Prevent fd leaks to the running shell or command (ghostty-org#5341)

Multiple fixes to prevent file descriptor leaks:

- libxev eventfd now uses CLOEXEC
- linux: cgroup clone now uses CLOEXEC for the cgroup fd
- termio pipe uses pipe2 with CLOEXEC
- pty master always sets CLOEXEC because the child doesn't need it
- termio exec now closes pty slave fd after fork

There still appear to be some fd leaks happening. They seem related to
GTK, they aren't things we're accessig directly. I still want to
investigate them but this at least cleans up the major sources of fd
leakage.

build.zig.zon

@@ -5,8 +5,8 @@
     .dependencies = .{
         // Zig libs
         .libxev = .{
-            .url = "https://github.com/mitchellh/libxev/archive/db6a52bafadf00360e675fefa7926e8e6c0e9931.tar.gz",
-            .hash = "12206029de146b685739f69b10a6f08baee86b3d0a5f9a659fa2b2b66c9602078bbf",
+            .url = "https://github.com/mitchellh/libxev/archive/aceef3d11efacd9d237c91632f930ed13a2834bf.tar.gz",
+            .hash = "12205b2b47fe61a4cde3a45ee4b9cddee75897739dbc196d6396e117cb1ce28e1ad0",
         },
         .mach_glfw = .{
             .url = "https://github.com/mitchellh/mach-glfw/archive/37c2995f31abcf7e8378fba68ddcf4a3faa02de0.tar.gz",

nix/zigCacheHash.nix

@@ -1,3 +1,3 @@
 # This file is auto-generated! check build-support/check-zig-cache-hash.sh for
 # more details.
-"sha256-H6o4Y09ATIylMUWuL9Y1fHwpuxSWyJ3Pl8fn4VeoDZo="
+"sha256-AvfYl8vLxxsRnf/ERpw5jQIro5rVd98q63hwFsgQOvo="

src/os/cgroup.zig

@@ -77,7 +77,22 @@ pub fn cloneInto(cgroup: []const u8) !posix.pid_t {
     // Get a file descriptor that refers to the cgroup directory in the cgroup
     // sysfs to pass to the kernel in clone3.
     const fd: linux.fd_t = fd: {
-        const rc = linux.open(path, linux.O{ .PATH = true, .DIRECTORY = true }, 0);
+        const rc = linux.open(
+            path,
+            .{
+                // Self-explanatory: we expect to open a directory, and
+                // we only need the path-level permissions.
+                .PATH = true,
+                .DIRECTORY = true,
+
+                // We don't want to leak this fd to the child process
+                // when we clone below since we're using this fd for
+                // a cgroup clone.
+                .CLOEXEC = true,
+            },
+            0,
+        );
+
         switch (posix.errno(rc)) {
             .SUCCESS => break :fd @as(linux.fd_t, @intCast(rc)),
             else => |errno| {

src/os/pipe.zig

@@ -3,10 +3,11 @@ const builtin = @import("builtin");
 const windows = @import("windows.zig");
 const posix = std.posix;
 
-/// pipe() that works on Windows and POSIX.
+/// pipe() that works on Windows and POSIX. For POSIX systems, this sets
+/// CLOEXEC on the file descriptors.
 pub fn pipe() ![2]posix.fd_t {
     switch (builtin.os.tag) {
-        else => return try posix.pipe(),
+        else => return try posix.pipe2(.{ .CLOEXEC = true }),
         .windows => {
             var read: windows.HANDLE = undefined;
             var write: windows.HANDLE = undefined;

src/pty.zig

@@ -94,6 +94,9 @@ const PosixPty = struct {
     };
 
     /// The file descriptors for the master and slave side of the pty.
+    /// The slave side is never closed automatically by this struct
+    /// so the caller is responsible for closing it if things
+    /// go wrong.
     master: Fd,
     slave: Fd,
 
@@ -117,6 +120,24 @@ const PosixPty = struct {
             _ = posix.system.close(slave_fd);
         }
 
+        // Set CLOEXEC on the master fd, only the slave fd should be inherited
+        // by the child process (shell/command).
+        cloexec: {
+            const flags = std.posix.fcntl(master_fd, std.posix.F.GETFD, 0) catch |err| {
+                log.warn("error getting flags for master fd err={}", .{err});
+                break :cloexec;
+            };
+
+            _ = std.posix.fcntl(
+                master_fd,
+                std.posix.F.SETFD,
+                flags | std.posix.FD_CLOEXEC,
+            ) catch |err| {
+                log.warn("error setting CLOEXEC on master fd err={}", .{err});
+                break :cloexec;
+            };
+        }
+
         // Enable UTF-8 mode. I think this is on by default on Linux but it
         // is NOT on by default on macOS so we ensure that it is always set.
         var attrs: c.termios = undefined;
@@ -126,15 +147,14 @@ const PosixPty = struct {
         if (c.tcsetattr(master_fd, c.TCSANOW, &attrs) != 0)
             return error.OpenptyFailed;
 
-        return Pty{
+        return .{
             .master = master_fd,
             .slave = slave_fd,
         };
     }
 
     pub fn deinit(self: *Pty) void {
         _ = posix.system.close(self.master);
-        _ = posix.system.close(self.slave);
         self.* = undefined;
     }
 
@@ -201,8 +221,6 @@ const PosixPty = struct {
         // Can close master/slave pair now
         posix.close(self.slave);
         posix.close(self.master);
-
-        // TODO: reset signals
     }
 };
 

src/termio/Exec.zig

@@ -1098,6 +1098,10 @@ const Subprocess = struct {
         });
         self.pty = pty;
         errdefer {
+            if (comptime builtin.os.tag != .windows) {
+                _ = posix.close(pty.slave);
+            }
+
             pty.deinit();
             self.pty = null;
         }
@@ -1182,6 +1186,13 @@ const Subprocess = struct {
             log.info("subcommand cgroup={s}", .{self.linux_cgroup orelse "-"});
         }
 
+        if (comptime builtin.os.tag != .windows) {
+            // Once our subcommand is started we can close the slave
+            // side. This prevents the slave fd from being leaked to
+            // future children.
+            _ = posix.close(pty.slave);
+        }
+
         self.command = cmd;
         return switch (builtin.os.tag) {
             .windows => .{