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).

doc/manual/rl-next/build-hook-default.md

@@ -0,0 +1,16 @@
+---
+synopsis: The "build-hook" setting" default is less useful when using `libnixstore` as a library
+prs:
+- 11178
+---
+
+*This is an obscure issue just affecting uses of `libnixstore` the library outside of the Nix executable.*
+
+As part the ongoing [rewrite of the build system](https://github.com/NixOS/nix/issues/2503) to use [Meson](https://mesonbuild.com/), we are also switching to packaging individual Nix components separately (and building them in separate derivations).
+This means that when building `libnixstore` we do not know where the Nix binaries will be installed --- `libnixstore` doesn't know about downstream consumers like the Nix binaries at all.
+This has a small adverse affect on remote building --- the `build-remote` executable that is specified from the `build-hook` setting will not be gotten from the (presumed) installation location, but instead looked up on the `PATH`.
+This means that other applications linking `libnixstore` that wish to use remote building must arrange for the `nix` command to be on the PATH (or manually overriding the setting) in order for that to work.
+
+Long term we don't envision this being a downside, because we plan to [get rid of `build-remote` and the build hook setting entirely](https://github.com/NixOS/nix/issues/1221).
+There is simply no need to add a second layer of remote-procedure-calling when we want to connect to a remote builder.
+When the build hook protocol did in principle support custom ways of remote building, that can also be accomplished with a customer service for the ssh or daemon/ssh-ng protocols, or with a custom store class.

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)
@@ -833,9 +820,18 @@ 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

@@ -328,7 +328,6 @@ prefix = get_option('prefix')
 path_opts = [
   # Meson built-ins.
   'datadir',
-  'bindir',
   'mandir',
   'libdir',
   'includedir',
@@ -373,7 +372,6 @@ 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

@@ -4,6 +4,7 @@
 #include "file-system.hh"
 #include "child.hh"
 #include "strings.hh"
+#include "executable-path.hh"
 
 namespace nix {
 
@@ -16,11 +17,18 @@ 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();
 
+    try {
+        buildHook = ExecutablePath::load().findPath(buildHook);
+    } catch (ExecutableLookupError & e) {
+        e.addTrace(nullptr, "while resolving the 'build-hook' setting'");
+        throw;
+    }
+
     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 +67,7 @@ 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/libutil/executable-path.cc

@@ -59,7 +59,7 @@ OsString ExecutablePath::render() const
 }
 
 std::optional<fs::path>
-ExecutablePath::find(const OsString & exe, std::function<bool(const fs::path &)> isExecutable) const
+ExecutablePath::findName(const OsString & exe, std::function<bool(const fs::path &)> isExecutable) const
 {
     // "If the pathname being sought contains a <slash>, the search
     // through the path prefixes shall not be performed."
@@ -75,4 +75,20 @@ ExecutablePath::find(const OsString & exe, std::function<bool(const fs::path &)>
     return std::nullopt;
 }
 
+fs::path ExecutablePath::findPath(const fs::path & exe, std::function<bool(const fs::path &)> isExecutable) const
+{
+    // "If the pathname being sought contains a <slash>, the search
+    // through the path prefixes shall not be performed."
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+    if (exe.filename() == exe) {
+        auto resOpt = findName(exe);
+        if (resOpt)
+            return *resOpt;
+        else
+            throw ExecutableLookupError("Could not find executable '%s'", exe.native());
+    } else {
+        return exe;
+    }
+}
+
 } // namespace nix

src/libutil/executable-path.hh

@@ -5,6 +5,8 @@
 
 namespace nix {
 
+MakeError(ExecutableLookupError, Error);
+
 struct ExecutablePath
 {
     std::vector<std::filesystem::path> directories;
@@ -54,10 +56,22 @@ struct ExecutablePath
      *
      * @return path to a resolved executable
      */
-    std::optional<std::filesystem::path> find(
+    std::optional<std::filesystem::path> findName(
         const OsString & exe,
         std::function<bool(const std::filesystem::path &)> isExecutable = isExecutableAmbient) const;
 
+    /**
+     * Like the `findName` but takes a path, not just an executable
+     * name.
+     *
+     * This implements the full POSIX spec: if the path is just a name,
+     * it searches like the above. Otherwise, it returns the path as is.
+     * If (in the name case) the search fails, an exception is thrown.
+     */
+    std::filesystem::path findPath(
+        const std::filesystem::path & exe,
+        std::function<bool(const std::filesystem::path &)> isExecutable = isExecutableAmbient) const;
+
     bool operator==(const ExecutablePath &) const = default;
 };
 

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();
+}
+
+}