Remove the need for a directory for db export

Plus on import use the context directory.  We can actually write there
just fine.

src/imex/transfer.rs

@@ -22,7 +22,7 @@
 //! getter can not connect to an impersonated provider and the provider does not offer the
 //! download to an impersonated getter.
 
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 use anyhow::{anyhow, bail, ensure, format_err, Context as _, Result};
 use async_channel::Receiver;
@@ -76,21 +76,25 @@ impl BackupProvider {
     /// the possible cancellation of the "ongoing process".
     ///
     /// [`Accounts::stop_io`]: crate::accounts::Accounts::stop_io
-    pub async fn prepare(context: &Context, dir: &Path) -> Result<Self> {
-        ensure!(
-            // TODO: Should we worry about path normalisation?
-            dir != context.get_blobdir(),
-            "Temporary database export directory should not be in blobdir"
-        );
+    pub async fn prepare(context: &Context) -> Result<Self> {
         e2ee::ensure_secret_key_exists(context)
             .await
             .context("Private key not available, aborting backup export")?;
 
         // Acquire global "ongoing" mutex.
         let cancel_token = context.alloc_ongoing().await?;
+        let context_dir = context
+            .get_blobdir()
+            .parent()
+            .ok_or(anyhow!("Context dir not found"))?;
+        let dbfile = context_dir.join(DBFILE_BACKUP_NAME);
+        if fs::metadata(&dbfile).await.is_ok() {
+            fs::remove_file(&dbfile).await?;
+            warn!(context, "Previous database export deleted");
+        }
         let res = tokio::select! {
             biased;
-            res = Self::prepare_inner(context, dir) => {
+            res = Self::prepare_inner(context, &dbfile) => {
                 match res {
                     Ok(slf) => Ok(slf),
                     Err(err) => {
@@ -112,6 +116,7 @@ impl BackupProvider {
             context.clone(),
             provider,
             cancel_token,
+            dbfile,
         ));
         Ok(Self { handle, ticket })
     }
@@ -119,11 +124,10 @@ impl BackupProvider {
     /// Creates the provider task.
     ///
     /// Having this as a function makes it easier to cancel it when needed.
-    async fn prepare_inner(context: &Context, dir: &Path) -> Result<(Provider, Ticket)> {
+    async fn prepare_inner(context: &Context, dbfile: &Path) -> Result<(Provider, Ticket)> {
         // Generate the token up front: we also use it to encrypt the database.
         let token = AuthToken::generate();
         context.emit_event(SendProgress::Started.into());
-        let dbfile = dir.join(DBFILE_BACKUP_NAME);
         export_database(context, &dbfile, token.to_string())
             .await
             .context("Database export failed")?;
@@ -131,7 +135,7 @@ impl BackupProvider {
 
         // Now we can be sure IO is not running.
         let mut files = vec![DataSource::with_name(
-            dbfile,
+            dbfile.to_owned(),
             format!("db/{DBFILE_BACKUP_NAME}"),
         )];
         let blobdir = BlobDirContents::new(context).await?;
@@ -165,6 +169,7 @@ impl BackupProvider {
         context: Context,
         mut provider: Provider,
         cancel_token: Receiver<()>,
+        dbfile: PathBuf,
     ) -> Result<()> {
         context.emit_event(SendProgress::ProviderListening.into());
         let mut events = provider.subscribe();
@@ -213,7 +218,9 @@ impl BackupProvider {
                 },
             }
         };
-        // TODO: delete the database?
+        if let Err(err) = fs::remove_file(&dbfile).await {
+            error!(context, "Failed to remove database export: {err:#}");
+        }
         context.emit_event(SendProgress::Completed.into());
         context.free_ongoing().await;
         res
@@ -361,12 +368,16 @@ async fn on_blob(
 ) -> Result<DataStream> {
     ensure!(!name.is_empty(), "Received a nameless blob");
     let path = if name.starts_with("db/") {
-        // We can only safely write to the blobdir.  But the blobdir could have a file named
-        // exactly like our special name.  We solve this by using an uppercase extension
-        // which is forbidden for normal blobs.
-        context
+        let context_dir = context
             .get_blobdir()
-            .join(format!("{DBFILE_BACKUP_NAME}.SPECIAL"))
+            .parent()
+            .ok_or(anyhow!("Context dir not found"))?;
+        let dbfile = context_dir.join(DBFILE_BACKUP_NAME);
+        if fs::metadata(&dbfile).await.is_ok() {
+            fs::remove_file(&dbfile).await?;
+            warn!(context, "Previous database export deleted");
+        }
+        dbfile
     } else {
         ensure!(name.starts_with("blob/"), "malformatted blob name");
         let blobname = name.rsplit('/').next().context("malformatted blob name")?;
@@ -456,8 +467,6 @@ impl From<ReceiveProgress> for EventType {
 mod tests {
     use std::time::Duration;
 
-    use testdir::testdir;
-
     use crate::chat::{get_chat_msgs, send_msg, ChatItem};
     use crate::message::{Message, Viewtype};
     use crate::test_utils::TestContextManager;
@@ -466,7 +475,6 @@ mod tests {
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_send_receive() {
-        let dir = testdir!();
         let mut tcm = TestContextManager::new();
 
         // Create first device.
@@ -479,7 +487,7 @@ mod tests {
         send_msg(&ctx0, self_chat.id, &mut msg).await.unwrap();
 
         // Prepare to transfer backup.
-        let provider = BackupProvider::prepare(&ctx0, &dir).await.unwrap();
+        let provider = BackupProvider::prepare(&ctx0).await.unwrap();
 
         // Set up second device.
         let ctx1 = tcm.unconfigured().await;