add dc_get_last_error() (#2788)

* add dc_get_last_error()

* make clippy happy

* simplify block_on() call

src/context.rs

@@ -76,6 +76,11 @@ pub struct InnerContext {
     pub(crate) id: u32,
 
     creation_time: SystemTime,
+
+    /// The text of the last error logged and emitted as an event.
+    /// If the ui wants to display an error after a failure,
+    /// `last_error` should be used to avoid races with the event thread.
+    pub(crate) last_error: RwLock<String>,
 }
 
 #[derive(Debug)]
@@ -147,6 +152,7 @@ impl Context {
             quota: RwLock::new(None),
             creation_time: std::time::SystemTime::now(),
             last_full_folder_scan: Mutex::new(None),
+            last_error: RwLock::new("".to_string()),
         };
 
         let ctx = Context {

src/log.rs

@@ -1,6 +1,7 @@
 //! # Logging.
 
 use crate::context::Context;
+use async_std::task::block_on;
 
 #[macro_export]
 macro_rules! info {
@@ -39,10 +40,28 @@ macro_rules! error {
     };
     ($ctx:expr, $msg:expr, $($args:expr),* $(,)?) => {{
         let formatted = format!($msg, $($args),*);
+        $ctx.set_last_error(&formatted);
         $ctx.emit_event($crate::EventType::Error(formatted));
     }};
 }
 
+impl Context {
+    /// Set last error string.
+    /// Implemented as blocking as used from macros in different, not always async blocks.
+    pub fn set_last_error(&self, error: &str) {
+        block_on(async move {
+            let mut last_error = self.last_error.write().await;
+            *last_error = error.to_string();
+        });
+    }
+
+    /// Get last error string.
+    pub async fn get_last_error(&self) -> String {
+        let last_error = &*self.last_error.read().await;
+        last_error.clone()
+    }
+}
+
 pub trait LogExt<T, E>
 where
     Self: std::marker::Sized,
@@ -134,3 +153,31 @@ impl<T, E: std::fmt::Display> LogExt<T, E> for Result<T, E> {
         self
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::test_utils::TestContext;
+    use anyhow::Result;
+
+    #[async_std::test]
+    async fn test_get_last_error() -> Result<()> {
+        let t = TestContext::new().await;
+
+        assert_eq!(t.get_last_error().await, "");
+
+        error!(t, "foo-error");
+        assert_eq!(t.get_last_error().await, "foo-error");
+
+        warn!(t, "foo-warning");
+        assert_eq!(t.get_last_error().await, "foo-error");
+
+        info!(t, "foo-info");
+        assert_eq!(t.get_last_error().await, "foo-error");
+
+        error!(t, "bar-error");
+        error!(t, "baz-error");
+        assert_eq!(t.get_last_error().await, "baz-error");
+
+        Ok(())
+    }
+}