Move NIX_BIN_DIR and all logic using it to the Nix executable itself

This is because with the split packages of the Meson build, we simply
have no idea what directory the binaries will be installed in when we
build the library.

In the process of doing so, consolidate and make more sophisticated the
logic to cope with a few corner cases (e.g. `NIX_BIN_DIR` exists, but no
binaries are inside it).

src/libcmd/repl.cc

@@ -7,7 +7,6 @@
 
 #include "ansicolor.hh"
 #include "shared.hh"
-#include "config-global.hh"
 #include "eval.hh"
 #include "eval-settings.hh"
 #include "attr-path.hh"
@@ -77,10 +76,14 @@ struct NixRepl
     int displ;
     StringSet varNames;
 
+    RunNix * runNixPtr;
+
+    void runNix(Path program, const Strings & args, const std::optional<std::string> & input = {});
+
     std::unique_ptr<ReplInteracter> interacter;
 
     NixRepl(const LookupPath & lookupPath, nix::ref<Store> store,ref<EvalState> state,
-            std::function<AnnotatedValues()> getValues);
+            std::function<AnnotatedValues()> getValues, RunNix * runNix);
     virtual ~NixRepl() = default;
 
     ReplExitStatus mainLoop() override;
@@ -125,32 +128,16 @@ std::string removeWhitespace(std::string s)
 
 
 NixRepl::NixRepl(const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-            std::function<NixRepl::AnnotatedValues()> getValues)
+            std::function<NixRepl::AnnotatedValues()> getValues, RunNix * runNix = nullptr)
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
     , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
+    , runNixPtr{runNix}
     , interacter(make_unique<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history"))
 {
 }
 
-void runNix(Path program, const Strings & args,
-    const std::optional<std::string> & input = {})
-{
-    auto subprocessEnv = getEnv();
-    subprocessEnv["NIX_CONFIG"] = globalConfig.toKeyValue();
-    //isInteractive avoid grabling interactive commands
-    runProgram2(RunOptions {
-        .program = settings.nixBinDir+ "/" + program,
-        .args = args,
-        .environment = subprocessEnv,
-        .input = input,
-        .isInteractive = true,
-    });
-
-    return;
-}
-
 static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positions, const DebugTrace & dt)
 {
     if (dt.isError)
@@ -836,9 +823,18 @@ void NixRepl::evalString(std::string s, Value & v)
 }
 
 
+void NixRepl::runNix(Path program, const Strings & args, const std::optional<std::string> & input)
+{
+    if (runNixPtr)
+        (*runNixPtr)(program, args, input);
+    else
+        throw Error("Cannot run '%s', no method of calling the Nix CLI provided", program);
+}
+
+
 std::unique_ptr<AbstractNixRepl> AbstractNixRepl::create(
    const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-   std::function<AnnotatedValues()> getValues)
+   std::function<AnnotatedValues()> getValues, RunNix * runNix)
 {
     return std::make_unique<NixRepl>(
         lookupPath,

src/libcmd/repl.hh

@@ -19,9 +19,19 @@ struct AbstractNixRepl
 
     typedef std::vector<std::pair<Value*,std::string>> AnnotatedValues;
 
+    using RunNix = void(Path program, const Strings & args, const std::optional<std::string> & input);
+
+    /**
+     * @param runNix Function to run the nix CLI to support various
+     * `:<something>` commands. Can not be provided, in which case
+     * everything else will still work fine, but those commands won't.
+     */
     static std::unique_ptr<AbstractNixRepl> create(
-        const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-        std::function<AnnotatedValues()> getValues);
+        const LookupPath & lookupPath,
+        nix::ref<Store> store,
+        ref<EvalState> state,
+        std::function<AnnotatedValues()> getValues,
+        RunNix * runNix = nullptr);
 
     static ReplExitStatus runSimple(
         ref<EvalState> evalState,

src/libstore/globals.cc

@@ -64,7 +64,6 @@ Settings::Settings()
     , nixStateDir(canonPath(getEnvNonEmpty("NIX_STATE_DIR").value_or(NIX_STATE_DIR)))
     , nixConfDir(canonPath(getEnvNonEmpty("NIX_CONF_DIR").value_or(NIX_CONF_DIR)))
     , nixUserConfFiles(getUserConfigFiles())
-    , nixBinDir(canonPath(getEnvNonEmpty("NIX_BIN_DIR").value_or(NIX_BIN_DIR)))
     , nixManDir(canonPath(NIX_MAN_DIR))
     , nixDaemonSocketFile(canonPath(getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH").value_or(nixStateDir + DEFAULT_SOCKET_PATH)))
 {
@@ -95,34 +94,6 @@ Settings::Settings()
     sandboxPaths = tokenizeString<StringSet>("/System/Library/Frameworks /System/Library/PrivateFrameworks /bin/sh /bin/bash /private/tmp /private/var/tmp /usr/lib");
     allowedImpureHostPrefixes = tokenizeString<StringSet>("/System/Library /usr/lib /dev /bin/sh");
 #endif
-
-    /* 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(AbstractConfig & config)

src/libstore/globals.hh

@@ -84,11 +84,6 @@ public:
      */
     std::vector<Path> nixUserConfFiles;
 
