alternator: add another unwrap_number() variant

We have an unwrap_number() function which in case of data errors (such
as the value not being a number) throws an exception with a given
string used in the message.

In this patch we add a variant of unwrap_number() - try_unwrap_number() -
which doesn't take a message, and doesn't throw exceptions - instead it
returns an empty std::optional if the given value is not a number.
This function is useful in places where we need to know if we got a
number or not, but both are fine but not errors. We'll use it in a
following patch to parse expiration times for the TTL feature.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

alternator/serialization.cc

@@ -270,6 +270,21 @@ big_decimal unwrap_number(const rjson::value& v, std::string_view diagnostic) {
     }
 }
 
+std::optional<big_decimal> try_unwrap_number(const rjson::value& v) {
+    if (!v.IsObject() || v.MemberCount() != 1) {
+        return std::nullopt;
+    }
+    auto it = v.MemberBegin();
+    if (it->name != "N" || !it->value.IsString()) {
+        return std::nullopt;
+    }
+    try {
+        return big_decimal(rjson::to_string_view(it->value));
+    } catch (const marshal_exception& e) {
+        return std::nullopt;
+    }
+}
+
 const std::pair<std::string, const rjson::value*> unwrap_set(const rjson::value& v) {
     if (!v.IsObject() || v.MemberCount() != 1) {
         return {"", nullptr};

alternator/serialization.hh

@@ -23,6 +23,7 @@
 
 #include <string>
 #include <string_view>
+#include <optional>
 #include "types.hh"
 #include "schema_fwd.hh"
 #include "keys.hh"
@@ -64,6 +65,10 @@ clustering_key ck_from_json(const rjson::value& item, schema_ptr schema);
 // raises ValidationException with diagnostic.
 big_decimal unwrap_number(const rjson::value& v, std::string_view diagnostic);
 
+// try_unwrap_number is like unwrap_number, but returns an unset optional
+// when the given v does not encode a number.
+std::optional<big_decimal> try_unwrap_number(const rjson::value& v);
+
 // Check if a given JSON object encodes a set (i.e., it is a {"SS": [...]}, or "NS", "BS"
 // and returns set's type and a pointer to that set. If the object does not encode a set,
 // returned value is {"", nullptr}