Fix build hook error for libstore library users

.gitignore

@@ -89,6 +89,7 @@ perl/Makefile.config
 /tests/ca/config.nix
 /tests/dyn-drv/config.nix
 /tests/repl-result-out
+/tests/test-libstoreconsumer/test-libstoreconsumer
 
 # /tests/lang/
 /tests/lang/*.out

Makefile

@@ -27,6 +27,7 @@ makefiles += \
   src/libstore/tests/local.mk \
   src/libexpr/tests/local.mk \
   tests/local.mk \
+  tests/test-libstoreconsumer/local.mk \
   tests/plugins/local.mk
 else
 makefiles += \

src/libstore/globals.cc

@@ -77,7 +77,30 @@ Settings::Settings()
     allowedImpureHostPrefixes = tokenizeString<StringSet>("/System/Library /usr/lib /dev /bin/sh");
 #endif
 
-    buildHook = getSelfExe().value_or("nix") + " __build-remote";
+    /* Set the build hook location
+
+       For builds we perform a self-invocation, so Nix has to be self-aware.
+       That is, it has to know where it is installed. We don't think it's sentient.
+
+       Normally, nix is installed according to `nixBinDir`, which is set at compile time,
+       but can be overridden. This makes for a great default that works even if this
+       code is linked as a library into some other program whose main is not aware
+       that it might need to be a build remote hook.
+
+       However, it may not have been installed at all. For example, if it's a static build,
+       there's a good chance that it has been moved out of its installation directory.
+       That makes `nixBinDir` useless. Instead, we'll query the OS for the path to the
+       current executable, using `getSelfExe()`.
+
+       As a last resort, we resort to `PATH`. Hopefully we find a `nix` there that's compatible.
+       If you're porting Nix to a new platform, that might be good enough for a while, but
+       you'll want to improve `getSelfExe()` to work on your platform.
+     */
+    std::string nixExePath = nixBinDir + "/nix";
+    if (!pathExists(nixExePath)) {
+        nixExePath = getSelfExe().value_or("nix");
+    }
+    buildHook = nixExePath + " __build-remote";
 }
 
 void loadConfFile()

tests/local.mk

@@ -134,6 +134,7 @@ nix_tests = \
   flakes/show.sh \
   impure-derivations.sh \
   path-from-hash-part.sh \
+  test-libstoreconsumer.sh \
   toString-path.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)
@@ -152,6 +153,7 @@ test-deps += \
   tests/common/vars-and-functions.sh \
   tests/config.nix \
   tests/ca/config.nix \
+  tests/test-libstoreconsumer/test-libstoreconsumer \
   tests/dyn-drv/config.nix
 
 ifeq ($(BUILD_SHARED_LIBS), 1)

tests/test-libstoreconsumer.sh

@@ -0,0 +1,6 @@
+source common.sh
+
+drv="$(nix-instantiate simple.nix)"
+cat "$drv"
+out="$(./test-libstoreconsumer/test-libstoreconsumer "$drv")"
+cat "$out/hello" | grep -F "Hello World!"

tests/test-libstoreconsumer/README.md

@@ -0,0 +1,6 @@
+
+A very simple C++ consumer of the libstore library.
+
+  - Keep it simple. Library consumers expect something simple.
+  - No build hook, or any other reinvocations.
+  - No more global state than necessary.

tests/test-libstoreconsumer/local.mk

@@ -0,0 +1,12 @@
+programs += test-libstoreconsumer
+
+test-libstoreconsumer_DIR := $(d)
+
+test-libstoreconsumer_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+
+test-libstoreconsumer_CXXFLAGS += -I src/libutil -I src/libstore
+
+test-libstoreconsumer_LIBS = libstore libutil
+
+test-libstoreconsumer_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) $(LOWDOWN_LIBS)

tests/test-libstoreconsumer/main.cc

@@ -0,0 +1,45 @@
+#include "globals.hh"
+#include "store-api.hh"
+#include "build-result.hh"
+#include <iostream>
+
+using namespace nix;
+
+int main (int argc, char **argv)
+{
+    try {
+        if (argc != 2) {
+            std::cerr << "Usage: " << argv[0] << " store/path/to/something.drv\n";
+            return 1;
+        }
+
+        std::string drvPath = argv[1];
+
+        initLibStore();
+
+        auto store = nix::openStore();
+
+        // build the derivation
+
+        std::vector<DerivedPath> paths {
+            DerivedPath::Built {
+                .drvPath = store->parseStorePath(drvPath),
+                .outputs = OutputsSpec::Names{"out"}
+            }
+        };
+
+        const auto results = store->buildPathsWithResults(paths, bmNormal, store);
+
+        for (const auto & result : results) {
+            for (const auto & [outputName, realisation] : result.builtOutputs) {
+                std::cout << store->printStorePath(realisation.outPath) << "\n";
+            }
+        }
+
+        return 0;
+
+    } catch (const std::exception & e) {
+        std::cerr << "Error: " << e.what() << "\n";
+        return 1;
+    }
+}