allow for custom block index path

useful for when you have a blocks dir and you want to benchmark rebuilding
the block index / chainstate with different db implementations.

can specify -blocksdir=<blocks directory> -blockindexdir=<my custom index dir>
and reuse the same blocksdir to test multiple chainstate / block index implementations

src/common/args.cpp

@@ -304,6 +304,16 @@ fs::path ArgsManager::GetBlocksDirPath() const
     return path;
 }
 
+fs::path ArgsManager::GetIndexDir() const
+{
+    LOCK(cs_args);
+    const fs::path indexdir{GetPathArg("-blockindexdir")};
+    if (!indexdir.empty()) {
+        return fs::absolute(indexdir);
+    } else {
+        return GetDataDirNet() / "blocks" / "index";
+    }
+}
 fs::path ArgsManager::GetDataDir(bool net_specific) const
 {
     LOCK(cs_args);

src/common/args.h

@@ -232,6 +232,7 @@ class ArgsManager
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
     fs::path GetDataDirNet() const { return GetDataDir(true); }
+    fs::path GetIndexDir() const;
 
     /**
      * Clear cached directory paths

src/init.cpp

@@ -461,6 +461,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
 #endif
     argsman.AddArg("-assumevalid=<hex>", strprintf("If this block is in the chain assume that it and its ancestors are valid and potentially skip their script verification (0 to verify all, default: %s, testnet3: %s, testnet4: %s, signet: %s)", defaultChainParams->GetConsensus().defaultAssumeValid.GetHex(), testnetChainParams->GetConsensus().defaultAssumeValid.GetHex(), testnet4ChainParams->GetConsensus().defaultAssumeValid.GetHex(), signetChainParams->GetConsensus().defaultAssumeValid.GetHex()), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-blocksdir=<dir>", "Specify directory to hold blocks subdirectory for *.dat files (default: <datadir>)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-blockindexdir=<dir>", "Specify directory to hold blocks index subdirectory (default: <datadir>/blocks/)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-blocksxor",
                    strprintf("Whether an XOR-key applies to blocksdir *.dat files. "
                              "The created XOR-key will be zeros for an existing blocksdir or when `-blocksxor=0` is "
@@ -1055,6 +1056,7 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         ChainstateManager::Options chainman_opts_dummy{
             .chainparams = chainparams,
             .datadir = args.GetDataDirNet(),
+            .indexdir = args.GetIndexDir(),
             .notifications = notifications,
         };
         auto chainman_result{ApplyArgsManOptions(args, chainman_opts_dummy)};
@@ -1205,6 +1207,7 @@ static ChainstateLoadResult InitAndLoadChainstate(
     ChainstateManager::Options chainman_opts{
         .chainparams = chainparams,
         .datadir = args.GetDataDirNet(),
+        .indexdir = args.GetIndexDir(),
         .notifications = *node.notifications,
         .signals = node.validation_signals.get(),
     };

src/kernel/chainstatemanager_opts.h

@@ -34,6 +34,7 @@ namespace kernel {
 struct ChainstateManagerOpts {
     const CChainParams& chainparams;
     fs::path datadir;
+    fs::path indexdir;
     std::optional<int32_t> check_block_index{};
     bool checkpoints_enabled{DEFAULT_CHECKPOINTS_ENABLED};
     //! If set, it will override the minimum work we will assume exists on some valid chain.

src/node/chainstate.cpp

@@ -42,7 +42,7 @@ static ChainstateLoadResult CompleteChainstateInitialization(
     // fails if it's still open from the previous loop. Close it first:
     pblocktree.reset();
     pblocktree = std::make_unique<BlockTreeDB>(DBParams{
-        .path = chainman.m_options.datadir / "blocks" / "index",
+        .path = chainman.m_options.indexdir,
         .cache_bytes = static_cast<size_t>(cache_sizes.block_tree_db),
         .memory_only = options.block_tree_db_in_memory,
         .wipe_data = options.wipe_block_tree_db,

src/test/validation_chainstatemanager_tests.cpp

@@ -386,6 +386,7 @@ struct SnapshotTestSetup : TestChain100Setup {
             const ChainstateManager::Options chainman_opts{
                 .chainparams = ::Params(),
                 .datadir = chainman.m_options.datadir,
+                .indexdir = chainman.m_options.indexdir,
                 .notifications = *m_node.notifications,
                 .signals = m_node.validation_signals.get(),
             };
@@ -795,6 +796,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_args, BasicTestingSetup)
         static const ChainstateManager::Options options{
             .chainparams = ::Params(),
             .datadir = {},
+            .indexdir = {},
             .notifications = notifications};
         return SetOptsFromArgs(*this->m_node.args, options, args);
     };