MXS-1929: Add config dump helper functions

The functions dump of parameters only if they differ from the
defaults. The check for equality is rather coarse but it should work as
long as all core objects use C++ types correctly e.g. integer are not used
to store boolean values (I'm looking at you, enable_root and
localhost_match_wildcard_host). The boolean type has a specialization to
convert the value to the string format used for all defaults in the core.

This also adds the missing return value checks to the dprintf calls and
reports errors if any are encountered.

server/core/config.cc

@@ -4666,4 +4666,32 @@ MXS_CONFIG_PARAMETER* ParamList::params()
     return m_ctx.parameters;
 }
 
+
+void dump_if_changed(const MXS_MODULE_PARAM* params, int file,
+                     const std::string& key, const std::string& value)
+{
+    for (int i = 0; params[i].name; i++)
+    {
+        if (params[i].name == key)
+        {
+            /**
+             * This detects only exact matches, not ones that are logically equivalent
+             * but lexicographically different e.g. 1 and true. This might not
+             * be a bad thing: it'll distinct user defined values from defaults.
+             */
+
+            if (!params[i].default_value || value != params[i].default_value)
+            {
+                if (dprintf(file, "%s=%s\n", key.c_str(), value.c_str()) == -1)
+                {
+                    MXS_ERROR("Failed to serialize service value: %d, %s",
+                              errno, mxs_strerror(errno));
+                }
+            }
+
+            break;
+        }
+    }
+}
+
 }

server/core/internal/config.hh

@@ -18,6 +18,8 @@
 
 #include <maxscale/config.h>
 
+#include <sstream>
+
 #include <maxscale/ssl.h>
 #include <maxscale/jansson.h>
 
@@ -214,3 +216,55 @@ bool is_normal_server_parameter(const char *param);
  *         `dest` are left in an undefined state
  */
 bool get_suffixed_size(const char* value, uint64_t* dest);
+
+namespace maxscale
+{
+
+// Internal function
+void dump_if_changed(const MXS_MODULE_PARAM* params, int file,
+                     const std::string& key, const std::string& value);
+
+
+/**
+ * Dump a parameter into a file descriptor
+ *
+ * The function detects only literal matches to the string format default values.
+ * If the string conversion results in an empty string, the value will not be
+ * dumped. This can only happen if an empty string is passed as the value.
+ *
+ * The function writes a single key-value pair into the file and terminates
+ * the line with a newline. This is intended to be used with configuration
+ * dumping code in the core.
+ *
+ * Note: Does not work with enum type parameters, they'll get converted into
+ * integers. Convert them to string format at the call site.
+ *
+ * @param params Module parameter to use
+ * @param file   File descriptor where the line is written
+ * @param key    Name of the parameter
+ * @param value  The parameter value
+ */
+template <class T>
+inline void dump_param(const MXS_MODULE_PARAM* params, int file, const std::string& key, const T value)
+{
+    std::stringstream ss;
+    ss << value;
+    auto strval = ss.str();
+
+    if (!strval.empty())
+    {
+        // Don't dump empty values
+        dump_if_changed(params, file, key, strval);
+    }
+}
+
+// Specialization required to dump booleans in the same format as they used in
+// the defaults. This requires that all defaults use either "true" or "false"
+// for the values.
+template <>
+inline void dump_param(const MXS_MODULE_PARAM* params, int file, const std::string& key, bool value)
+{
+    dump_if_changed(params, file, key, value ? "true" : "false");
+}
+
+}