Don't scan from stale history before a kept clone fetches

crates/workspace-host/src/alerts.rs

@@ -205,7 +205,8 @@ impl PullScan {
     }
     /// Scans the next commit on the head's first-parent line after `mark`, with any commits a
     /// merge brought, oldest first: where the mark goes, `None` when it is at the head. Walking
-    /// that line keeps the mark an ancestor of what is left, so it always moves forward.
+    /// that line keeps the mark an ancestor of what is left, so it always moves forward. A head
+    /// behind the mark, such as a kept clone's before its first fetch, has nothing new yet.
     fn advance(
         &self,
         check: &str,
@@ -214,6 +215,9 @@ impl PullScan {
     ) -> std::result::Result<Option<String>, String> {
         let failed = |error: anyhow::Error| format!("{error:#}");
         let clone = self.clone.as_path();
+        if publish::is_ancestor(clone, &self.head, mark) {
+            return Ok(None);
+        }
         let start = self.start(mark);
         let range = format!("{start}..{}", self.head);
         let walk = ["rev-list", "--first-parent", "--reverse", &range];
@@ -1050,6 +1054,43 @@ mod tests {
         });
     }
 
+    #[test]
+    fn a_kept_clones_first_scan_waits_for_its_fetch_and_scans_no_old_history() {
+        let (_folder, github) = fixture();
+        let mut mac = Mac::join(&github);
+        let sage = crate::storage::tests::space_named(&mac.host, "Sage");
+        mac.host.remove_space(&sage.id).unwrap();
+        github.commit(&in_folder(NOTE), PHONE);
+        join_sage(&mut mac.host, &github).unwrap();
+        mac.scan();
+        assert_eq!(mac.mark(SENSITIVE_INFO), head(&github), "the mark waits");
+        mac.pull(&github);
+        assert_eq!(mac.alerts(), [], "what it missed is old history");
+        assert_eq!(mac.mark(SENSITIVE_INFO), head(&github));
+    }
+
+    #[test]
+    fn a_rewrite_before_a_kept_clones_first_fetch_scans_what_follows() {
+        let (_folder, github) = fixture();
+        let mut mac = Mac::join(&github);
+        let base = head(&github);
+        let sage = crate::storage::tests::space_named(&mac.host, "Sage");
+        mac.host.remove_space(&sage.id).unwrap();
+        github.commit(&in_folder(NOTE), PHONE);
+        join_sage(&mut mac.host, &github).unwrap();
+        git(&github.bare, &["update-ref", "refs/heads/main", &base]);
+        let rewritten = in_folder("second-brain/notes/Rewritten.md");
+        github.commit(&rewritten, PHONE);
+        mac.pull(&github);
+        let alerts = mac.alerts();
+        assert_eq!(alerts.len(), 1, "{alerts:?}");
+        assert_eq!(
+            alerts[0].path, rewritten,
+            "the dropped commit is not scanned"
+        );
+        assert_eq!(mac.mark(SENSITIVE_INFO), head(&github));
+    }
+
     #[test]
     fn a_rewrite_restarts_at_the_merge_base() {
         let (_folder, github) = fixture();

crates/workspace-host/src/storage.rs

@@ -1639,6 +1639,11 @@ impl Host {
         if !clone.exists() {
             fs::create_dir_all(clone.parent().context("A clone is in a folder")?)?;
             fs::rename(staged.staging.keep(), &clone)?;
+        } else if joined_at.is_some() {
+            // A kept clone takes the head it joined at, where its pull scan starts.
+            let staging = (staged.staging.path().to_str()).context("A staging path is UTF-8")?;
+            let fetch = ["fetch", "--quiet", "--no-tags", "--recurse-submodules=no"];
+            runtime::git_output(&clone, &[&fetch[..], &[staging, "HEAD"]].concat())?;
         }
         let head = runtime::git_output(&clone, &["rev-parse", "HEAD"])?;
         let head = head.trim();