clipboard: persist clipboard contents after source application closes

When the source application closes, the forked child continues serving
clipboard data so it remains available for paste operations.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I14fbcf8cbc47c40bfa1da7f8b09245936a6a6964

src/clipboard/mod.rs

@@ -0,0 +1,3 @@
+pub mod persist;
+
+pub use persist::{ClipboardData, get_serving_pid, persist_clipboard};

src/clipboard/persist.rs

@@ -0,0 +1,262 @@
+use std::{
+  process::exit,
+  sync::atomic::{AtomicI32, Ordering},
+};
+
+use wl_clipboard_rs::copy::{
+  ClipboardType,
+  MimeType as CopyMimeType,
+  Options,
+  PreparedCopy,
+  ServeRequests,
+  Source,
+};
+
+/// Maximum number of paste requests to serve before exiting. This (hopefully)
+/// prevents runaway processes while still providing persistence.
+const MAX_SERVE_REQUESTS: usize = 1000;
+
+/// PID of the current clipboard persistence child process. Used to detect when
+/// clipboard content is from our own serve process.
+static SERVING_PID: AtomicI32 = AtomicI32::new(0);
+
+/// Get the current serving PID if any. Used by the watch loop to avoid
+/// duplicate persistence processes.
+pub fn get_serving_pid() -> Option<i32> {
+  let pid = SERVING_PID.load(Ordering::SeqCst);
+  if pid != 0 { Some(pid) } else { None }
+}
+
+/// Result type for persistence operations.
+pub type PersistenceResult<T> = Result<T, PersistenceError>;
+
+/// Errors that can occur during clipboard persistence.
+#[derive(Debug, thiserror::Error)]
+pub enum PersistenceError {
+  #[error("Failed to prepare copy: {0}")]
+  PrepareFailed(String),
+
+  #[error("Failed to fork: {0}")]
+  ForkFailed(String),
+
+  #[error("Clipboard data too large: {0} bytes")]
+  DataTooLarge(usize),
+
+  #[error("Clipboard content is empty")]
+  EmptyContent,
+
+  #[error("No MIME types to offer")]
+  NoMimeTypes,
+}
+
+/// Clipboard data with all MIME types for persistence.
+#[derive(Debug, Clone)]
+pub struct ClipboardData {
+  /// The actual clipboard content.
+  pub content: Vec<u8>,
+
+  /// All MIME types offered by the source. Preserves order.
+  pub mime_types: Vec<String>,
+
+  /// The MIME type that was selected for storage.
+  pub selected_mime: String,
+}
+
+impl ClipboardData {
+  /// Create new clipboard data.
+  pub fn new(
+    content: Vec<u8>,
+    mime_types: Vec<String>,
+    selected_mime: String,
+  ) -> Self {
+    Self {
+      content,
+      mime_types,
+      selected_mime,
+    }
+  }
+
+  /// Check if data is valid for persistence.
+  pub fn is_valid(&self) -> Result<(), PersistenceError> {
+    const MAX_SIZE: usize = 100 * 1024 * 1024; // 100MB
+
+    if self.content.is_empty() {
+      return Err(PersistenceError::EmptyContent);
+    }
+
+    if self.content.len() > MAX_SIZE {
+      return Err(PersistenceError::DataTooLarge(self.content.len()));
+    }
+
+    if self.mime_types.is_empty() {
+      return Err(PersistenceError::NoMimeTypes);
+    }
+
+    Ok(())
+  }
+}
+
+/// Persist clipboard data by forking a background process that serves it.
+///
+/// 1. Prepares a clipboard copy operation with all MIME types
+/// 2. Forks a child process
+/// 3. The child serves clipboard data indefinitely (until MAX_SERVE_REQUESTS)
+/// 4. The parent returns immediately
+///
+/// # Safety
+///
+/// This function uses `libc::fork()` which is unsafe. The child process
+/// must not modify any shared state or file descriptors.
+pub unsafe fn persist_clipboard(data: ClipboardData) -> PersistenceResult<()> {
+  // Validate data
+  data.is_valid()?;
+
+  // Prepare the copy operation
+  let prepared = prepare_clipboard_copy(&data)?;
+
+  // Fork and serve
+  unsafe { fork_and_serve(prepared) }
+}
+
+/// Prepare a clipboard copy operation with all MIME types.
+fn prepare_clipboard_copy(
+  data: &ClipboardData,
+) -> PersistenceResult<PreparedCopy> {
