libstore-c: Add new derivation and store path functions

Add several new functions to the C API:

StorePath operations:
- nix_store_path_hash: Extract the hash part from a store path
- nix_store_create_from_parts: Construct a store path from hash and name

Derivation operations:
- nix_derivation_clone: Clone a derivation
- nix_derivation_to_json: Serialize a derivation to JSON

Store operations:
- nix_store_drv_from_store_path: Load a derivation from a store path

Test the new functions, and improve documentation of some existing
functions to better distinguish them, also.

Co-authored-by: Tristan Ross <tristan.ross@determinate.systems>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore-c/nix_api_store.cc

@@ -1,3 +1,6 @@
+#include <cstring>
+#include <span>
+
 #include "nix_api_store.h"
 #include "nix_api_store_internal.h"
 #include "nix_api_util.h"
@@ -8,6 +11,7 @@
 #include "nix/store/store-open.hh"
 #include "nix/store/build-result.hh"
 #include "nix/store/local-fs-store.hh"
+#include "nix/util/base-nix-32.hh"
 
 #include "nix/store/globals.hh"
 
@@ -222,6 +226,60 @@ StorePath * nix_store_path_clone(const StorePath * p)
     }
 }
 
+} // extern "C"
+
+template<size_t S>
+static auto to_cpp_array(const uint8_t (&r)[S])
+{
+    return reinterpret_cast<const std::array<std::byte, S> &>(r);
+}
+
+extern "C" {
+
+nix_err
+nix_store_path_hash(nix_c_context * context, const StorePath * store_path, nix_store_path_hash_part * hash_part_out)
+{
+    try {
+        auto hashPart = store_path->path.hashPart();
+        // Decode from Nix32 (base32) encoding to raw bytes
+        auto decoded = nix::BaseNix32::decode(hashPart);
+
+        assert(decoded.size() == sizeof(hash_part_out->bytes));
+        std::memcpy(hash_part_out->bytes, decoded.data(), sizeof(hash_part_out->bytes));
+        return NIX_OK;
+    }
+    NIXC_CATCH_ERRS
+}
+
+StorePath * nix_store_create_from_parts(
+    nix_c_context * context, const nix_store_path_hash_part * hash, const char * name, size_t name_len)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        // Encode the 20 raw bytes to Nix32 (base32) format
+        auto hashStr = nix::BaseNix32::encode(std::span<const std::byte>{to_cpp_array(hash->bytes)});
+
+        // Construct the store path basename: <hash>-<name>
+        std::string baseName;
+        baseName += hashStr;
+        baseName += "-";
+        baseName += std::string_view{name, name_len};
+
+        return new StorePath{nix::StorePath(std::move(baseName))};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_derivation * nix_derivation_clone(const nix_derivation * d)
+{
+    try {
+        return new nix_derivation{d->drv};
+    } catch (...) {
+        return nullptr;
+    }
+}
+
 nix_derivation * nix_derivation_from_json(nix_c_context * context, Store * store, const char * json)
 {
     if (context)
@@ -232,6 +290,20 @@ nix_derivation * nix_derivation_from_json(nix_c_context * context, Store * store
     NIXC_CATCH_ERRS_NULL
 }
 
+nix_err nix_derivation_to_json(
+    nix_c_context * context, const nix_derivation * drv, nix_get_string_callback callback, void * userdata)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto result = static_cast<nlohmann::json>(drv->drv).dump();
+        if (callback) {
+            callback(result.data(), result.size(), userdata);
+        }
+    }
+    NIXC_CATCH_ERRS
+}
+
 StorePath * nix_add_derivation(nix_c_context * context, Store * store, nix_derivation * derivation)
 {
     if (context)
@@ -256,4 +328,14 @@ nix_err nix_store_copy_closure(nix_c_context * context, Store * srcStore, Store
     NIXC_CATCH_ERRS
 }
 
+nix_derivation * nix_store_drv_from_store_path(nix_c_context * context, Store * store, const StorePath * path)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        return new nix_derivation{store->ptr->derivationFromPath(path->path)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
 } // extern "C"

src/libstore-c/nix_api_store.h

@@ -106,7 +106,7 @@ nix_err
 nix_store_get_storedir(nix_c_context * context, Store * store, nix_get_string_callback callback, void * user_data);
 
