config_cache fixes (#3145)

* add simple backup export/import test

this test fails on current master
until the context is recrated.

* avoid config_cache races

adds needed SQL-statements to config_cache locking.

otherwise, another thread may alter the database
eg. between SELECT and the config_cache update -
resulting in the wrong value being written to config_cache.

* also update config_cache on initializing tables

VERSION_CFG is also set later, however,
not doing it here will result in bugs when we change DBVERSION at some point.

as this alters only VERSION_CFG and that is executed sequentially anyway,
race conditions between SQL and config_cache
seems not to be an issue in this case.

* clear config_cache after backup import

import replaces the whole database,
so config_cache needs to be invalidated as well.

we do that before import,
so in case a backup is imported only partly,
the cache does not add additional problems.

* update CHANGELOG

src/imex.rs

@@ -460,6 +460,8 @@ async fn import_backup(
         context.get_dbfile().display()
     );
 
+    context.sql.config_cache.write().await.clear();
+
     let archive = Archive::new(backup_file);
 
     let mut entries = archive.entries()?;
@@ -901,6 +903,54 @@ mod tests {
         }
     }
 
+    #[async_std::test]
+    async fn test_export_and_import_backup() -> Result<()> {
+        let backup_dir = tempfile::tempdir().unwrap();
+
+        let context1 = TestContext::new_alice().await;
+        assert!(context1.is_configured().await?);
+
+        let context2 = TestContext::new().await;
+        assert!(!context2.is_configured().await?);
+        assert!(has_backup(&context2, backup_dir.path().as_ref())
+            .await
+            .is_err());
+
+        // export from context1
+        assert!(imex(
+            &context1,
+            ImexMode::ExportBackup,
+            backup_dir.path().as_ref(),
+            None,
+        )
+        .await
+        .is_ok());
+        let _event = context1
+            .evtracker
+            .get_matching(|evt| matches!(evt, EventType::ImexProgress(1000)))
+            .await;
+
+        // import to context2
+        let backup = has_backup(&context2, backup_dir.path().as_ref()).await?;
+        assert!(
+            imex(&context2, ImexMode::ImportBackup, backup.as_ref(), None)
+                .await
+                .is_ok()
+        );
+        let _event = context2
+            .evtracker
+            .get_matching(|evt| matches!(evt, EventType::ImexProgress(1000)))
+            .await;
+
+        assert!(context2.is_configured().await?);
+        assert_eq!(
+            context2.get_config(Config::Addr).await?,
+            Some("alice@example.org".to_string())
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn test_normalize_setup_code() {
         let norm = normalize_setup_code("123422343234423452346234723482349234");