Add an environment variable for custom temp dirs

This would be useful if you need to do some elaborate namespacing setup
and want all sockets and other temporary files in a single directory
instead of in `${XDG_RUNTIME_DIR:/tmp}`.

This resolves #139.

CHANGELOG.md

@@ -6,6 +6,15 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Added an environment variable to choose a different directory for yabridge to
+  store its sockets and other temporary files in. This is only needed when
+  running the Wine process under a separate namespace. If you don't know that
+  you need this, then you probably don't need this!
+
 ## [3.6.0] - 2021-10-15
 
 ### Added

README.md

@@ -668,7 +668,9 @@ the yabridge [Discord](https://discord.gg/pyNeweqadf).
 - If you're using a `WINELOADER` that runs the Wine process under a separate
   namespace while the host is not sandboxed, then you'll have to use the
   `YABRIDGE_NO_WATCHDOG` environment variable to disable the watchdog timer. If
-  you know what this means then you probably know what you're doing.
+  you know what this means then you probably know what you're doing. In that
+  case, you may also want to use `YABRIDGE_TEMP_PATH` to choose a different
+  directory for yabridge to store its sockets and other temporary files in.
 
 ## Performance tuning
 

src/common/utils.cpp

@@ -32,9 +32,18 @@ using namespace std::literals::string_view_literals;
  */
 constexpr char disable_watchdog_timer_env_var[] = "YABRIDGE_NO_WATCHDOG";
 
+/**
+ * If this environment variable is set, yabridge will store its sockets and
+ * other temporary files here instead of in `$XDG_RUNTIME_DIR` or `/tmp`. This
+ * is only relevant when using some namespacing setup for sandboxing.
+ */
+constexpr char temp_dir_override_env_var[] = "YABRIDGE_TEMP_PATH";
+
 fs::path get_temporary_directory() {
     bp::environment env = boost::this_process::environment();
-    if (!env["XDG_RUNTIME_DIR"].empty()) {
+    if (!env[temp_dir_override_env_var].empty()) {
+        return env[temp_dir_override_env_var].to_string();
+    } else if (!env["XDG_RUNTIME_DIR"].empty()) {
         return env["XDG_RUNTIME_DIR"].to_string();
     } else {
         return fs::temp_directory_path();