Add API to get next fresh message

Rust: Context.get_next_fresh_msg()
C: dc_get_next_fresh_msg()
JSON-RPC: get_next_fresh_msg method

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### API-Changes
 - Add Python API to send reactions #3762
+- Add API to get next fresh message
 
 ### Fixes
 - Make sure malformed messsages will never block receiving further messages anymore #3771

deltachat-ffi/deltachat.h

@@ -1278,6 +1278,16 @@ int             dc_estimate_deletion_cnt    (dc_context_t* context, int from_ser
 dc_array_t*     dc_get_fresh_msgs            (dc_context_t* context);
 
 
+/**
+ * Returns the ID of the fresh message of any chat.
+ *
+ * @memberof dc_context_t
+ * @param context The context object as returned from dc_context_new().
+ * @return Message ID of the next fresh message. Returns 0 on error.
+ */
+dc_array_t*     dc_get_next_fresh_msg        (dc_context_t* context);
+
+
 /**
  * Mark all messages in a chat as _noticed_.
  * _Noticed_ messages are no longer _fresh_ and do not count as being unseen

deltachat-ffi/src/lib.rs

@@ -1243,6 +1243,20 @@ pub unsafe extern "C" fn dc_get_fresh_msgs(
     })
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_get_next_fresh_msg(context: *mut dc_context_t) -> u32 {
+    if context.is_null() {
+        eprintln!("ignoring careless call to dc_get_next_fresh_msg()");
+        return 0;
+    }
+    let ctx = &*context;
+
+    block_on(ctx.get_next_fresh_msg())
+        .log_err(ctx, "Failed to get next fresh message")
+        .unwrap_or_default()
+        .to_u32()
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_marknoticed_chat(context: *mut dc_context_t, chat_id: u32) {
     if context.is_null() {

deltachat-jsonrpc/src/api/mod.rs

@@ -372,6 +372,15 @@ impl CommandApi {
             .collect())
     }
 
+    /// Returns the message ID of the oldest fresh message.
+    ///
+    /// If there are no such messages, waits until there is one.
+    async fn get_next_fresh_msg(&self, account_id: u32) -> Result<u32> {
+        let ctx = self.get_context(account_id).await?;
+        let msg_id = ctx.get_next_fresh_msg().await?;
+        Ok(msg_id.to_u32())
+    }
+
     /// Get the number of _fresh_ messages in a chat.
     /// Typically used to implement a badge with a number in the chatlist.
     ///

src/context.rs

@@ -9,7 +9,7 @@ use std::time::{Duration, Instant, SystemTime};
 
 use anyhow::{ensure, Result};
 use async_channel::{self as channel, Receiver, Sender};
-use tokio::sync::{Mutex, RwLock};
+use tokio::sync::{Mutex, Notify, RwLock};
 
 use crate::chat::{get_chat_cnt, ChatId};
 use crate::config::Config;
@@ -205,6 +205,11 @@ pub struct InnerContext {
     pub(crate) translated_stockstrings: StockStrings,
     pub(crate) events: Events,
 
+    /// Notify about fresh messages.
+    ///
+    /// This notification is used only internally for [`InternalContext.get_next_fresh_msg()`].
+    pub(crate) notify_fresh_message: Notify,
+
     pub(crate) scheduler: RwLock<Option<Scheduler>>,
     pub(crate) ratelimit: RwLock<Ratelimit>,
 
@@ -354,6 +359,7 @@ impl Context {
             wrong_pw_warning_mutex: Mutex::new(()),
             translated_stockstrings: stockstrings,
             events,
+            notify_fresh_message: Notify::new(),
             scheduler: RwLock::new(None),
             ratelimit: RwLock::new(Ratelimit::new(Duration::new(60, 0), 6.0)), // Allow to send 6 messages immediately, no more than once every 10 seconds.
             quota: RwLock::new(None),
@@ -451,9 +457,12 @@ impl Context {
         self.emit_event(EventType::MsgsChanged { chat_id, msg_id });
     }
 
-    /// Emits an IncomingMsg event with specified chat and message ids
+    /// Emits an IncomingMsg event with specified chat and message ids.
+    ///
+    /// Notifies `get_next_fresh_msg()` that there might be a new fresh message.
     pub fn emit_incoming_msg(&self, chat_id: ChatId, msg_id: MsgId) {
         self.emit_event(EventType::IncomingMsg { chat_id, msg_id });
+        self.notify_fresh_message.notify_one();
     }
 
     /// Returns a receiver for emitted events.
@@ -751,6 +760,19 @@ impl Context {
         Ok(list)
     }
 
+    /// Returns oldest fresh message in unmuted and unblocked chats.
+    ///
+    /// If there are no such messages, waits until there is one.
+    pub async fn get_next_fresh_msg(&self) -> Result<MsgId> {
+        loop {
+            if let Some(msg_id) = self.get_fresh_msgs().await?.last() {
+                return Ok(*msg_id);
+            } else {
+                self.notify_fresh_message.notified().await;
+            }
+        }
+    }
+
     /// Searches for messages containing the query string.
     ///
     /// If `chat_id` is provided this searches only for messages in this chat, if `chat_id`