Add isInitialized to nix::Value

Add a method to check if a value has been initialized. This helps avoid
segfaults when calling `type()`.
Useful in the context of the new C API.

Closes #10524

src/libexpr/value.hh

@@ -23,6 +23,7 @@ class BindingsBuilder;
 
 
 typedef enum {
+    tUnset,
     tInt = 1,
     tBool,
     tString,
@@ -166,7 +167,7 @@ public:
 struct Value
 {
 private:
-    InternalType internalType;
+    InternalType internalType = tUnset;
 
     friend std::string showType(const Value & v);
 
@@ -270,6 +271,7 @@ public:
     inline ValueType type(bool invalidIsThunk = false) const
     {
         switch (internalType) {
+            case tUnset: break;
             case tInt: return nInt;
             case tBool: return nBool;
             case tString: return nString;
@@ -294,6 +296,11 @@ public:
         internalType = newType;
     }
 
+    inline bool isInitialized()
+    {
+        return internalType != tUnset;
+    }
+
     inline void mkInt(NixInt n)
     {
         finishValue(tInt, { .integer = n });

tests/unit/libexpr/value/value.cc

@@ -0,0 +1,25 @@
+#include "value.hh"
+
+#include "tests/libstore.hh"
+
+namespace nix {
+
+class ValueTest : public LibStoreTest
+{};
+
+TEST_F(ValueTest, unsetValue)
+{
+    Value unsetValue;
+    ASSERT_EQ(false, unsetValue.isInitialized());
+    ASSERT_EQ(nThunk, unsetValue.type(true));
+    ASSERT_DEATH(unsetValue.type(), "");
+}
+
+TEST_F(ValueTest, vInt)
+{
+    Value vInt;
+    vInt.mkInt(42);
+    ASSERT_EQ(true, vInt.isInitialized());
+}
+
+} // namespace nix