Connect/bind Unix domain sockets in a child process

In the child process, we can do a chdir() and avoid the problem of the
path not fitting into sockaddr_un.

src/libstore/uds-remote-store.cc

@@ -65,16 +65,7 @@ ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
         throw SysError("cannot create Unix domain socket");
     closeOnExec(conn->fd.get());
 
-    string socketPath = path ? *path : settings.nixDaemonSocketFile;
+    nix::connect(conn->fd.get(), path ? *path : settings.nixDaemonSocketFile);
-
-    struct sockaddr_un addr;
-    addr.sun_family = AF_UNIX;
-    if (socketPath.size() + 1 >= sizeof(addr.sun_path))
-        throw Error("socket path '%1%' is too long", socketPath);
-    strcpy(addr.sun_path, socketPath.c_str());
-
-    if (::connect(conn->fd.get(), (struct sockaddr *) &addr, sizeof(addr)) == -1)
-        throw SysError("cannot connect to daemon at '%1%'", socketPath);
 
     conn->from.fd = conn->fd.get();
     conn->to.fd = conn->fd.get();

src/libutil/util.cc

@@ -1686,16 +1686,7 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
 
     closeOnExec(fdSocket.get());
 
-    struct sockaddr_un addr;
+    bind(fdSocket.get(), path);
-    addr.sun_family = AF_UNIX;
-    if (path.size() + 1 >= sizeof(addr.sun_path))
-        throw Error("socket path '%1%' is too long", path);
-    strcpy(addr.sun_path, path.c_str());
-
-    unlink(path.c_str());
-
-    if (bind(fdSocket.get(), (struct sockaddr *) &addr, sizeof(addr)) == -1)
-        throw SysError("cannot bind to socket '%1%'", path);
 
     if (chmod(path.c_str(), mode) == -1)
         throw SysError("changing permissions on '%1%'", path);
@@ -1707,6 +1698,66 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
 }
 
 
+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([&]() {
+            auto 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);
+            strcpy(addr.sun_path, base.c_str());
+            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 {
+        strcpy(addr.sun_path, path.c_str());
+        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([&]() {
+            auto 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);
+            strcpy(addr.sun_path, base.c_str());
+            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 ar '%s'", path);
+    } else {
+        strcpy(addr.sun_path, path.c_str());
+        if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+            throw SysError("cannot connect to socket at '%s'", path);
+    }
+}
+
+
 string showBytes(uint64_t bytes)
 {
     return fmt("%.2f MiB", bytes / (1024.0 * 1024.0));

src/libutil/util.hh

@@ -259,7 +259,7 @@ void killUser(uid_t uid);
    pid to the caller. */
 struct ProcessOptions
 {
-    string errorPrefix = "error: ";
+    string errorPrefix = "";
     bool dieWithParent = true;
     bool runExitHandlers = false;
     bool allowVfork = true;
@@ -574,6 +574,12 @@ void commonChildInit(Pipe & logPipe);
 /* Create a Unix domain socket in listen mode. */
 AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode);
 
+/* Bind a Unix domain socket to a path. */
+void bind(int fd, const std::string & path);
+
+/* Connect to a Unix domain socket. */
+void connect(int fd, const std::string & path);
+
 
 // A Rust/Python-like enumerate() iterator adapter.
 // Borrowed from http://reedbeta.com/blog/python-like-enumerate-in-cpp17.