add --block-notify to monerod and --tx-notify to monero-wallet-{cli,rpc}

Those take a command line of the form "A [B]", with A being the
name (and optional path, if not in the caller's CWD, but fully
qualified path is recommended, avoids possible security issues)
to a program, and optional arguments. Any occurence of the two
character string "%s" will be replaced by the hash of the block
or transaction which triggered the notification.

Tokenization is barebones. If you want things like pipes, calls
to paths with spaces, etc, then use a script (though exec time
will suffer).

block-notify is called when a new block is added onto the chain.

tx-notify is called when a new transaction happens with the
wallet as source and/or destination.

It is the notification program's responsibility to determine what
to do in those cases.

Note that this is asynchronous, so it is very possible that:
- the notification programs will be run out of order
- several events happen before the notification for the first one

A Windows port would be nice if someone wants to make one.

src/cryptonote_core/blockchain.cpp

@@ -52,6 +52,7 @@
 #include "cryptonote_core.h"
 #include "ringct/rctSigs.h"
 #include "common/perf_timer.h"
+#include "common/notify.h"
 #if defined(PER_BLOCK_CHECKPOINT)
 #include "blocks/blocks.h"
 #endif
@@ -3552,6 +3553,10 @@ leave:
   get_difficulty_for_next_block(); // just to cache it
   invalidate_block_template_cache();
 
+  std::shared_ptr<tools::Notify> block_notify = m_block_notify;
+  if (block_notify)
+    block_notify->notify(epee::string_tools::pod_to_hex(id).c_str());
+
   return true;
 }
 //------------------------------------------------------------------

src/cryptonote_core/blockchain.h

@@ -55,6 +55,8 @@
 #include "cryptonote_basic/hardfork.h"
 #include "blockchain_db/blockchain_db.h"
 
+namespace tools { class Notify; }
+
 namespace cryptonote
 {
   class tx_memory_pool;
@@ -705,6 +707,13 @@ namespace cryptonote
     void set_user_options(uint64_t maxthreads, bool sync_on_blocks, uint64_t sync_threshold,
         blockchain_db_sync_mode sync_mode, bool fast_sync);
 
+    /**
+     * @brief sets a block notify object to call for every new block
+     *
+     * @param notify the notify object to cal at every new block
+     */
+    void set_block_notify(const std::shared_ptr<tools::Notify> &notify) { m_block_notify = notify; }
+
     /**
      * @brief Put DB in safe sync mode
      */
@@ -1032,6 +1041,8 @@ namespace cryptonote
     uint64_t m_btc_expected_reward;
     bool m_btc_valid;
 
+    std::shared_ptr<tools::Notify> m_block_notify;
+
     /**
      * @brief collects the keys for all outputs being "spent" as an input
      *

src/cryptonote_core/cryptonote_core.cpp

@@ -53,6 +53,7 @@ using namespace epee;
 #include "ringct/rctTypes.h"
 #include "blockchain_db/blockchain_db.h"
 #include "ringct/rctSigs.h"
+#include "common/notify.h"
 #include "version.h"
 
 #undef MONERO_DEFAULT_LOG_CATEGORY
@@ -167,6 +168,11 @@ namespace cryptonote
   , "Set maximum txpool weight in bytes."
   , DEFAULT_TXPOOL_MAX_WEIGHT
   };
+  static const command_line::arg_descriptor<std::string> arg_block_notify = {
+    "block-notify"
+  , "Run a program for each new block, '%s' will be replaced by the block hash"
+  , ""
+  };
 
   //-----------------------------------------------------------------------------------------------
   core::core(i_cryptonote_protocol* pprotocol):
@@ -276,6 +282,7 @@ namespace cryptonote
     command_line::add_arg(desc, arg_offline);
     command_line::add_arg(desc, arg_disable_dns_checkpoints);
     command_line::add_arg(desc, arg_max_txpool_weight);
+    command_line::add_arg(desc, arg_block_notify);
 
     miner::init_options(desc);
     BlockchainDB::init_options(desc);
@@ -545,6 +552,16 @@ namespace cryptonote
     m_blockchain_storage.set_user_options(blocks_threads,
         sync_on_blocks, sync_threshold, sync_mode, fast_sync);
 
+    try
+    {
+      if (!command_line::is_arg_defaulted(vm, arg_block_notify))
+        m_blockchain_storage.set_block_notify(std::shared_ptr<tools::Notify>(new tools::Notify(command_line::get_arg(vm, arg_block_notify).c_str())));
+    }
+    catch (const std::exception &e)
+    {
+      MERROR("Failed to parse block notify spec");
+    }
+
     const std::pair<uint8_t, uint64_t> regtest_hard_forks[3] = {std::make_pair(1, 0), std::make_pair(Blockchain::get_hard_fork_heights(MAINNET).back().version, 1), std::make_pair(0, 0)};
     const cryptonote::test_options regtest_test_options = {
       regtest_hard_forks