Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

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

@@ -0,0 +1,88 @@
+#include "globals.hh"
+#include "hook-instance.hh"
+#include "file-system.hh"
+#include "child.hh"
+
+namespace nix {
+
+HookInstance::HookInstance()
+{
+    debug("starting build hook '%s'", concatStringsSep(" ", settings.buildHook.get()));
+
+    auto buildHookArgs = settings.buildHook.get();
+
+    if (buildHookArgs.empty())
+        throw Error("'build-hook' setting is empty");
+
+    auto buildHook = canonPath(buildHookArgs.front());
+    buildHookArgs.pop_front();
+
+    Strings args;
+    args.push_back(std::string(baseNameOf(buildHook)));
+
+    for (auto & arg : buildHookArgs)
+        args.push_back(arg);
+
+    args.push_back(std::to_string(verbosity));
+
+    /* Create a pipe to get the output of the child. */
+    fromHook.create();
+
+    /* Create the communication pipes. */
+    toHook.create();
+
+    /* Create a pipe to get the output of the builder. */
+    builderOut.create();
+
+    /* Fork the hook. */
+    pid = startProcess([&]() {
+
+        if (dup2(fromHook.writeSide.get(), STDERR_FILENO) == -1)
+            throw SysError("cannot pipe standard error into log file");
+
+        commonChildInit();
+
+        if (chdir("/") == -1) throw SysError("changing into /");
+
+        /* Dup the communication pipes. */
+        if (dup2(toHook.readSide.get(), STDIN_FILENO) == -1)
+            throw SysError("dupping to-hook read side");
+
+        /* Use fd 4 for the builder's stdout/stderr. */
+        if (dup2(builderOut.writeSide.get(), 4) == -1)
+            throw SysError("dupping builder's stdout/stderr");
+
+        /* Hack: pass the read side of that fd to allow build-remote
+           to read SSH error messages. */
+        if (dup2(builderOut.readSide.get(), 5) == -1)
+            throw SysError("dupping builder's stdout/stderr");
+
+        execv(buildHook.c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("executing '%s'", buildHook);
+    });
+
+    pid.setSeparatePG(true);
+    fromHook.writeSide = -1;
+    toHook.readSide = -1;
+
+    sink = FdSink(toHook.writeSide.get());
+    std::map<std::string, Config::SettingInfo> settings;
+    globalConfig.getSettings(settings);
+    for (auto & setting : settings)
+        sink << 1 << setting.first << setting.second.value;
+    sink << 0;
+}
+
+
+HookInstance::~HookInstance()
+{
+    try {
+        toHook.writeSide = -1;
+        if (pid != -1) pid.kill();
+    } catch (...) {
+        ignoreException();
+    }
+}
+
+}