C api: nix_export_std_string -> nix_observe_string

src/libutil/c/nix_api_util.cc

@@ -63,16 +63,17 @@ const char * nix_version_get()
 }
 
 // Implementations
-nix_err nix_setting_get(nix_c_context * context, const char * key, char * value, int n)
+
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         std::map<std::string, nix::AbstractConfig::SettingInfo> settings;
         nix::globalConfig.getSettings(settings);
-        if (settings.contains(key))
-            return nix_export_std_string(context, settings[key].value, value, n);
-        else {
+        if (settings.contains(key)) {
+            return call_nix_observe_string(settings[key].value, callback, user_data);
+        } else {
             return nix_set_err_msg(context, NIX_ERR_KEY, "Setting not found");
         }
     }
@@ -114,24 +115,24 @@ const char * nix_err_msg(nix_c_context * context, const nix_c_context * read_con
     return nullptr;
 }
 
-nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, char * value, int n)
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
         return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
     }
-    return nix_export_std_string(context, read_context->name, value, n);
+    return call_nix_observe_string(read_context->name, callback, user_data);
 }
 
-nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, char * value, int n)
+nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
 {
     if (context)
         context->last_err_code = NIX_OK;
     if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
         return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
     }
-    return nix_export_std_string(context, read_context->info->msg.str(), value, n);
+    return call_nix_observe_string(read_context->info->msg.str(), callback, user_data);
 }
 
 nix_err nix_err_code(const nix_c_context * read_context)
@@ -140,12 +141,8 @@ nix_err nix_err_code(const nix_c_context * read_context)
 }
 
 // internal
-nix_err nix_export_std_string(nix_c_context * context, const std::string_view str, char * dest, unsigned int n)
+nix_err call_nix_observe_string(const std::string str, void * callback, void * user_data)
 {
-    size_t i = str.copy(dest, n - 1);
-    dest[i] = 0;
-    if (i == n - 1) {
-        return nix_set_err_msg(context, NIX_ERR_OVERFLOW, "Provided buffer too short");
-    } else
-        return NIX_OK;
+    ((nix_observe_string) callback)(str.c_str(), str.size(), user_data);
+    return NIX_OK;
 }

src/libutil/c/nix_api_util.h

@@ -119,6 +119,15 @@ typedef int nix_err;
  */
 typedef struct nix_c_context nix_c_context;
 
+/**
+ * @brief Called to get the value of a string owned by Nix.
+ *
+ * @param[in] start the string to copy.
+ * @param[in] n the string length.
+ * @param[in] user_data optional, arbitrary data, passed to the nix_observe_string when it's called.
+ */
+typedef void (*nix_observe_string)(const char * start, unsigned int n, void * user_data);
+
 // Function prototypes
 
 /**
@@ -160,14 +169,13 @@ nix_err nix_libutil_init(nix_c_context * context);
  *
  * @param[out] context optional, Stores error information
  * @param[in] key The key of the setting to retrieve.
- * @param[out] value A pointer to a buffer where the value of the setting will
- * be stored.
- * @param[in] n The size of the buffer pointed to by value.
- * @return NIX_ERR_KEY if the setting is unknown, NIX_ERR_OVERFLOW if the
- * provided buffer is too short, or NIX_OK if the setting was retrieved
+ * @param[in] callback Called with the setting value.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
+ * @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was retrieved
  * successfully.
  */
-nix_err nix_setting_get(nix_c_context * context, const char * key, char * value, int n);
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data);
 
 /**
  * @brief Sets a setting in the nix global configuration.
@@ -227,12 +235,14 @@ const char * nix_err_msg(nix_c_context * context, const nix_c_context * ctx, uns
  *
  * @param[out] context optional, the context to store errors in if this function
  * fails
- * @param[in] read_context the context to retrieve the error message from
- * @param[out] value The allocated area to write the error string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] read_context the context to retrieve the error message from.
+ * @param[in] callback Called with the error message.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return NIX_OK if there were no errors, an error code otherwise.
  */
-nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, char * value, int n);
+nix_err
+nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
 
 /**
  * @brief Retrieves the error name from a context.
@@ -245,11 +255,12 @@ nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_con
  * @param context optional, the context to store errors in if this function
  * fails
  * @param[in] read_context the context to retrieve the error message from
- * @param[out] value The allocated area to write the error string to.
- * @param[in] n Maximum size of the returned string.
+ * @param[in] callback Called with the error name.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_observe_string
  * @return NIX_OK if there were no errors, an error code otherwise.
  */
-nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, char * value, int n);
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
 
 /**
  * @brief Retrieves the most recent error code from a nix_c_context

src/libutil/c/nix_api_util_internal.h

@@ -20,16 +20,16 @@ nix_err nix_context_error(nix_c_context * context);
 /**
  * Internal use only.
  *
- * Export a std::string across the C api boundary
+ * Helper to invoke nix_observe_string
  * @param context optional, the context to store errors in if this function
  * fails
- * @param str The string to export
- * @param value The allocated area to write the string to.
- * @param n Maximum size of the returned string.
- * @return NIX_OK if there were no errors, NIX_ERR_OVERFLOW if the string length
- * exceeds `n`.
+ * @param str The string to observe
+ * @param callback Called with the observed string.
+ * @param user_data optional, arbitrary data, passed to the callback when it's called.
+ * @return NIX_OK if there were no errors.
+ * @see nix_observe_string
  */
-nix_err nix_export_std_string(nix_c_context * context, const std::string_view str, char * dest, unsigned int n);
+nix_err call_nix_observe_string(const std::string str, void * callback, void * user_data);
 
 #define NIXC_CATCH_ERRS \
     catch (...) \