libstore-c: Add C FFI for computing filesystem closures

Adds nix_store_compute_fs_closure to compute the set of all paths reachable
from a given set of store paths through references. This provides the
foundational API needed for closure-based GC operations.

Includes:
- nix_store_path_callback typedef for result iteration
- nix_store_compute_fs_closure function with options for:
  - Reverse dependency computation (flip_direction)
  - Including derivation outputs
  - Including derivers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: domenkozar

src/libstore-c/nix_api_store.cc

@@ -10,6 +10,7 @@
 #include "nix/store/local-fs-store.hh"
 #include "nix/store/indirect-root-store.hh"
 #include "nix/store/local-store.hh"
+#include "nix/store/gc-store.hh"
 
 #include "nix/store/globals.hh"
 
@@ -341,4 +342,51 @@ nix_err nix_store_delete_path(nix_c_context * context, Store * store, const char
     NIXC_CATCH_ERRS
 }
 
+nix_err nix_store_compute_fs_closure(
+    nix_c_context * context,
+    Store * store,
+    StorePath ** paths,
+    size_t num_paths,
+    bool flip_direction,
+    bool include_outputs,
+    bool include_derivers,
+    nix_store_path_callback callback,
+    void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        if (!store)
+            return context ? context->last_err_code = NIX_ERR_KEY : NIX_ERR_KEY;
+
+        if (num_paths == 0 || !paths) {
+            // Empty input, nothing to do
+            return NIX_OK;
+        }
+
+        // Convert StorePath** array to StorePathSet
+        nix::StorePathSet startPaths;
+        for (size_t i = 0; i < num_paths; i++) {
+            if (!paths[i])
+                return context ? context->last_err_code = NIX_ERR_KEY : NIX_ERR_KEY;
+            startPaths.insert(paths[i]->path);
+        }
+
+        // Compute the closure
+        nix::StorePathSet closure;
+        store->ptr->computeFSClosure(startPaths, closure, flip_direction, include_outputs, include_derivers);
+
+        // Invoke callback for each path in the closure
+        if (callback) {
+            for (const auto & path : closure) {
+                StorePath sp{path};
+                callback(&sp, user_data);
+            }
+        }
+
+        return NIX_OK;
+    }
+    NIXC_CATCH_ERRS
+}
+
 } // extern "C"

src/libstore-c/nix_api_store.h

@@ -13,6 +13,7 @@
 
 #include "nix_api_util.h"
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus
@@ -375,6 +376,45 @@ nix_err nix_store_add_indirect_root(nix_c_context * context, Store * store, cons
  */
 nix_err nix_store_delete_path(nix_c_context * context, Store * store, const char * path, uint64_t * bytes_freed);
 
+/**
+ * @brief Callback for iterating over store paths
+ *
+ * Called once for each store path in a result set.
+ *
+ * @param[in] path The store path
+ * @param[in] user_data User-provided data passed to the function
+ */
+typedef void (*nix_store_path_callback)(const StorePath * path, void * user_data);
+
+/**
+ * @brief Compute the filesystem closure of store paths.
+ *
+ * The closure is the set of all paths reachable from the input paths
+ * through references. This function is useful for determining all dependencies
+ * of given paths before performing operations on them.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] store Nix Store reference
+ * @param[in] paths Array of starting store paths (cannot be NULL if num_paths > 0)
+ * @param[in] num_paths Number of paths in the array
+ * @param[in] flip_direction If true, compute reverse dependencies (dependents) instead of forward
+ * @param[in] include_outputs If true, include outputs of derivations
+ * @param[in] include_derivers If true, include derivers of store paths
+ * @param[in] callback Function called for each path in the computed closure
+ * @param[in] user_data Arbitrary data passed to the callback
+ * @return NIX_OK on success, error code on failure
+ */
+nix_err nix_store_compute_fs_closure(
+    nix_c_context * context,
+    Store * store,
+    StorePath ** paths,
+    size_t num_paths,
+    bool flip_direction,
+    bool include_outputs,
+    bool include_derivers,
+    nix_store_path_callback callback,
+    void * user_data);
+
 // cffi end
 #ifdef __cplusplus
 }