Fix build hook error for libstore library users

A library shouldn't require changes to the caller's argument handling,
especially if it doesn't have to, and indeed we don't have to.

This changes the lookup order to prioritize the hardcoded path to nix
if it exists. The static executable still finds itself through /proc
and the like.

8 files changed, 97 insertions, 1 deletions

diff --git a/.gitignore b/.gitignore
index 7ae1071d0..29d9106ae 100644
--- a/.gitignore
+++ b/.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
diff --git a/Makefile b/Makefile
index d6b49473a..c6220482a 100644
--- a/Makefile
+++ b/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 += \
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 32e9a6ea9..d53377239 100644
--- a/src/libstore/globals.cc
+++ b/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()
diff --git a/tests/local.mk b/tests/local.mk
index 9e340e2e2..a37365efe 100644
--- a/tests/local.mk
+++ b/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 ($(BUILD_SHARED_LIBS), 1)
diff --git a/tests/test-libstoreconsumer.sh b/tests/test-libstoreconsumer.sh
new file mode 100644
index 000000000..8a77cf5a1
--- /dev/null
+++ b/tests/test-libstoreconsumer.sh
@@ -0,0 +1,6 @@
+source common.sh
+
+drv="$(nix-instantiate simple.nix)"
+cat "$drv"
+out="$(./test-libstoreconsumer/test-libstoreconsumer "$drv")"
+cat "$out/hello" | grep -F "Hello World!"
diff --git a/tests/test-libstoreconsumer/README.md b/tests/test-libstoreconsumer/README.md
new file mode 100644
index 000000000..ded69850f
--- /dev/null
+++ b/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.
diff --git a/tests/test-libstoreconsumer/local.mk b/tests/test-libstoreconsumer/local.mk
new file mode 100644
index 000000000..cd2d0c7f8
--- /dev/null
+++ b/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)
diff --git a/tests/test-libstoreconsumer/main.cc b/tests/test-libstoreconsumer/main.cc
new file mode 100644
index 000000000..31b6d8ef1
--- /dev/null
+++ b/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;
+    }
+}