Add a mock backend for testing (including on other compositors)

Should help determine which issues are cosmic-comp bugs.

src/utils.rs

@@ -0,0 +1,51 @@
+use rustix::{io::Errno, shm::ShmOFlags};
+use std::{
+    os::fd::OwnedFd,
+    time::{SystemTime, UNIX_EPOCH},
+};
+
+#[cfg(target_os = "linux")]
+fn create_memfd() -> rustix::io::Result<OwnedFd> {
+    let fd = rustix::io::retry_on_intr(|| {
+        rustix::fs::memfd_create(
+            "cosmic-workspaces-shm",
+            rustix::fs::MemfdFlags::CLOEXEC | rustix::fs::MemfdFlags::ALLOW_SEALING,
+        )
+    })?;
+    let _ = rustix::fs::fcntl_add_seals(
+        &fd,
+        rustix::fs::SealFlags::SHRINK | rustix::fs::SealFlags::SEAL,
+    );
+    Ok(fd)
+}
+
+pub fn create_memfile() -> rustix::io::Result<OwnedFd> {
+    #[cfg(target_os = "linux")]
+    if let Ok(fd) = create_memfd() {
+        return Ok(fd);
+    }
+
+    loop {
+        let flags = ShmOFlags::CREATE | ShmOFlags::EXCL | ShmOFlags::RDWR;
+
+        let time = SystemTime::now();
+        let name = format!(
+            "/cosmic-workspaces-shm-{}",
+            time.duration_since(UNIX_EPOCH).unwrap().subsec_nanos()
+        );
+
+        match rustix::shm::shm_open(&name, flags, 0600.into()) {
+            Ok(fd) => match rustix::shm::shm_unlink(&name) {
+                Ok(_) => return Ok(fd),
+                Err(errno) => {
+                    return Err(errno.into());
+                }
+            },
+            #[allow(unreachable_patterns)]
+            Err(Errno::EXIST | Errno::EXIST) => {
+                continue;
+            }
+            Err(err) => return Err(err.into()),
+        }
+    }
+}