Split up util.{hh,cc}

All OS and IO operations should be moved out, leaving only some misc
portable pure functions.

This is useful to avoid copious CPP when doing things like Windows and
Emscripten ports.

Newly exposed functions to break cycles:

 - `restoreSignals`
 - `updateWindowSize`

src/libutil/unix-domain-socket.cc

@@ -0,0 +1,100 @@
+#include "file-system.hh"
+#include "processes.hh"
+#include "unix-domain-socket.hh"
+
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+namespace nix {
+
+AutoCloseFD createUnixDomainSocket()
+{
+    AutoCloseFD fdSocket = socket(PF_UNIX, SOCK_STREAM
+        #ifdef SOCK_CLOEXEC
+        | SOCK_CLOEXEC
+        #endif
+        , 0);
+    if (!fdSocket)
+        throw SysError("cannot create Unix domain socket");
+    closeOnExec(fdSocket.get());
+    return fdSocket;
+}
+
+
+AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
+{
+    auto fdSocket = nix::createUnixDomainSocket();
+
+    bind(fdSocket.get(), path);
+
+    if (chmod(path.c_str(), mode) == -1)
+        throw SysError("changing permissions on '%1%'", path);
+
+    if (listen(fdSocket.get(), 100) == -1)
+        throw SysError("cannot listen on socket '%1%'", path);
+
+    return fdSocket;
+}
+
+
+void bind(int fd, const std::string & path)
+{
+    unlink(path.c_str());
+
+    struct sockaddr_un addr;
+    addr.sun_family = AF_UNIX;
+
+    if (path.size() + 1 >= sizeof(addr.sun_path)) {
+        Pid pid = startProcess([&]() {
+            Path dir = dirOf(path);
+            if (chdir(dir.c_str()) == -1)
+                throw SysError("chdir to '%s' failed", dir);
+            std::string base(baseNameOf(path));
+            if (base.size() + 1 >= sizeof(addr.sun_path))
+                throw Error("socket path '%s' is too long", base);
+            memcpy(addr.sun_path, base.c_str(), base.size() + 1);
+            if (bind(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+                throw SysError("cannot bind to socket '%s'", path);
+            _exit(0);
+        });
+        int status = pid.wait();
+        if (status != 0)
+            throw Error("cannot bind to socket '%s'", path);
+    } else {
+        memcpy(addr.sun_path, path.c_str(), path.size() + 1);
+        if (bind(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+            throw SysError("cannot bind to socket '%s'", path);
+    }
+}
+
+
+void connect(int fd, const std::string & path)
+{
+    struct sockaddr_un addr;
+    addr.sun_family = AF_UNIX;
+
+    if (path.size() + 1 >= sizeof(addr.sun_path)) {
+        Pid pid = startProcess([&]() {
+            Path dir = dirOf(path);
+            if (chdir(dir.c_str()) == -1)
+                throw SysError("chdir to '%s' failed", dir);
+            std::string base(baseNameOf(path));
+            if (base.size() + 1 >= sizeof(addr.sun_path))
+                throw Error("socket path '%s' is too long", base);
+            memcpy(addr.sun_path, base.c_str(), base.size() + 1);
+            if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+                throw SysError("cannot connect to socket at '%s'", path);
+            _exit(0);
+        });
+        int status = pid.wait();
+        if (status != 0)
+            throw Error("cannot connect to socket at '%s'", path);
+    } else {
+        memcpy(addr.sun_path, path.c_str(), path.size() + 1);
+        if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+            throw SysError("cannot connect to socket at '%s'", path);
+    }
+}
+
+}