compositor: implement wayland socket handover (#6930)

* compositor: implement wayland socket handover

This commit implements the compositor side of the Wayland socket
handover protocol as described in the [KDE Wiki]. The CLI options are
chosen so that they are compatible with Kwin.

[KDE Wiki]: https://invent.kde.org/plasma/kwin/-/wikis/Restarting

* main: verify that --wayland-fd is a valid file descriptor

* main: fail if only one of --socket and --wayland-fd is passed

5 files changed, 74 insertions, 12 deletions

diff --git a/docs/Hyprland.1 b/docs/Hyprland.1
index f43d2c5d..5ef24fd5 100644
--- a/docs/Hyprland.1
+++ b/docs/Hyprland.1
@@ -32,6 +32,10 @@ Show command usage.
 .TP
 \f[B]-c\f[R], \f[B]--config\f[R]
 Specify config file to use.
+\f[B]--socket\f[R]
+Sets the Wayland socket name (for Wayland socket handover)
+\f[B]--wayland-fd\f[R]
+Sets the Wayland socket file descriptor (for Wayland socket handover)
 .SH BUGS
 .TP
 Submit bug reports and request features online at:
diff --git a/docs/Hyprland.1.rst b/docs/Hyprland.1.rst
index 54126501..c73b4343 100644
--- a/docs/Hyprland.1.rst
+++ b/docs/Hyprland.1.rst
@@ -41,6 +41,12 @@ OPTIONS
 **-c**, **--config**
     Specify config file to use.
 
+**--socket**
+    Sets the Wayland socket name (for Wayland socket handover)
+
+**--wayland-fd**
+    Sets the Wayland socket file descriptor (for Wayland socket handover)
+
 BUGS
 ====
 
diff --git a/src/Compositor.cpp b/src/Compositor.cpp
index d1c51075..7ffccf36 100644
--- a/src/Compositor.cpp
+++ b/src/Compositor.cpp
@@ -27,6 +27,7 @@
 #include <hyprutils/string/String.hpp>
 using namespace Hyprutils::String;
 
+#include <fcntl.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/resource.h>
@@ -533,19 +534,28 @@ void CCompositor::prepareFallbackOutput() {
     wlr_headless_add_output(headless, 1920, 1080);
 }
 
-void CCompositor::startCompositor() {
+void CCompositor::startCompositor(std::string socketName, int socketFd) {
     initAllSignals();
 
-    // get socket, avoid using 0
-    for (int candidate = 1; candidate <= 32; candidate++) {
-        const auto CANDIDATESTR = ("wayland-" + std::to_string(candidate));
-        const auto RETVAL       = wl_display_add_socket(m_sWLDisplay, CANDIDATESTR.c_str());
+    if (!socketName.empty() && socketFd != -1) {
+        fcntl(socketFd, F_SETFD, FD_CLOEXEC);
+        const auto RETVAL = wl_display_add_socket_fd(m_sWLDisplay, socketFd);
         if (RETVAL >= 0) {
-            m_szWLDisplaySocket = CANDIDATESTR;
-            Debug::log(LOG, "wl_display_add_socket for {} succeeded with {}", CANDIDATESTR, RETVAL);
-            break;
-        } else {
-            Debug::log(WARN, "wl_display_add_socket for {} returned {}: skipping candidate {}", CANDIDATESTR, RETVAL, candidate);
+            m_szWLDisplaySocket = socketName;
+            Debug::log(LOG, "wl_display_add_socket_fd for {} succeeded with {}", socketName, RETVAL);
+        } else
+            Debug::log(WARN, "wl_display_add_socket_fd for {} returned {}: skipping", socketName, RETVAL);
+    } else {
+        // get socket, avoid using 0
+        for (int candidate = 1; candidate <= 32; candidate++) {
+            const auto CANDIDATESTR = ("wayland-" + std::to_string(candidate));
+            const auto RETVAL       = wl_display_add_socket(m_sWLDisplay, CANDIDATESTR.c_str());
+            if (RETVAL >= 0) {
+                m_szWLDisplaySocket = CANDIDATESTR;
+                Debug::log(LOG, "wl_display_add_socket for {} succeeded with {}", CANDIDATESTR, RETVAL);
+                break;
+            } else
+                Debug::log(WARN, "wl_display_add_socket for {} returned {}: skipping candidate {}", CANDIDATESTR, RETVAL, candidate);
         }
     }
 
