fix: reject ambiguous --source/--track, clear index cache on throw

- ingest_knowledge: --track is repeatable but --source writes to exactly one
  index, so taking args.tracks[0] let argument order silently decide the
  destination. That combination is now an argparse error. The --source help
  also claimed it "overrides --track" while --index-name documented the
  default as the track index; rewritten so the precedence (--index-name, else
  the single --track, else the default track) is stated once. Multiple --track
  without --source is unaffected.

- moss-vscode: clearIndexCache() only ran in the else-branch, so a rebuild()
  that threw after deleting the previous documents — from readFileForIndex()
  or addDocs() — jumped to catch and left the on-disk cache describing
  documents that no longer exist. Moved into a finally keyed on
  indexer.isIndexed(), which the throw path cannot skip.

Verified: argparse across 6 invocations (multi-track rejected with exit 2,
single track honoured, --index-name precedence intact, multi-track still fine
without --source); and the try/catch/finally shape across cancelled, thrown
and successful runs — the throw path now clears, the success path still does
not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: samanyugoyal2010

apps/moss-interview-coach/backend/ingest_knowledge.py

@@ -243,7 +243,11 @@ def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
         "--source",
         type=Path,
         default=None,
-        help="JSON file or markdown directory (overrides --track; uses --index-name).",
+        help=(
+            "JSON file or markdown directory to ingest instead of the built-in "
+            "rubrics. Destination index comes from --index-name, else from the "
+            "single --track given, else the default track."
+        ),
     )
     parser.add_argument(
         "--index-name",
@@ -263,7 +267,15 @@ def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
             "with --source, omit to skip the sample query."
         ),
     )
-    return parser.parse_args(argv)
+    args = parser.parse_args(argv)
+    # --track is repeatable, but --source writes to exactly one index. Silently
+    # taking the first would make argument order decide the destination.
+    if args.source is not None and args.tracks and len(args.tracks) > 1:
+        parser.error(
+            "--source ingests into a single index, so pass at most one --track "
+            "(or use --index-name to name the destination explicitly)."
+        )
+    return args
 
 
 def main() -> None:

apps/moss-vscode/src/extension.ts

@@ -501,14 +501,6 @@ async function runCreateIndex(
       vscode.window.showInformationMessage(
         `Moss index ready — ${files} files indexed (saved for next time).`,
       );
-    } else {
-      // rebuild() deletes the previous documents before scanning, so a
-      // cancelled or failed run leaves the on-disk cache describing documents
-      // that no longer exist. persistIndex() refuses to write while unindexed,
-      // so drop the cache instead — otherwise the next launch restores stale
-      // metadata for an index that is gone.
-      await clearIndexCache(context).catch(() => undefined);
-      log("Index not created; cleared stale index cache.");
     }
   } catch (err) {
     const message = err instanceof Error ? err.message : String(err);
@@ -519,6 +511,18 @@ async function runCreateIndex(
       statusBarItem.text = "$(error) Moss: error";
       statusBarItem.tooltip = message;
     }
+  } finally {
+    // rebuild() deletes the previous documents before scanning, so *any* run
+    // that does not end in a ready index leaves the on-disk cache describing
+    // documents that no longer exist — cancelled, or thrown from
+    // readFileForIndex()/addDocs() midway. persistIndex() refuses to write
+    // while unindexed, so drop the cache here instead, in a finally so the
+    // throw path cannot skip it; otherwise the next launch restores stale
+    // metadata for an index that is gone.
+    if (!indexer.isIndexed()) {
+      await clearIndexCache(context).catch(() => undefined);
+      log("Index not ready; cleared stale index cache.");
+    }
   }
 }