 /**
- * @brief Parse a Nix store path into a StorePath
+ * @brief Parse a Nix store path that includes the store dir into a StorePath
  *
  * @note Don't forget to free this path using nix_store_path_free()!
  * @param[out] context Optional, stores error information
@@ -188,9 +188,16 @@ nix_store_get_version(nix_c_context * context, Store * store, nix_get_string_cal
 /**
  * @brief Create a `nix_derivation` from a JSON representation of that derivation.
  *
+ * @note Unlike `nix_derivation_to_json`, this needs a `Store`. This is because
+ * over time we expect the internal representation of derivations in Nix to
+ * differ from accepted derivation formats. The store argument is here to help
+ * any logic needed to convert from JSON to the internal representation, in
+ * excess of just parsing.
+ *
  * @param[out] context Optional, stores error information.
  * @param[in] store nix store reference.
  * @param[in] json JSON of the derivation as a string.
+ * @return A new derivation, or NULL on error. Free with `nix_derivation_free` when done using the `nix_derivation`.
  */
 nix_derivation * nix_derivation_from_json(nix_c_context * context, Store * store, const char * json);
 
@@ -242,6 +249,16 @@ nix_err nix_store_get_fs_closure(
     void * userdata,
     void (*callback)(nix_c_context * context, void * userdata, const StorePath * store_path));
 
+/**
+ * @brief Returns the derivation associated with the store path
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] store The nix store
+ * @param[in] path The nix store path
+ * @return A new derivation, or NULL on error. Free with `nix_derivation_free` when done using the `nix_derivation`.
+ */
+nix_derivation * nix_store_drv_from_store_path(nix_c_context * context, Store * store, const StorePath * path);
+
 // cffi end
 #ifdef __cplusplus
 }

src/libstore-c/nix_api_store/derivation.h

@@ -20,6 +20,14 @@ extern "C" {
 /** @brief Nix Derivation */
 typedef struct nix_derivation nix_derivation;
 
+/**
+ * @brief Copy a `nix_derivation`
+ *
+ * @param[in] d the derivation to copy
+ * @return a new `nix_derivation`
+ */
+nix_derivation * nix_derivation_clone(const nix_derivation * d);
+
 /**
  * @brief Deallocate a `nix_derivation`
  *
@@ -28,6 +36,17 @@ typedef struct nix_derivation nix_derivation;
  */
 void nix_derivation_free(nix_derivation * drv);
 
+/**
+ * @brief Gets the derivation as a JSON string
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] drv The derivation
+ * @param[in] callback Called with the JSON string
+ * @param[in] userdata Arbitrary data passed to the callback
+ */
+nix_err nix_derivation_to_json(
+    nix_c_context * context, const nix_derivation * drv, nix_get_string_callback callback, void * userdata);
+
 // cffi end
 #ifdef __cplusplus
 }

src/libstore-c/nix_api_store/store_path.h

@@ -10,6 +10,9 @@
  * @brief Store path operations
  */
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "nix_api_util.h"
 
 #ifdef __cplusplus
@@ -44,6 +47,45 @@ void nix_store_path_free(StorePath * p);
  */
 void nix_store_path_name(const StorePath * store_path, nix_get_string_callback callback, void * user_data);
 
+/**
+ * @brief A store path hash
+ *
+ * Once decoded from "nix32" encoding, a store path hash is 20 raw bytes.
+ */
+typedef struct nix_store_path_hash_part
+{
+    uint8_t bytes[20];
+} nix_store_path_hash_part;
+
+/**
+ * @brief Get the path hash (e.g. "<hash>" in /nix/store/<hash>-<name>)
+ *
+ * The hash is returned as raw bytes, decoded from "nix32" encoding.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] store_path the path to get the hash from
+ * @param[out] hash_part_out the decoded hash as 20 raw bytes
+ * @return NIX_OK on success, error code on failure
+ */
+nix_err
+nix_store_path_hash(nix_c_context * context, const StorePath * store_path, nix_store_path_hash_part * hash_part_out);
+
+/**
+ * @brief Create a StorePath from its constituent parts (hash and name)
+ *
+ * This function constructs a store path from a hash and name, without needing
+ * a Store reference or the store directory prefix.
+ *
+ * @note Don't forget to free this path using nix_store_path_free()!
+ * @param[out] context Optional, stores error information
+ * @param[in] hash The store path hash (20 raw bytes)
+ * @param[in] name The store path name (the part after the hash)
+ * @param[in] name_len Length of the name string
+ * @return owned store path, NULL on error
+ */
+StorePath * nix_store_create_from_parts(
+    nix_c_context * context, const nix_store_path_hash_part * hash, const char name[/*name_len*/], size_t name_len);
+
 // cffi end
 #ifdef __cplusplus
 }