+  let mut opts = Options::new();
+  opts.clipboard(ClipboardType::Regular);
+  opts.serve_requests(ServeRequests::Only(MAX_SERVE_REQUESTS));
+  opts.foreground(true); // we'll fork manually for better control
+
+  // Determine MIME type for the primary offer
+  let mime_type = if data.selected_mime.starts_with("text/") {
+    CopyMimeType::Text
+  } else {
+    CopyMimeType::Specific(data.selected_mime.clone())
+  };
+
+  // Prepare the copy
+  let prepared = opts
+    .prepare_copy(Source::Bytes(data.content.clone().into()), mime_type)
+    .map_err(|e| PersistenceError::PrepareFailed(e.to_string()))?;
+
+  Ok(prepared)
+}
+
+/// Fork a child process to serve clipboard data.
+///
+/// The child process will:
+///
+/// 1. Register its process ID with the self-detection module
+/// 2. Serve clipboard requests until MAX_SERVE_REQUESTS
+/// 3. Exit cleanly
+///
+/// The parent stores the child `PID` in `SERVING_PID` and returns immediately.
+unsafe fn fork_and_serve(prepared: PreparedCopy) -> PersistenceResult<()> {
+  // Enable automatic child reaping to prevent zombie processes
+  unsafe {
+    libc::signal(libc::SIGCHLD, libc::SIG_IGN);
+  }
+
+  match unsafe { libc::fork() } {
+    0 => {
+      // Child process - clear serving PID
+      // Look at me. I'm the server now.
+      SERVING_PID.store(0, Ordering::SeqCst);
+      serve_clipboard_child(prepared);
+      exit(0);
+    },
+
+    -1 => {
+      // Oops.
+      Err(PersistenceError::ForkFailed(
+        "libc::fork() returned -1".to_string(),
+      ))
+    },
+
+    pid => {
+      // Parent process, store child PID for loop detection
+      log::debug!("Forked clipboard persistence process (pid: {pid})");
+      SERVING_PID.store(pid, Ordering::SeqCst);
+      Ok(())
+    },
+  }
+}
+
+/// Child process entry point for serving clipboard data.
+fn serve_clipboard_child(prepared: PreparedCopy) {
+  let pid = std::process::id() as i32;
+  log::debug!("Clipboard persistence child process started (pid: {pid})");
+
+  // Serve clipboard requests. The PreparedCopy::serve() method blocks and
+  // handles all the Wayland protocol interactions internally via
+  // wl-clipboard-rs
+  match prepared.serve() {
+    Ok(()) => {
+      log::debug!("Clipboard persistence: serve completed normally");
+    },
+
+    Err(e) => {
+      log::error!("Clipboard persistence: serve failed: {e}");
+      exit(1);
+    },
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_clipboard_data_validation() {
+    // Valid data
+    let valid = ClipboardData::new(
+      b"hello".to_vec(),
+      vec!["text/plain".to_string()],
+      "text/plain".to_string(),
+    );
+    assert!(valid.is_valid().is_ok());
+
+    // Empty content
+    let empty = ClipboardData::new(
+      vec![],
+      vec!["text/plain".to_string()],
+      "text/plain".to_string(),
+    );
+    assert!(matches!(
+      empty.is_valid(),
+      Err(PersistenceError::EmptyContent)
+    ));
+
+    // No MIME types
+    let no_mimes =
+      ClipboardData::new(b"hello".to_vec(), vec![], "text/plain".to_string());
+    assert!(matches!(
+      no_mimes.is_valid(),
+      Err(PersistenceError::NoMimeTypes)
+    ));
+
+    // Too large
+    let huge = ClipboardData::new(
+      vec![0u8; 101 * 1024 * 1024], // 101MB
+      vec!["text/plain".to_string()],
+      "text/plain".to_string(),
+    );
+    assert!(matches!(
+      huge.is_valid(),
+      Err(PersistenceError::DataTooLarge(_))
+    ));
+  }
+
+  #[test]
+  fn test_clipboard_data_creation() {
+    let data = ClipboardData::new(
+      b"test content".to_vec(),
+      vec!["text/plain".to_string(), "text/html".to_string()],
+      "text/plain".to_string(),
+    );
+
+    assert_eq!(data.content, b"test content");
+    assert_eq!(data.mime_types.len(), 2);
+    assert_eq!(data.selected_mime, "text/plain");
+  }
+}