Add mallocinfo mode to `getmemoryinfo` RPC

This adds a mode argument to `getmemoryinfo`. By default the output
will remain the same. However if a mode argument of `mallocinfo` is
provided the result of glibc `malloc_info` (if available) will
be returned as a string, as-is.

This is useful for tracking heap usage over time or troubleshooting
memory fragmentation issues.

configure.ac

@@ -566,6 +566,13 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/socket.h>]],
  [ AC_MSG_RESULT(no)]
 )
 
+dnl Check for malloc_info (for memory statistics information in getmemoryinfo)
+AC_MSG_CHECKING(for getmemoryinfo)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <malloc.h>]],
+ [[ int f = malloc_info(0, NULL); ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_MALLOC_INFO, 1,[Define this symbol if you have malloc_info]) ],
+ [ AC_MSG_RESULT(no)]
+)
 
 AC_MSG_CHECKING([for visibility attribute])
 AC_LINK_IFELSE([AC_LANG_SOURCE([

src/rpc/misc.cpp

@@ -20,6 +20,9 @@
 #endif
 
 #include <stdint.h>
+#ifdef HAVE_MALLOC_INFO
+#include <malloc.h>
+#endif
 
 #include <boost/assign/list_of.hpp>
 
@@ -485,16 +488,39 @@ static UniValue RPCLockedMemoryInfo()
     return obj;
 }
 
+#ifdef HAVE_MALLOC_INFO
+static std::string RPCMallocInfo()
+{
+    char *ptr = nullptr;
+    size_t size = 0;
+    FILE *f = open_memstream(&ptr, &size);
+    if (f) {
+        malloc_info(0, f);
+        fclose(f);
+        if (ptr) {
+            std::string rv(ptr, size);
+            free(ptr);
+            return rv;
+        }
+    }
+    return "";
+}
+#endif
+
 UniValue getmemoryinfo(const JSONRPCRequest& request)
 {
     /* Please, avoid using the word "pool" here in the RPC interface or help,
      * as users will undoubtedly confuse it with the other "memory pool"
      */
-    if (request.fHelp || request.params.size() != 0)
+    if (request.fHelp || request.params.size() > 1)
         throw std::runtime_error(
-            "getmemoryinfo\n"
+            "getmemoryinfo (\"mode\")\n"
             "Returns an object containing information about memory usage.\n"
-            "\nResult:\n"
+            "Arguments:\n"
+            "1. \"mode\" determines what kind of information is returned. This argument is optional, the default mode is \"stats\".\n"
+            "  - \"stats\" returns general statistics about memory usage in the daemon.\n"
+            "  - \"mallocinfo\" returns an XML string describing low-level heap state (only available if compiled with glibc 2.10+).\n"
+            "\nResult (mode \"stats\"):\n"
             "{\n"
             "  \"locked\": {               (json object) Information about locked memory manager\n"
             "    \"used\": xxxxx,          (numeric) Number of bytes used\n"
@@ -505,13 +531,27 @@ UniValue getmemoryinfo(const JSONRPCRequest& request)
             "    \"chunks_free\": xxxxx,   (numeric) Number unused chunks\n"
             "  }\n"
             "}\n"
+            "\nResult (mode \"mallocinfo\"):\n"
+            "\"<malloc version=\"1\">...\"\n"
             "\nExamples:\n"
             + HelpExampleCli("getmemoryinfo", "")
             + HelpExampleRpc("getmemoryinfo", "")
         );
-    UniValue obj(UniValue::VOBJ);
-    obj.push_back(Pair("locked", RPCLockedMemoryInfo()));
-    return obj;
+
+    std::string mode = (request.params.size() < 1 || request.params[0].isNull()) ? "stats" : request.params[0].get_str();
+    if (mode == "stats") {
+        UniValue obj(UniValue::VOBJ);
+        obj.push_back(Pair("locked", RPCLockedMemoryInfo()));
+        return obj;
+    } else if (mode == "mallocinfo") {
+#ifdef HAVE_MALLOC_INFO
+        return RPCMallocInfo();
+#else
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "mallocinfo is only available when compiled with glibc 2.10+");
+#endif
+    } else {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "unknown mode " + mode);
+    }
 }
 
 UniValue echo(const JSONRPCRequest& request)
@@ -531,7 +571,7 @@ static const CRPCCommand commands[] =
 { //  category              name                      actor (function)         okSafeMode
   //  --------------------- ------------------------  -----------------------  ----------
     { "control",            "getinfo",                &getinfo,                true,  {} }, /* uses wallet if enabled */
-    { "control",            "getmemoryinfo",          &getmemoryinfo,          true,  {} },
+    { "control",            "getmemoryinfo",          &getmemoryinfo,          true,  {"mode"} },
     { "util",               "validateaddress",        &validateaddress,        true,  {"address"} }, /* uses wallet if enabled */
     { "util",               "createmultisig",         &createmultisig,         true,  {"nrequired","keys"} },
     { "util",               "verifymessage",          &verifymessage,          true,  {"address","signature","message"} },