Allow configuring a custom user agent for DuckDB (#989)

Author: JelteF

docs/settings.md

@@ -147,6 +147,13 @@ Allows mixed transactions between DuckDB and PostgreSQL. This experimental setti
 - **Default**: `false`
 - **Access**: General
 
+### `duckdb.custom_user_agent`
+
+Appends a custom string to the DuckDB user agent (this is appended after `pg_duckdb`).
+
+- **Default**: `""` (empty string)
+- **Access**: Superuser-only
+
 ## File System and Storage
 
 ### `duckdb.temporary_directory`

include/pgduckdb/pgduckdb_guc.hpp

@@ -27,4 +27,5 @@ extern char *duckdb_extension_directory;
 extern char *duckdb_max_temp_directory_size;
 extern char *duckdb_default_collation;
 extern char *duckdb_azure_transport_option_type;
+extern char *duckdb_custom_user_agent;
 } // namespace pgduckdb

src/pgduckdb_duckdb.cpp

@@ -100,7 +100,12 @@ DuckDBManager::Initialize() {
 	std::filesystem::create_directories(duckdb_extension_directory);
 
 	duckdb::DBConfig config;
-	config.SetOptionByName("custom_user_agent", "pg_duckdb");
+	std::string user_agent = "pg_duckdb";
+	if (!IsEmptyString(duckdb_custom_user_agent)) {
+		user_agent += " ";
+		user_agent += duckdb_custom_user_agent;
+	}
+	config.SetOptionByName("custom_user_agent", user_agent);
 	config.SetOptionByName("default_null_order", "postgres");
 
 	SET_DUCKDB_OPTION(allow_unsigned_extensions);

src/pgduckdb_guc.cpp

@@ -141,6 +141,7 @@ char *duckdb_extension_directory = MakeDirName("extensions");
 char *duckdb_max_temp_directory_size = strdup("");
 char *duckdb_default_collation = strdup("");
 char *duckdb_azure_transport_option_type = strdup("");
+char *duckdb_custom_user_agent = strdup("");
 
 void
 InitGUC() {
@@ -243,6 +244,9 @@ InitGUC() {
 	                           "Set the azure_transport_option_type for DuckDB Azure extension. Can be used to "
 	                           "workaround issue #882: https://github.com/duckdb/pg_duckdb/issues/882",
 	                           &duckdb_azure_transport_option_type, PGC_SUSET);
+
+	DefineCustomDuckDBVariable("duckdb.custom_user_agent", "Additional user agent string to append to 'pg_duckdb'",
+	                           &duckdb_custom_user_agent, PGC_SUSET);
 }
 
 #if PG_VERSION_NUM < 160000