rest: Add GetChainman function and use it

This is not the cleanest change but:

1. It fixes the erroneous use of RPC's Ensure*() in rest.cpp, which
   cause crashes in REST contexts.

   RPC code wraps all calls in a try/except, REST code does not.
   Ensure*(), being part of RPC, expects that its throw's will get
   caught by a try/except. But if you use Ensure*() in REST code, since
   it doesn't have a try/except wrap, a crash will happen.

2. It is consistent with other functions like GetMemPool.

Someone can probably make this a bit prettier.

Author: dongcarl

src/rest.cpp

@@ -107,6 +107,27 @@ static CTxMemPool* GetMemPool(const std::any& context, HTTPRequest* req)
     return node_context->mempool.get();
 }
 
+/**
+ * Get the node context chainstatemanager.
+ *
+ * @param[in]  req The HTTP request, whose status code will be set if node
+ *                 context chainstatemanager is not found.
+ * @returns        Pointer to the chainstatemanager or nullptr if none found.
+ */
+static ChainstateManager* GetChainman(const std::any& context, HTTPRequest* req)
+{
+    auto node_context = util::AnyPtr<NodeContext>(context);
+    if (!node_context || !node_context->chainman) {
+        RESTERR(req, HTTP_INTERNAL_SERVER_ERROR,
+                strprintf("%s:%d (%s)\n"
+                          "Internal bug detected: Chainman disabled or instance not found!\n"
+                          "You may report this issue here: %s\n",
+                          __FILE__, __LINE__, __func__, PACKAGE_BUGREPORT));
+        return nullptr;
+    }
+    return node_context->chainman;
+}
+
 static RetFormat ParseDataFormat(std::string& param, const std::string& strReq)
 {
     const std::string::size_type pos = strReq.rfind('.');
@@ -181,7 +202,9 @@ static bool rest_headers(const std::any& context,
     std::vector<const CBlockIndex *> headers;
     headers.reserve(count);
     {
-        ChainstateManager& chainman = EnsureAnyChainman(context);
+        ChainstateManager* maybe_chainman = GetChainman(context, req);
+        if (!maybe_chainman) return false;
+        ChainstateManager& chainman = *maybe_chainman;
         LOCK(cs_main);
         CChain& active_chain = chainman.ActiveChain();
         tip = active_chain.Tip();
@@ -252,7 +275,9 @@ static bool rest_block(const std::any& context,
     CBlockIndex* pblockindex = nullptr;
     CBlockIndex* tip = nullptr;
     {
-        ChainstateManager& chainman = EnsureAnyChainman(context);
+        ChainstateManager* maybe_chainman = GetChainman(context, req);
+        if (!maybe_chainman) return false;
+        ChainstateManager& chainman = *maybe_chainman;
         LOCK(cs_main);
         tip = chainman.ActiveChain().Tip();
         pblockindex = chainman.m_blockman.LookupBlockIndex(hash);
@@ -541,7 +566,9 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
     std::string bitmapStringRepresentation;
     std::vector<bool> hits;
     bitmap.resize((vOutPoints.size() + 7) / 8);
-    ChainstateManager& chainman = EnsureAnyChainman(context);
+    ChainstateManager* maybe_chainman = GetChainman(context, req);
+    if (!maybe_chainman) return false;
+    ChainstateManager& chainman = *maybe_chainman;
     {
         auto process_utxos = [&vOutPoints, &outs, &hits](const CCoinsView& view, const CTxMemPool& mempool) {
             for (const COutPoint& vOutPoint : vOutPoints) {
@@ -644,7 +671,9 @@ static bool rest_blockhash_by_height(const std::any& context, HTTPRequest* req,
 
     CBlockIndex* pblockindex = nullptr;
     {
-        ChainstateManager& chainman = EnsureAnyChainman(context);
+        ChainstateManager* maybe_chainman = GetChainman(context, req);
+        if (!maybe_chainman) return false;
+        ChainstateManager& chainman = *maybe_chainman;
         LOCK(cs_main);
         const CChain& active_chain = chainman.ActiveChain();
         if (blockheight > active_chain.Height()) {