schemagen: fix nullable root types (#13)

The C++ code generator now handles schemas where the top-level type is
nullable ("type": ["object", "null"], ["string", "null"], or
["array", "null"]).

Changes to weaseljson_schemagen.py:
- Rename the inner object struct when the root is a nullable object, so
  the `using Root = std::optional<...>` alias no longer conflicts with
  `struct Root`.
- Treat nullable root objects and arrays as container roots, emplacing
  the inner value before pushing the root frame and pointing the frame at
  the contained value.
- For nullable root scalars/enums, engage() now returns a pointer to the
  value inside the optional rather than to the optional wrapper itself.
- cbNull() now safely accepts a top-level null when the root is nullable
  and rejects it otherwise.

Regression tests added:
- nullable_object.schema.json + test_nullable_root.cpp
- nullable_string.schema.json
- nullable_array.schema.json

Closes #13

contrib/schemagen/test_nullable_root.cpp

@@ -0,0 +1,158 @@
+// Regression test for issue #13: nullable root types.
+#include <cassert>
+#include <cstdio>
+#include <string>
+
+#include "nullable_array.h"
+#include "nullable_object.h"
+#include "nullable_string.h"
+
+static int failures = 0;
+#define CHECK(cond)                                                            \
+  do {                                                                         \
+    if (!(cond)) {                                                             \
+      printf("FAIL %s:%d: %s\n", __FILE__, __LINE__, #cond);                   \
+      ++failures;                                                              \
+    }                                                                          \
+  } while (0)
+
+static WeaselJsonStatus parseStrided(nullable_object::RootBuilder &b,
+                                     std::string in) {
+  for (size_t i = 0; i < in.size(); ++i) {
+    char c = in[i];
+    WeaselJsonStatus s = b.feed(&c, 1);
+    if (s != WeaselJson_AGAIN)
+      return s;
+  }
+  return b.finish();
+}
+
+static WeaselJsonStatus parseStrided(nullable_string::RootBuilder &b,
+                                     std::string in) {
+  for (size_t i = 0; i < in.size(); ++i) {
+    char c = in[i];
+    WeaselJsonStatus s = b.feed(&c, 1);
+    if (s != WeaselJson_AGAIN)
+      return s;
+  }
+  return b.finish();
+}
+
+static WeaselJsonStatus parseStrided(nullable_array::RootBuilder &b,
+                                     std::string in) {
+  for (size_t i = 0; i < in.size(); ++i) {
+    char c = in[i];
+    WeaselJsonStatus s = b.feed(&c, 1);
+    if (s != WeaselJson_AGAIN)
+      return s;
+  }
+  return b.finish();
+}
+
+static void expectReject(nullable_object::RootBuilder &b, std::string in,
+                         const char *what) {
+  WeaselJsonStatus s = parseStrided(b, in);
+  if (s == WeaselJson_REJECT) {
+    printf("ok   reject: %s\n", what);
+  } else {
+    printf("FAIL expected reject (%s) got status %d for: %s\n", what, s,
+           in.c_str());
+    ++failures;
+  }
+}
+
+int main() {
+  // ---- nullable root object: valid document ----
+  {
+    nullable_object::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, R"({"x":"hello"})");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_object::Root r = b.take();
+      CHECK(r.has_value());
+      CHECK(r->x == "hello");
+      printf("ok   nullable root object accepts object\n");
+    }
+  }
+
+  // ---- nullable root object: null document ----
+  {
+    nullable_object::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, "null");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_object::Root r = b.take();
+      CHECK(!r.has_value());
+      printf("ok   nullable root object accepts null\n");
+    }
+  }
+
+  // ---- nullable root object: schema checks still run ----
+  {
+    nullable_object::RootBuilder b;
+    expectReject(b, R"({"x":"hello","extra":1})",
+                 "unknown key in strict nullable root object");
+  }
+  {
+    nullable_object::RootBuilder b;
+    expectReject(b, R"({})", "missing required field in nullable root object");
+  }
+
+  // ---- nullable root string: valid value ----
+  {
+    nullable_string::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, R"("hello")");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_string::Root r = b.take();
+      CHECK(r.has_value());
+      CHECK(*r == "hello");
+      printf("ok   nullable root string accepts string\n");
+    }
+  }
+
+  // ---- nullable root string: null value ----
+  {
+    nullable_string::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, "null");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_string::Root r = b.take();
+      CHECK(!r.has_value());
+      printf("ok   nullable root string accepts null\n");
+    }
+  }
+
+  // ---- nullable root array: valid value ----
+  {
+    nullable_array::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, "[1,2,3]");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_array::Root r = b.take();
+      CHECK(r.has_value());
+      CHECK(r->size() == 3);
+      CHECK((*r)[0] == 1 && (*r)[1] == 2 && (*r)[2] == 3);
+      printf("ok   nullable root array accepts array\n");
+    }
+  }
+
+  // ---- nullable root array: null value ----
+  {
+    nullable_array::RootBuilder b;
+    WeaselJsonStatus s = parseStrided(b, "null");
+    CHECK(s == WeaselJson_OK);
+    if (s == WeaselJson_OK) {
+      nullable_array::Root r = b.take();
+      CHECK(!r.has_value());
+      printf("ok   nullable root array accepts null\n");
+    }
+  }
+
+  if (failures == 0) {
+    printf("\nALL TESTS PASSED\n");
+    return 0;
+  }
+  printf("\n%d FAILURE(S)\n", failures);
+  return 1;
+}