashEntries.entrySet()) {
@@ -80,7 +80,7 @@ public Set<String> updateMempoolTransactions() {
         return updatedScriptHashes;
     }
 
-    public Set<String> purgeTransaction(String txid) {
+    public synchronized Set<String> purgeTransaction(String txid) {
         Set<String> updatedScriptHashes = new HashSet<>();
 
         for(Map.Entry<String, Set<TxEntry>> scriptHashEntry : scriptHashEntries.entrySet()) {
@@ -98,7 +98,7 @@ public Set<String> purgeTransaction(String txid) {
         return updatedScriptHashes;
     }
 
-    public String getStatus(String scriptHash) {
+    public synchronized String getStatus(String scriptHash) {
         Set<TxEntry> entries = scriptHashEntries.get(scriptHash);
         if(entries == null || entries.isEmpty()) {
             return null;
@@ -112,7 +112,7 @@ public String getStatus(String scriptHash) {
         return Utils.bytesToHex(Sha256Hash.hash(scriptHashStatus.toString().getBytes(StandardCharsets.UTF_8)));
     }
 
-    public Address getFundingAddress(HashIndex spentOutput) {
+    public synchronized Address getFundingAddress(HashIndex spentOutput) {
         return fundingAddresses.get(spentOutput);
     }
 
@@ -124,16 +124,16 @@ public Map<String, MempoolEntry> getMempoolEntries() {
         return mempoolEntries;
     }
 
-    public Set<TxEntry> getHistory(String scriptHash) {
+    public synchronized List<TxEntry> getHistory(String scriptHash) {
         Set<TxEntry> entries = scriptHashEntries.get(scriptHash);
         if(entries == null) {
-            return Collections.emptySet();
+            return Collections.emptyList();
         }
 
-        return entries;
+        return new ArrayList<>(entries);
     }
 
-    public String getBlockHash(int height) {
+    public synchronized String getBlockHash(int height) {
         return blockHeightHashes.get(height);
     }
 

### src/test/java/com/sparrowwallet/sparrow/net/cormorant/electrum/RequestHandlerTest.java
@@ -104,6 +104,7 @@ public OutputStream getOutputStream() {
 
             endOfInput.countDown();
             handler.get(5, TimeUnit.SECONDS);
+            assertTrue(socket.isClosed(), "The handler must close the client socket when it exits");
 
             List<String> lines = written.toString(StandardCharsets.UTF_8).lines().toList();
             assertEquals(2, lines.size(), "The response and the notification must each arrive as one whole line");

### src/test/java/com/sparrowwallet/sparrow/net/cormorant/index/StoreTest.java
@@ -0,0 +1,41 @@
+package com.sparrowwallet.sparrow.net.cormorant.index;
+
+import com.sparrowwallet.drongo.address.Address;
+import com.sparrowwallet.drongo.address.InvalidAddressException;
+import com.sparrowwallet.sparrow.net.cormorant.bitcoind.Category;
+import com.sparrowwallet.sparrow.net.cormorant.bitcoind.ListTransaction;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class StoreTest {
+    private static final String FIRST_TXID = "0000000000000000000000000000000000000000000000000000000000000001";
+    private static final String SECOND_TXID = "0000000000000000000000000000000000000000000000000000000000000002";
+
+    @Test
+    public void testHistoryIsUnaffectedByLaterUpdates() throws InvalidAddressException {
+        Store store = new Store();
+        Address address = Address.fromString("bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kv8f3t4");
+        store.addAddressTransaction(address, transaction(address, FIRST_TXID, 0));
+        store.addAddressTransaction(address, transaction(address, SECOND_TXID, 1));
+        String scriptHash = Store.getScriptHash(address);
+
+        //A client connection serialises the history after it is returned, while the polling thread goes on updating the store
+        Iterator<TxEntry> history = store.getHistory(scriptHash).iterator();
+        store.purgeTransaction(FIRST_TXID);
+
+        List<String> served = new ArrayList<>();
+        history.forEachRemaining(txEntry -> served.add(txEntry.tx_hash));
+        assertEquals(List.of(FIRST_TXID, SECOND_TXID), served, "The history returned must be the one at the time it was requested");
+        assertEquals(List.of(SECOND_TXID), store.getHistory(scriptHash).stream().map(txEntry -> txEntry.tx_hash).toList());
+    }
+
+    private ListTransaction transaction(Address address, String txid, int blockIndex) {
+        return new ListTransaction(address.toString(), List.of(), Category.receive, 1.0, 0, 0.0, 1, "0000000000000000000000000000000000000000000000000000000000000003",
+                blockIndex, 0, 840000, txid, 0, 0, List.of(), List.of());
+    }
+}