C API: nix_get_string now accepts a callback to return the value

doc/external-api/README.md

@@ -40,10 +40,19 @@ Nix expression `builtins.nixVersion`.
 #include <nix_api_expr.h>
 #include <nix_api_value.h>
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 
 // NOTE: This example lacks all error handling. Production code must check for
 // errors, as some return values will be undefined.
-int main() {
+
+void my_get_string_cb(const char * start, unsigned int n, char ** user_data)
+{
+    *user_data = strdup(start);
+}
+
+int main()
+{
     nix_libexpr_init(NULL);
 
     Store * store = nix_store_open(NULL, "dummy://", NULL);
@@ -52,8 +61,12 @@ int main() {
 
     nix_expr_eval_from_string(NULL, state, "builtins.nixVersion", ".", value);
     nix_value_force(NULL, state, value);
-   printf("Nix version: %s\n", nix_get_string(NULL, value));
 
+    char * version;
+    nix_get_string(NULL, value, my_get_string_cb, version);
+    printf("Nix version: %s\n", version);
+
+    free(version);
     nix_gc_decref(NULL, value);
     nix_state_free(state);
     nix_store_free(store);

src/libexpr-c/nix_api_value.cc

@@ -166,16 +166,16 @@ bool nix_get_bool(nix_c_context * context, const Value * value)
     NIXC_CATCH_ERRS_RES(false);
 }
 
-const char * nix_get_string(nix_c_context * context, const Value * value)
+nix_err nix_get_string(nix_c_context * context, const Value * value, nix_get_string_callback callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_not_null(value);
         assert(v.type() == nix::nString);
-        return v.c_str();
+        call_nix_get_string_callback(v.c_str(), callback, user_data);
     }
-    NIXC_CATCH_ERRS_NULL
+    NIXC_CATCH_ERRS
 }
 
 const char * nix_get_path_string(nix_c_context * context, const Value * value)

src/libexpr-c/nix_api_value.h

@@ -178,10 +178,13 @@ bool nix_get_bool(nix_c_context * context, const Value * value);
  *
  * @param[out] context Optional, stores error information
  * @param[in] value Nix value to inspect
+ * @param[in] callback Called with the string value.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
  * @return string
- * @return NULL in case of error.
+ * @return error code, NIX_OK on success.
  */
-const char * nix_get_string(nix_c_context * context, const Value * value);
+nix_err
+nix_get_string(nix_c_context * context, const Value * value, nix_get_string_callback callback, void * user_data);
 
 /** @brief Get path as string
  * @param[out] context Optional, stores error information
@@ -482,7 +485,6 @@ const StorePath * nix_realised_string_get_store_path(nix_realised_string * reali
  */
 void nix_realised_string_free(nix_realised_string * realised_string);
 
-
 // cffi end
 #ifdef __cplusplus
 }

tests/unit/libexpr/nix_api_expr.cc

@@ -17,9 +17,10 @@ TEST_F(nix_api_expr_test, nix_expr_eval_from_string)
 {
     nix_expr_eval_from_string(nullptr, state, "builtins.nixVersion", ".", value);
     nix_value_force(nullptr, state, value);
-    auto result = nix_get_string(nullptr, value);
+    std::string result;
+    nix_get_string(nullptr, value, OBSERVE_STRING(result));
 
-    ASSERT_STREQ(PACKAGE_VERSION, result);
+    ASSERT_STREQ(PACKAGE_VERSION, result.c_str());
 }
 
 TEST_F(nix_api_expr_test, nix_expr_eval_add_numbers)
@@ -47,7 +48,8 @@ TEST_F(nix_api_expr_test, nix_expr_eval_drv)
     nix_value_call(ctx, stateResult, valueFn, value, valueResult);
     ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(nullptr, valueResult));
 
-    std::string p = nix_get_string(nullptr, valueResult);
+    std::string p;
+    nix_get_string(nullptr, valueResult, OBSERVE_STRING(p));
     std::string pEnd = "-myname";
     ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
 
@@ -69,7 +71,8 @@ TEST_F(nix_api_expr_test, nix_build_drv)
     nix_expr_eval_from_string(nullptr, state, expr, ".", value);
 