-    /**
-     * The directory where the main programs are stored.
-     */
-    Path nixBinDir;
-
     /**
      * The directory where the man pages are stored.
      */
@@ -246,7 +241,7 @@ public:
         )",
         {"build-timeout"}};
 
-    Setting<Strings> buildHook{this, {}, "build-hook",
+    Setting<Strings> buildHook{this, {"nix", "__build-remote"}, "build-hook",
         R"(
           The path to the helper program that executes remote builds.
 

src/libstore/local.mk

@@ -71,7 +71,6 @@ libstore_CXXFLAGS += \
  -DNIX_STATE_DIR=\"$(NIX_ROOT)$(localstatedir)/nix\" \
  -DNIX_LOG_DIR=\"$(NIX_ROOT)$(localstatedir)/log/nix\" \
  -DNIX_CONF_DIR=\"$(NIX_ROOT)$(sysconfdir)/nix\" \
- -DNIX_BIN_DIR=\"$(NIX_ROOT)$(bindir)\" \
  -DNIX_MAN_DIR=\"$(NIX_ROOT)$(mandir)\" \
  -DLSOF=\"$(NIX_ROOT)$(lsof)\"
 

src/libstore/meson.build

@@ -327,7 +327,6 @@ prefix = get_option('prefix')
 path_opts = [
   # Meson built-ins.
   'datadir',
-  'bindir',
   'mandir',
   'libdir',
   'includedir',
@@ -372,7 +371,6 @@ cpp_str_defines = {
   'NIX_STATE_DIR': state_dir / 'nix',
   'NIX_LOG_DIR':   log_dir,
   'NIX_CONF_DIR':  sysconfdir / 'nix',
-  'NIX_BIN_DIR':   bindir,
   'NIX_MAN_DIR':   mandir,
 }
 

src/libstore/unix/build/hook-instance.cc

@@ -16,11 +16,23 @@ HookInstance::HookInstance()
     if (buildHookArgs.empty())
         throw Error("'build-hook' setting is empty");
 
-    auto buildHook = canonPath(buildHookArgs.front());
+    std::filesystem::path buildHook = buildHookArgs.front();
     buildHookArgs.pop_front();
 
+    if (buildHook.is_absolute()) {
+        buildHook = std::filesystem::weakly_canonical(buildHook);
+    } else {
+        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":")) {
+            auto resolved = dir / buildHook;
+            if (std::filesystem::exists(resolved)) {
+                buildHook = resolved;
+                break;
+            }
+        }
+    }
+
     Strings args;
-    args.push_back(std::string(baseNameOf(buildHook)));
+    args.push_back(buildHook.filename().string());
 
     for (auto & arg : buildHookArgs)
         args.push_back(arg);
@@ -59,7 +71,7 @@ HookInstance::HookInstance()
         if (dup2(builderOut.readSide.get(), 5) == -1)
             throw SysError("dupping builder's stdout/stderr");
 
-        execv(buildHook.c_str(), stringsToCharPtrs(args).data());
+        execv(buildHook.native().c_str(), stringsToCharPtrs(args).data());
 
         throw SysError("executing '%s'", buildHook);
     });

src/nix-channel/nix-channel.cc

@@ -7,6 +7,7 @@
 #include "eval-settings.hh" // for defexpr
 #include "users.hh"
 #include "tarball.hh"
+#include "bin-dir.hh"
 
 #include <fcntl.h>
 #include <regex>
@@ -67,7 +68,7 @@ static void removeChannel(const std::string & name)
     channels.erase(name);
     writeChannels();
 
-    runProgram(settings.nixBinDir + "/nix-env", true, { "--profile", profile, "--uninstall", name });
+    runProgram(getNixBin("nix-env").string(), true, { "--profile", profile, "--uninstall", name });
 }
 
 static Path nixDefExpr;
@@ -118,7 +119,7 @@ static void update(const StringSet & channelNames)
 
             bool unpacked = false;
             if (std::regex_search(filename, std::regex("\\.tar\\.(gz|bz2|xz)$"))) {
-                runProgram(settings.nixBinDir + "/nix-build", false, { "--no-out-link", "--expr", "import " + unpackChannelPath +
+                runProgram(getNixBin("nix-build").string(), false, { "--no-out-link", "--expr", "import " + unpackChannelPath +
                             "{ name = \"" + cname + "\"; channelName = \"" + name + "\"; src = builtins.storePath \"" + filename + "\"; }" });
                 unpacked = true;
             }
@@ -143,7 +144,7 @@ static void update(const StringSet & channelNames)
     for (auto & expr : exprs)
         envArgs.push_back(std::move(expr));
     envArgs.push_back("--quiet");
-    runProgram(settings.nixBinDir + "/nix-env", false, envArgs);
+    runProgram(getNixBin("nix-env").string(), false, envArgs);
 
