refactor: Add helper function secp256k1_context_is_proper()

src/secp256k1.c

@@ -75,6 +75,15 @@ static const secp256k1_context secp256k1_context_static_ = {
 const secp256k1_context *secp256k1_context_static = &secp256k1_context_static_;
 const secp256k1_context *secp256k1_context_no_precomp = &secp256k1_context_static_;
 
+/* Helper function that determines if a context is proper, i.e., is not the static context or a copy thereof.
+ *
+ * This is intended for "context" functions such as secp256k1_context_clone. Function which need specific
+ * features of a context should still check for these features directly. For example, a function that needs
+ * ecmult_gen should directly check for the existence of the ecmult_gen context. */
+static int secp256k1_context_is_proper(const secp256k1_context* ctx) {
+    return secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx);
+}
+
 void secp256k1_selftest(void) {
     if (!secp256k1_selftest_passes()) {
         secp256k1_callback_call(&default_error_callback, "self test failed");
@@ -171,6 +180,9 @@ void secp256k1_context_destroy(secp256k1_context* ctx) {
 }
 
 void secp256k1_context_set_illegal_callback(secp256k1_context* ctx, void (*fun)(const char* message, void* data), const void* data) {
+    /* We compare pointers instead of checking secp256k1_context_is_proper() here
+       because setting callbacks is allowed on *copies* of the static context:
+       it's harmless and makes testing easier. */
     ARG_CHECK_NO_RETURN(ctx != secp256k1_context_static);
     if (fun == NULL) {
         fun = secp256k1_default_illegal_callback_fn;
@@ -180,6 +192,9 @@ void secp256k1_context_set_illegal_callback(secp256k1_context* ctx, void (*fun)(
 }
 
 void secp256k1_context_set_error_callback(secp256k1_context* ctx, void (*fun)(const char* message, void* data), const void* data) {
+    /* We compare pointers instead of checking secp256k1_context_is_proper() here
+       because setting callbacks is allowed on *copies* of the static context:
+       it's harmless and makes testing easier. */
     ARG_CHECK_NO_RETURN(ctx != secp256k1_context_static);
     if (fun == NULL) {
         fun = secp256k1_default_error_callback_fn;