Enable the unix:// store on Windows

Windows now has some basic Unix Domain Socket support, see
https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/

Building `nix daemon` on Windows I've left for later, because the daemon
currently forks per connection but this is not an option on Windows. But
we can get the client part working right away.

src/libstore/indirect-root-store.cc

@@ -0,0 +1,44 @@
+#include "indirect-root-store.hh"
+
+namespace nix {
+
+void IndirectRootStore::makeSymlink(const Path & link, const Path & target)
+{
+    /* Create directories up to `gcRoot'. */
+    createDirs(dirOf(link));
+
+    /* Create the new symlink. */
+    Path tempLink = fmt("%1%.tmp-%2%-%3%", link, getpid(), rand());
+    createSymlink(target, tempLink);
+
+    /* Atomically replace the old one. */
+    renameFile(tempLink, link);
+}
+
+
+Path IndirectRootStore::addPermRoot(const StorePath & storePath, const Path & _gcRoot)
+{
+    Path gcRoot(canonPath(_gcRoot));
+
+    if (isInStore(gcRoot))
+        throw Error(
+                "creating a garbage collector root (%1%) in the Nix store is forbidden "
+                "(are you running nix-build inside the store?)", gcRoot);
+
+    /* Register this root with the garbage collector, if it's
+       running. This should be superfluous since the caller should
+       have registered this root yet, but let's be on the safe
+       side. */
+    addTempRoot(storePath);
+
+    /* Don't clobber the link if it already exists and doesn't
+       point to the Nix store. */
+    if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
+        throw Error("cannot create symlink '%1%'; already exists", gcRoot);
+    makeSymlink(gcRoot, printStorePath(storePath));
+    addIndirectRoot(gcRoot);
+
+    return gcRoot;
+}
+
+}

src/libstore/indirect-root-store.hh

@@ -67,6 +67,9 @@ struct IndirectRootStore : public virtual LocalFSStore
      * The form this weak-reference takes is implementation-specific.
      */
     virtual void addIndirectRoot(const Path & path) = 0;
+
+protected:
+    void makeSymlink(const Path & link, const Path & target);
 };
 
 }

src/libstore/local.mk

@@ -21,6 +21,9 @@ libstore_LDFLAGS += $(SQLITE3_LIBS) $(LIBCURL_LIBS) $(THREAD_LDFLAGS)
 ifdef HOST_LINUX
   libstore_LDFLAGS += -ldl
 endif
+ifdef HOST_WINDOWS
+  libstore_LDFLAGS += -lws2_32
+endif
 
 $(foreach file,$(libstore_FILES),$(eval $(call install-data-in,$(d)/$(file),$(datadir)/nix/sandbox)))
 

src/libstore/uds-remote-store.cc

@@ -2,16 +2,20 @@
 #include "unix-domain-socket.hh"
 #include "worker-protocol.hh"
 
+#include <cstring>
 #include <sys/types.h>
 #include <sys/stat.h>
-#include <sys/socket.h>
-#include <sys/un.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
 
-#include <cstring>
-
+#ifdef _WIN32
+# include <winsock2.h>
+# include <afunix.h>
+#else
+# include <sys/socket.h>
+# include <sys/un.h>
+#endif
 
 namespace nix {
 
@@ -57,7 +61,7 @@ std::string UDSRemoteStore::getUri()
 
 void UDSRemoteStore::Connection::closeWrite()
 {
-    shutdown(fd.get(), SHUT_WR);
+    shutdown(toSocket(fd.get()), SHUT_WR);
 }
 
 
@@ -68,7 +72,7 @@ ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
     /* Connect to a daemon that does the privileged work for us. */
     conn->fd = createUnixDomainSocket();
 
-    nix::connect(conn->fd.get(), path ? *path : settings.nixDaemonSocketFile);
+    nix::connect(toSocket(conn->fd.get()), path ? *path : settings.nixDaemonSocketFile);
 
     conn->from.fd = conn->fd.get();
     conn->to.fd = conn->fd.get();

src/libstore/unix/gc.cc

@@ -35,20 +35,6 @@ static std::string gcSocketPath = "/gc-socket/socket";
 static std::string gcRootsDir = "gcroots";
 
 
-static void makeSymlink(const Path & link, const Path & target)
-{
-    /* Create directories up to `gcRoot'. */
-    createDirs(dirOf(link));
-
-    /* Create the new symlink. */
-    Path tempLink = fmt("%1%.tmp-%2%-%3%", link, getpid(), rand());
-    createSymlink(target, tempLink);
-
-    /* Atomically replace the old one. */
-    renameFile(tempLink, link);
-}
-
-
 void LocalStore::addIndirectRoot(const Path & path)
 {
     std::string hash = hashString(HashAlgorithm::SHA1, path).to_string(HashFormat::Nix32, false);
@@ -57,32 +43,6 @@ void LocalStore::addIndirectRoot(const Path & path)
 }
 
 
-Path IndirectRootStore::addPermRoot(const StorePath & storePath, const Path & _gcRoot)
-{
-    Path gcRoot(canonPath(_gcRoot));
-
-    if (isInStore(gcRoot))
-        throw Error(
-                "creating a garbage collector root (%1%) in the Nix store is forbidden "
-                "(are you running nix-build inside the store?)", gcRoot);
-
-    /* Register this root with the garbage collector, if it's
-       running. This should be superfluous since the caller should
-       have registered this root yet, but let's be on the safe
-       side. */
-    addTempRoot(storePath);
-
-    /* Don't clobber the link if it already exists and doesn't
-       point to the Nix store. */
-    if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
-        throw Error("cannot create symlink '%1%'; already exists", gcRoot);
-    makeSymlink(gcRoot, printStorePath(storePath));
-    addIndirectRoot(gcRoot);
-
-    return gcRoot;
-}
-
-
 void LocalStore::createTempRootsFile()
 {
     auto fdTempRoots(_fdTempRoots.lock());