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 <[email protected]>
Co-authored-by: Robert Hensing <[email protected]>

Author: 3 people

src/libstore-c/nix_api_store.cc

@@ -10,6 +10,10 @@
 #include "nix/store/local-fs-store.hh"
 
 #include "nix/store/globals.hh"
+#include "nix/util/base-nix-32.hh"
+
+#include <cstring>
+#include <span>
 
 extern "C" {
 
@@ -218,6 +222,47 @@ StorePath * nix_store_path_clone(const StorePath * p)
     return new StorePath{p->path};
 }
 
+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() == 20);
+        std::memcpy(hash_part_out->bytes, decoded.data(), 20);
+        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>(reinterpret_cast<const std::byte *>(hash->bytes), 20));
+
+        // 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)
+{
+    return new nix_derivation{d->drv};
+}
+
 nix_derivation * nix_derivation_from_json(nix_c_context * context, Store * store, const char * json)
 {
     if (context)
@@ -228,6 +273,20 @@ 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)
@@ -252,4 +311,14 @@ 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
@@ -188,6 +188,12 @@ 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.
@@ -242,6 +248,16 @@ 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
+ */
+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
 }

src/libstore-tests/nix_api_store.cc

@@ -1,5 +1,7 @@
 #include <fstream>
 
+#include <nlohmann/json.hpp>
+
 #include "nix_api_util.h"
 #include "nix_api_store.h"
 
@@ -92,6 +94,69 @@ TEST_F(nix_api_store_test, DoesNotCrashWhenContextIsNull)
     nix_store_path_free(path);
 }
 
+TEST_F(nix_api_store_test, nix_store_path_hash)
+{
+    StorePath * path = nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
+    ASSERT_NE(path, nullptr);
+
+    nix_store_path_hash_part hash;
+    auto ret = nix_store_path_hash(ctx, path, &hash);
+    assert_ctx_ok();
+    ASSERT_EQ(ret, NIX_OK);
+
+    // Verify it's 20 bytes
+    static_assert(sizeof(hash.bytes) == 20);
+
+    // The hash should be non-zero
+    bool allZero = true;
+    for (int i = 0; i < 20; i++) {
+        if (hash.bytes[i] != 0) {
+            allZero = false;
+            break;
+        }
+    }
+    ASSERT_FALSE(allZero);
+
+    nix_store_path_free(path);
+}
+
+TEST_F(nix_api_store_test, nix_store_create_from_parts_roundtrip)
+{
+    // Parse a path
+    StorePath * original = nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
+    EXPECT_NE(original, nullptr);
+
+    // Get its hash
+    nix_store_path_hash_part hash;
+    auto ret = nix_store_path_hash(ctx, original, &hash);
+    assert_ctx_ok();
+    ASSERT_EQ(ret, NIX_OK);
+
+    // Get its name
+    std::string name;
+    nix_store_path_name(original, OBSERVE_STRING(name));
+
+    // Reconstruct from parts
+    StorePath * reconstructed = nix_store_create_from_parts(ctx, &hash, name.c_str(), name.size());
+    assert_ctx_ok();
+    ASSERT_NE(reconstructed, nullptr);
+
+    // Should be equal
+    EXPECT_EQ(original->path, reconstructed->path);
+
+    nix_store_path_free(original);
+    nix_store_path_free(reconstructed);
+}
+
+TEST_F(nix_api_store_test, nix_store_create_from_parts_invalid_name)
+{
+    nix_store_path_hash_part hash = {};
+    // Invalid name with spaces
+    StorePath * path = nix_store_create_from_parts(ctx, &hash, "invalid name", 12);
+    ASSERT_EQ(path, nullptr);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+}
+
 TEST_F(nix_api_store_test, get_version)
 {
     std::string str;
@@ -795,4 +860,97 @@ TEST_F(NixApiStoreTestWithRealisedPath, nix_store_get_fs_closure_error_propagati
     ASSERT_EQ(call_count, 1); // Should have been called exactly once, then aborted
 }
 
+/**
+ * @brief Helper function to load JSON from a test data file
+ *
+ * @param filename Relative path from _NIX_TEST_UNIT_DATA
+ * @return JSON string contents of the file
+ */
+static std::string load_json_from_test_data(const char * filename)
+{
+    std::filesystem::path unitTestData{getenv("_NIX_TEST_UNIT_DATA")};
+    std::ifstream t{unitTestData / filename};
+    std::stringstream buffer;
+    buffer << t.rdbuf();
+    return buffer.str();
+}
+
+TEST_F(nix_api_store_test, nix_derivation_to_json_roundtrip)
+{
+    // Load JSON from test data
+    auto originalJson = load_json_from_test_data("derivation/invariants/filled-in-deferred-empty-env-var-pre.json");
+
+    // Parse to derivation
+    auto * drv = nix_derivation_from_json(ctx, store, originalJson.c_str());
+    assert_ctx_ok();
+    ASSERT_NE(drv, nullptr);
+
+    // Convert back to JSON
+    std::string convertedJson;
+    auto ret = nix_derivation_to_json(ctx, drv, OBSERVE_STRING(convertedJson));
+    assert_ctx_ok();
+    ASSERT_EQ(ret, NIX_OK);
+    ASSERT_FALSE(convertedJson.empty());
+
+    // Parse both JSON strings to compare (ignoring whitespace differences)
+    auto originalParsed = nlohmann::json::parse(originalJson);
+    auto convertedParsed = nlohmann::json::parse(convertedJson);
+
+    // Remove parts that will be different due to filling-in.
+    originalParsed.at("outputs").erase("out");
+    originalParsed.at("env").erase("out");
+    convertedParsed.at("outputs").erase("out");
+    convertedParsed.at("env").erase("out");
+
+    // They should be equivalent
+    ASSERT_EQ(originalParsed, convertedParsed);
+
+    nix_derivation_free(drv);
+}
+
+TEST_F(nix_api_store_test, nix_derivation_store_round_trip)
+{
+    // Load a derivation from JSON
+    auto json = load_json_from_test_data("derivation/invariants/filled-in-deferred-empty-env-var-pre.json");
+    auto * drv = nix_derivation_from_json(ctx, store, json.c_str());
+    assert_ctx_ok();
+    ASSERT_NE(drv, nullptr);
+
+    // Add to store
+    auto * drvPath = nix_add_derivation(ctx, store, drv);
+    assert_ctx_ok();
+    ASSERT_NE(drvPath, nullptr);
+
+    // Retrieve from store
+    auto * drv2 = nix_store_drv_from_store_path(ctx, store, drvPath);
+    assert_ctx_ok();
+    ASSERT_NE(drv2, nullptr);
+
+    // The round trip should make the same derivation
+    ASSERT_EQ(drv->drv, drv2->drv);
+
+    nix_store_path_free(drvPath);
+    nix_derivation_free(drv);
+    nix_derivation_free(drv2);
+}
+
+TEST_F(nix_api_store_test, nix_derivation_clone)
+{
+    // Load a derivation from JSON
+    auto json = load_json_from_test_data("derivation/invariants/filled-in-deferred-empty-env-var-pre.json");
+    auto * drv = nix_derivation_from_json(ctx, store, json.c_str());
+    assert_ctx_ok();
+    ASSERT_NE(drv, nullptr);
+
+    // Clone the derivation
+    auto * drv2 = nix_derivation_clone(drv);
+    ASSERT_NE(drv2, nullptr);
+
+    // The clone should be equal
+    ASSERT_EQ(drv->drv, drv2->drv);
+
+    nix_derivation_free(drv);
+    nix_derivation_free(drv2);
+}
+
 } // namespace nixC