Handle corrupt wallets gracefully.

Corrupt wallets used to cause a DB_RUNRECOVERY uncaught exception and a
crash. This commit does three things:

1) Runs a BDB verify early in the startup process, and if there is a
low-level problem with the database:
  + Moves the bad wallet.dat to wallet.timestamp.bak
  + Runs a 'salvage' operation to get key/value pairs, and
    writes them to a new wallet.dat
  + Continues with startup.

2) Much more tolerant of serialization errors. All errors in deserialization
are reported by tolerated EXCEPT for errors related to reading keypairs
or master key records-- those are reported and then shut down, so the user
can get help (or recover from a backup).

3) Adds a new -salvagewallet option, which:
 + Moves the wallet.dat to wallet.timestamp.bak
 + extracts ONLY keypairs and master keys into a new wallet.dat
 + soft-sets -rescan, to recreate transaction history

This was tested by randomly corrupting testnet wallets using a little
python script I wrote (https://gist.github.com/3812689)

src/db.cpp

@@ -136,6 +136,69 @@ void CDBEnv::MakeMock()
     fMockDb = true;
 }
 
+CDBEnv::VerifyResult CDBEnv::Verify(std::string strFile, bool (*recoverFunc)(CDBEnv& dbenv, std::string strFile))
+{
+    LOCK(cs_db);
+    assert(mapFileUseCount.count(strFile) == 0);
+
+    Db db(&dbenv, 0);
+    int result = db.verify(strFile.c_str(), NULL, NULL, 0);
+    if (result == 0)
+        return VERIFY_OK;
+    else if (recoverFunc == NULL)
+        return RECOVER_FAIL;
+
+    // Try to recover:
+    bool fRecovered = (*recoverFunc)(*this, strFile);
+    return (fRecovered ? RECOVER_OK : RECOVER_FAIL);
+}
+
+bool CDBEnv::Salvage(std::string strFile, bool fAggressive,
+                     std::vector<CDBEnv::KeyValPair >& vResult)
+{
+    LOCK(cs_db);
+    assert(mapFileUseCount.count(strFile) == 0);
+
+    u_int32_t flags = DB_SALVAGE;
+    if (fAggressive) flags |= DB_AGGRESSIVE;
+
+    stringstream strDump;
+
+    Db db(&dbenv, 0);
+    int result = db.verify(strFile.c_str(), NULL, &strDump, flags);
+    if (result != 0)
+    {
+        printf("ERROR: db salvage failed\n");
+        return false;
+    }
+
+    // Format of bdb dump is ascii lines:
+    // header lines...
+    // HEADER=END
+    // hexadecimal key
+    // hexadecimal value
+    // ... repeated
+    // DATA=END
+
+    string strLine;
+    while (!strDump.eof() && strLine != "HEADER=END")
+        getline(strDump, strLine); // Skip past header
+
+    std::string keyHex, valueHex;
+    while (!strDump.eof() && keyHex != "DATA=END")
+    {
+        getline(strDump, keyHex);
+        if (keyHex != "DATA_END")
+        {
+            getline(strDump, valueHex);
+            vResult.push_back(make_pair(ParseHex(keyHex),ParseHex(valueHex)));
+        }
+    }
+
+    return (result == 0);
+}
+
+
 void CDBEnv::CheckpointLSN(std::string strFile)
 {
     dbenv.txn_checkpoint(0, 0, 0);
@@ -257,6 +320,15 @@ void CDBEnv::CloseDb(const string& strFile)
     }
 }
 
+bool CDBEnv::RemoveDb(const string& strFile)
+{
+    this->CloseDb(strFile);
+
+    LOCK(cs_db);
+    int rc = dbenv.dbremove(NULL, strFile.c_str(), NULL, DB_AUTO_COMMIT);
+    return (rc == 0);
+}
+
 bool CDB::Rewrite(const string& strFile, const char* pszSkip)
 {
     while (!fShutdown)