diff --git a/src/Compositor.hpp b/src/Compositor.hpp
index 17db2c8b..4aec323f 100644
--- a/src/Compositor.hpp
+++ b/src/Compositor.hpp
@@ -78,7 +78,7 @@ class CCompositor {
     std::unordered_map<std::string, uint64_t> m_mMonitorIDMap;
 
     void                                      initServer();
-    void                                      startCompositor();
+    void                                      startCompositor(std::string socketName, int socketFd);
     void                                      cleanup();
     void                                      createLockFile();
     void                                      removeLockFile();
diff --git a/src/main.cpp b/src/main.cpp
index 7e6fee02..1ac3ab8b 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -4,6 +4,7 @@
 #include "config/ConfigManager.hpp"
 #include "init/initHelpers.hpp"
 
+#include <fcntl.h>
 #include <iostream>
 #include <iterator>
 #include <vector>
@@ -16,6 +17,8 @@ void help() {
     std::cout << "\nArguments:\n";
     std::cout << "  --help              -h       - Show this message again\n";
     std::cout << "  --config FILE       -c FILE  - Specify config file to use\n";
+    std::cout << "  --socket NAME                - Sets the Wayland socket name (for Wayland socket handover)\n";
+    std::cout << "  --wayland-fd FD              - Sets the Wayland socket fd (for Wayland socket handover)\n";
     std::cout << "  --i-am-really-stupid         - Omits root user privileges check (why would you do that?)\n";
 }
 
@@ -37,6 +40,8 @@ int main(int argc, char** argv) {
 
     // parse some args
     std::string              configPath;
+    std::string              socketName;
+    int                      socketFd   = -1;
     bool                     ignoreSudo = false;
 
     std::vector<std::string> args{argv + 1, argv + argc};
@@ -46,6 +51,36 @@ int main(int argc, char** argv) {
             std::cout << "[ WARNING ] Running Hyprland with superuser privileges might damage your system\n";
 
             ignoreSudo = true;
+        } else if (it->compare("--socket") == 0) {
+            if (std::next(it) == args.end()) {
+                help();
+
+                return 1;
+            }
+
+            socketName = *std::next(it);
+            it++;
+        } else if (it->compare("--wayland-fd") == 0) {
+            if (std::next(it) == args.end()) {
+                help();
+
+                return 1;
+            }
+
+            try {
+                socketFd = std::stoi(std::next(it)->c_str());
+
+                // check if socketFd is a valid file descriptor
+                if (fcntl(socketFd, F_GETFD) == -1)
+                    throw std::exception();
+            } catch (...) {
+                std::cerr << "[ ERROR ] Invalid Wayland FD!\n";
+                help();
+
+                return 1;
+            }
+
+            it++;
         } else if (it->compare("-c") == 0 || it->compare("--config") == 0) {
             if (std::next(it) == args.end()) {
                 help();
@@ -93,6 +128,13 @@ int main(int argc, char** argv) {
         std::cout << "Superuser privileges check is omitted. I hope you know what you're doing.\n";
     }
 
+    if (socketName.empty() ^ (socketFd == -1)) {
+        std::cerr << "[ ERROR ] Hyprland was launched with only one of --socket and --wayland-fd.\n";
+        std::cerr << "          Hint: Pass both --socket and --wayland-fd to perform Wayland socket handover.\n";
+
+        return 1;
+    }
+
     std::cout << "Welcome to Hyprland!\n";
 
     // let's init the compositor.
@@ -113,7 +155,7 @@ int main(int argc, char** argv) {
     Debug::log(LOG, "Hyprland init finished.");
 
     // If all's good to go, start.
-    g_pCompositor->startCompositor();
+    g_pCompositor->startCompositor(socketName, socketFd);
 
     g_pCompositor->m_bIsShuttingDown = true;