     // Make the channels appear in nix-env.
     struct stat st;
@@ -244,7 +245,7 @@ static int main_nix_channel(int argc, char ** argv)
             case cListGenerations:
                 if (!args.empty())
                     throw UsageError("'--list-generations' expects no arguments");
-                std::cout << runProgram(settings.nixBinDir + "/nix-env", false, {"--profile", profile, "--list-generations"}) << std::flush;
+                std::cout << runProgram(getNixBin("nix-env").string(), false, {"--profile", profile, "--list-generations"}) << std::flush;
                 break;
             case cRollback:
                 if (args.size() > 1)
@@ -256,7 +257,7 @@ static int main_nix_channel(int argc, char ** argv)
                 } else {
                     envArgs.push_back("--rollback");
                 }
-                runProgram(settings.nixBinDir + "/nix-env", false, envArgs);
+                runProgram(getNixBin("nix-env").string(), false, envArgs);
                 break;
         }
 

src/nix/bin-dir.cc

@@ -0,0 +1,38 @@
+#include "current-process.hh"
+#include "file-system.hh"
+#include "globals.hh"
+#include "bin-dir.hh"
+
+namespace nix {
+
+namespace fs = std::filesystem;
+
+fs::path getNixBin(std::optional<std::string_view> binaryNameOpt)
+{
+    auto getBinaryName = [&] { return binaryNameOpt ? *binaryNameOpt : "nix"; };
+
+    // If the environment variable is set, use it unconditionally
+    if (auto envOpt = getEnvNonEmpty("NIX_BIN_DIR"))
+        return fs::path{*envOpt} / std::string{getBinaryName()};
+
+    // Use some-times avaiable OS tricks to get to the path of this Nix, and try that
+    if (auto selfOpt = getSelfExe()) {
+        fs::path path{*selfOpt};
+        if (binaryNameOpt)
+            path = path.parent_path() / std::string{*binaryNameOpt};
+        if (fs::exists(path))
+            return path;
+    }
+
+    // If `nix` exists at the hardcoded fallback path, use it.
+    {
+        auto path = fs::path{NIX_BIN_DIR} / std::string{getBinaryName()};
+        if (fs::exists(path))
+            return path;
+    }
+
+    // return just the name, hoping the exe is on the `PATH`
+    return getBinaryName();
+}
+
+}

src/nix/bin-dir.hh

@@ -0,0 +1,31 @@
+#pragma once
+///@file
+
+#include <filesystem>
+
+namespace nix {
+
+/**
+ * Get a path to the given Nix binary.
+ *
+ * Normally, nix is installed according to `NIX_BIN_DIR`, which is set
+ * at compile time, but can be overridden.
+ *
+ * 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 `NIX_BIN_DIR` 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.
+ *
+ * @param binary_name the exact binary name we're looking up. Might be
+ * `nix-*` instead of `nix` for the legacy CLI commands. Optional to use
+ * current binary name.
+ */
+std::filesystem::path getNixBin(std::optional<std::string_view> binary_name = {});
+
+}

src/nix/local.mk

@@ -26,6 +26,8 @@ endif
 
 nix_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libexpr) $(INCLUDE_libflake) $(INCLUDE_libmain) -I src/libcmd -I doc/manual $(INCLUDE_nix)
 
+nix_CXXFLAGS += -DNIX_BIN_DIR=\"$(NIX_ROOT)$(bindir)\"
+
 nix_LIBS = libexpr libmain libfetchers libflake libstore libutil libcmd
 
 nix_LDFLAGS = $(THREAD_LDFLAGS) $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) $(LOWDOWN_LIBS)

src/nix/main.cc

@@ -19,6 +19,7 @@
 #include "network-proxy.hh"
 #include "eval-cache.hh"
 #include "flake/flake.hh"
+#include "bin-dir.hh"
 
 #include <sys/types.h>
 #include <regex>
@@ -366,6 +367,17 @@ void mainWrapped(int argc, char * * argv)
     initGC();
     flake::initLib(flakeSettings);
 
+    /* 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.
+     */
+    settings.buildHook.setDefault(Strings {
+        getNixBin({}),
+        "__build-remote",
+    });
+
     #if __linux__
     if (isRootUser()) {
         try {