Replace shared_ptr with mutex-protected strings for thread safety

- Replace std::shared_ptr<std::string> with mutex-protected std::string
- Add atomic flags for fast-path checking to avoid lock contention
- Use relaxed memory ordering for atomic flags as suggested
- Ensures thread-safe access to cache_home and tls_ca_file configuration
- All formatting done with clang-format per project standards

Co-authored-by: bbockelm <1093447+bbockelm@users.noreply.github.com>

Author: Copilot

src/scitokens_internal.cpp

@@ -1131,7 +1131,9 @@ configurer::Configuration::set_cache_home(const std::string dir_path) {
     // If setting to "", then we should treat as though it is unsetting the
     // config
     if (dir_path.length() == 0) { // User is configuring to empty string
-        get_cache_home_ref() = std::make_shared<std::string>(dir_path);
+        std::lock_guard<std::mutex> lock(get_cache_home_mutex());
+        get_cache_home_string() = dir_path;
+        get_cache_home_set().store(false, std::memory_order_relaxed);
         return std::make_pair(true, "");
     }
 
@@ -1154,20 +1156,38 @@ configurer::Configuration::set_cache_home(const std::string dir_path) {
 
     // Now it exists and we can write to it, set the value and let
     // scitokens_cache handle the rest
-    get_cache_home_ref() = std::make_shared<std::string>(cleaned_dir_path);
+    {
+        std::lock_guard<std::mutex> lock(get_cache_home_mutex());
+        get_cache_home_string() = cleaned_dir_path;
+        get_cache_home_set().store(true, std::memory_order_relaxed);
+    }
     return std::make_pair(true, "");
 }
 
 void configurer::Configuration::set_tls_ca_file(const std::string ca_file) {
-    get_tls_ca_file_ref() = std::make_shared<std::string>(ca_file);
+    std::lock_guard<std::mutex> lock(get_tls_ca_file_mutex());
+    get_tls_ca_file_string() = ca_file;
+    get_tls_ca_file_set().store(!ca_file.empty(), std::memory_order_relaxed);
 }
 
 std::string configurer::Configuration::get_cache_home() {
-    return *get_cache_home_ref();
+    // Fast path: check if the value has been set
+    if (!get_cache_home_set().load(std::memory_order_relaxed)) {
+        return "";
+    }
+    // Slow path: acquire lock and read the value
+    std::lock_guard<std::mutex> lock(get_cache_home_mutex());
+    return get_cache_home_string();
 }
 
 std::string configurer::Configuration::get_tls_ca_file() {
-    return *get_tls_ca_file_ref();
+    // Fast path: check if the value has been set
+    if (!get_tls_ca_file_set().load(std::memory_order_relaxed)) {
+        return "";
+    }
+    // Slow path: acquire lock and read the value
+    std::lock_guard<std::mutex> lock(get_tls_ca_file_mutex());
+    return get_tls_ca_file_string();
 }
 
 // bool configurer::Configuration::check_dir(const std::string dir_path) {

src/scitokens_internal.h

@@ -59,14 +59,31 @@ class Configuration {
         static std::atomic_int instance{4 * 24 * 3600};
         return instance;
     }
-    static std::shared_ptr<std::string> &get_cache_home_ref() {
-        static std::shared_ptr<std::string> instance =
-            std::make_shared<std::string>("");
+
+    // Thread-safe accessors for string configurations
+    static std::mutex &get_cache_home_mutex() {
+        static std::mutex instance;
+        return instance;
+    }
+    static std::string &get_cache_home_string() {
+        static std::string instance;
+        return instance;
+    }
+    static std::atomic<bool> &get_cache_home_set() {
+        static std::atomic<bool> instance{false};
+        return instance;
+    }
+
+    static std::mutex &get_tls_ca_file_mutex() {
+        static std::mutex instance;
+        return instance;
+    }
+    static std::string &get_tls_ca_file_string() {
+        static std::string instance;
         return instance;
     }
-    static std::shared_ptr<std::string> &get_tls_ca_file_ref() {
-        static std::shared_ptr<std::string> instance =
-            std::make_shared<std::string>("");
+    static std::atomic<bool> &get_tls_ca_file_set() {
+        static std::atomic<bool> instance{false};
         return instance;
     }