melib/imap: don't examine unloaded mailboxes for updates

In examine_updates() which is periodically executed in the IMAP watch
thread, the mailbox's contents get fetched regardless if the user has
fetched the mailbox before. That means eg a large mailbox that was
unused by the user might perform a large fetch all of a sudden without
the user's knowledge/touch.

Add `warm` property in ImapMailbox that states whether the mailbox has
been loaded before in current execution.

Closes #88 IMAP: don't examine unloaded mailboxes for updates, just for message count stats

melib/src/backends/imap.rs

@@ -348,6 +348,7 @@ impl MailBackend for ImapType {
                 let f = &state.uid_store.mailboxes.lock().await[&mailbox_hash];
                 f.exists.lock().unwrap().clear();
                 f.unseen.lock().unwrap().clear();
+                f.set_warm(true);
                 if f.no_select {
                     yield vec![];
                     return;

melib/src/backends/imap/mailbox.rs

@@ -43,12 +43,31 @@ pub struct ImapMailbox {
     pub permissions: Arc<Mutex<MailboxPermissions>>,
     pub exists: Arc<Mutex<LazyCountSet>>,
     pub unseen: Arc<Mutex<LazyCountSet>>,
+    pub warm: Arc<Mutex<bool>>,
 }
 
 impl ImapMailbox {
     pub fn imap_path(&self) -> &str {
         &self.imap_path
     }
+
+    /// Establish that mailbox contents have been fetched at least once during this execution
+    #[inline(always)]
+    pub fn set_warm(&self, new_value: bool) {
+        *self.warm.lock().unwrap() = new_value;
+    }
+
+    /// Mailbox contents have been fetched at least once during this execution
+    #[inline(always)]
+    pub fn is_warm(&self) -> bool {
+        *self.warm.lock().unwrap()
+    }
+
+    /// Mailbox contents have not been fetched at all during this execution
+    #[inline(always)]
+    pub fn is_cold(&self) -> bool {
+        !self.is_warm()
+    }
 }
 
 impl BackendMailbox for ImapMailbox {

melib/src/backends/imap/watch.rs

@@ -244,6 +244,20 @@ pub async fn examine_updates(
                 uidvalidities.insert(mailbox_hash, select_response.uidvalidity);
             }
         }
+        if mailbox.is_cold() {
+            /* Mailbox hasn't been loaded yet */
+            if let Ok(mut exists_lck) = mailbox.exists.lock() {
+                exists_lck.clear();
+                exists_lck.set_not_yet_seen(select_response.exists);
+            }
+            if let Ok(mut unseen_lck) = mailbox.unseen.lock() {
+                unseen_lck.clear();
+                unseen_lck.set_not_yet_seen(select_response.unseen);
+            }
+            mailbox.set_warm(true);
+            return Ok(());
+        }
+
         if debug!(select_response.recent > 0) {
             /* UID SEARCH RECENT */
             conn.send_command(b"UID SEARCH RECENT").await?;