     Value * drvPathValue = nix_get_attr_byname(nullptr, value, state, "drvPath");
-    const char * drvPath = nix_get_string(nullptr, drvPathValue);
+    std::string drvPath;
+    nix_get_string(nullptr, drvPathValue, OBSERVE_STRING(drvPath));
 
     std::string p = drvPath;
     std::string pEnd = "-myname.drv";
@@ -78,18 +81,19 @@ TEST_F(nix_api_expr_test, nix_build_drv)
     // NOTE: .drvPath should be usually be ignored. Output paths are more versatile.
     //       See https://github.com/NixOS/nix/issues/6507
     //       Use e.g. nix_string_realise to realise the output.
-    StorePath * drvStorePath = nix_store_parse_path(ctx, store, drvPath);
+    StorePath * drvStorePath = nix_store_parse_path(ctx, store, drvPath.c_str());
     ASSERT_EQ(true, nix_store_is_valid_path(ctx, store, drvStorePath));
 
     Value * outPathValue = nix_get_attr_byname(ctx, value, state, "outPath");
-    const char * outPath = nix_get_string(ctx, outPathValue);
+    std::string outPath;
+    nix_get_string(ctx, outPathValue, OBSERVE_STRING(outPath));
 
     p = outPath;
     pEnd = "-myname";
     ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
     ASSERT_EQ(true, drvStorePath->path.isDerivation());
 
-    StorePath * outStorePath = nix_store_parse_path(ctx, store, outPath);
+    StorePath * outStorePath = nix_store_parse_path(ctx, store, outPath.c_str());
     ASSERT_EQ(false, nix_store_is_valid_path(ctx, store, outStorePath));
 
     nix_store_realise(ctx, store, drvStorePath, nullptr, nullptr);

tests/unit/libexpr/nix_api_external.cc

@@ -6,7 +6,9 @@
 #include "nix_api_expr_internal.h"
 #include "nix_api_value.h"
 #include "nix_api_external.h"
+
 #include "tests/nix_api_expr.hh"
+#include "tests/string_callback.hh"
 
 #include <gtest/gtest.h>
 
@@ -58,6 +60,9 @@ TEST_F(nix_api_expr_test, nix_expr_eval_external)
 
     nix_value_call(ctx, state, valueFn, value, valueResult);
 
-    ASSERT_STREQ("nix-external<MyExternalValueDesc( 42 )>", nix_get_string(nullptr, valueResult));
+    std::string string_value;
+    nix_get_string(nullptr, valueResult, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("nix-external<MyExternalValueDesc( 42 )>", string_value.c_str());
 }
+
 }

tests/unit/libexpr/nix_api_value.cc

@@ -6,6 +6,7 @@
 #include "nix_api_value.h"
 
 #include "tests/nix_api_expr.hh"
+#include "tests/string_callback.hh"
 
 #include <cstdlib>
 #include <gtest/gtest.h>
@@ -53,13 +54,15 @@ TEST_F(nix_api_expr_test, nix_value_set_get_bool)
 
 TEST_F(nix_api_expr_test, nix_value_set_get_string)
 {
-    ASSERT_EQ(nullptr, nix_get_string(ctx, nullptr));
+    std::string string_value;
-    ASSERT_DEATH(nix_get_string(ctx, value), "");
+    ASSERT_EQ(NIX_ERR_UNKNOWN, nix_get_string(ctx, nullptr, OBSERVE_STRING(string_value)));
+    ASSERT_DEATH(nix_get_string(ctx, value, OBSERVE_STRING(string_value)), "");
 
     const char * myString = "some string";
     nix_init_string(ctx, value, myString);
 
-    ASSERT_STREQ(myString, nix_get_string(ctx, value));
+    nix_get_string(ctx, value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ(myString, string_value.c_str());
     ASSERT_STREQ("a string", nix_get_typename(ctx, value));
     ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(ctx, value));
 }
@@ -162,11 +165,14 @@ TEST_F(nix_api_expr_test, nix_build_and_init_attr)
     ASSERT_EQ(false, nix_has_attr_byname(ctx, value, state, "no-value"));
 
     out_value = nix_get_attr_byname(ctx, value, state, "b");
-    ASSERT_STREQ("foo", nix_get_string(ctx, out_value));
+    std::string string_value;
+    nix_get_string(ctx, out_value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("foo", string_value.c_str());
     nix_gc_decref(nullptr, out_value);
 
     out_value = nix_get_attr_byidx(ctx, value, state, 1, out_name);
-    ASSERT_STREQ("foo", nix_get_string(ctx, out_value));
+    nix_get_string(ctx, out_value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("foo", string_value.c_str());
     ASSERT_STREQ("b", *out_name);
     nix_gc_decref(nullptr, out_value);