Use ModifyCoins instead of mutable GetCoins.

Replace the mutable non-copying GetCoins method with a ModifyCoins, which
returns an encapsulated iterator, so we can keep track of concurrent
modifications (as iterators can be invalidated by those) and run cleanup
code after a modification is finished.

This also removes the overloading of the 'GetCoins' name.

src/coins.h

@@ -83,11 +83,26 @@ public:
     // as new tx version will probably only be introduced at certain heights
     int nVersion;
 
-    // construct a CCoins from a CTransaction, at a given height
-    CCoins(const CTransaction &tx, int nHeightIn) : fCoinBase(tx.IsCoinBase()), vout(tx.vout), nHeight(nHeightIn), nVersion(tx.nVersion) {
+    void FromTx(const CTransaction &tx, int nHeightIn) {
+        fCoinBase = tx.IsCoinBase();
+        vout = tx.vout;
+        nHeight = nHeightIn;
+        nVersion = tx.nVersion;
         ClearUnspendable();
     }
 
+    // construct a CCoins from a CTransaction, at a given height
+    CCoins(const CTransaction &tx, int nHeightIn) {
+        FromTx(tx, nHeightIn);
+    }
+
+    void Clear() {
+        fCoinBase = false;
+        std::vector<CTxOut>().swap(vout);
+        nHeight = 0;
+        nVersion = 0;
+    }
+
     // empty constructor
     CCoins() : fCoinBase(false), vout(0), nHeight(0), nVersion(0) { }
 
@@ -323,10 +338,31 @@ public:
 };
 
 
+class CCoinsViewCache;
+
+/** A reference to a mutable cache entry. Encapsulating it allows us to run
+ *  cleanup code after the modification is finished, and keeping track of
+ *  concurrent modifications. */
+class CCoinsModifier
+{
+private:
+    CCoinsViewCache& cache;
+    CCoinsMap::iterator it;
+    CCoinsModifier(CCoinsViewCache& cache_, CCoinsMap::iterator it_);
+
+public:
+    CCoins* operator->() { return &it->second; }
+    CCoins& operator*() { return it->second; }
+    ~CCoinsModifier();
+    friend class CCoinsViewCache;
+};
+
 /** CCoinsView that adds a memory cache for transactions to another CCoinsView */
 class CCoinsViewCache : public CCoinsViewBacked
 {
 protected:
+    /* Whether this cache has an active modifier. */
+    bool hasModifier;
 
     /* Make mutable so that we can "fill the cache" even from Get-methods
        declared as "const".  */
@@ -335,6 +371,7 @@ protected:
 
 public:
     CCoinsViewCache(CCoinsView &baseIn, bool fDummy = false);
+    ~CCoinsViewCache();
 
     // Standard CCoinsView methods
     bool GetCoins(const uint256 &txid, CCoins &coins) const;
@@ -349,8 +386,10 @@ public:
     // allowed while accessing the returned pointer.
     const CCoins* AccessCoins(const uint256 &txid) const;
 
-    // Return a modifiable reference to a CCoins. Check HaveCoins first.
-    CCoins &GetCoins(const uint256 &txid);
+    // Return a modifiable reference to a CCoins. If no entry with the given
+    // txid exists, a new one is created. Simultaneous modifications are not
+    // allowed.
+    CCoinsModifier ModifyCoins(const uint256 &txid);
 
     // Push the modifications applied to this cache to its base.
     // Failure to call this method before destruction will cause the changes to be forgotten.
@@ -377,6 +416,8 @@ public:
 
     const CTxOut &GetOutputFor(const CTxIn& input) const;
 
+    friend class CCoinsModifier;
+
 private:
     CCoinsMap::iterator FetchCoins(const uint256 &txid);
     CCoinsMap::const_iterator FetchCoins(const uint256 &txid) const;