[hahn-kev-bot]

---

AI summary

Summary

FwLite's flash-drive sync will use a JsonSyncable folder, and a drive pulled mid-write leaves a client_<id>.jsonl file ending in a partial line. Before this change, that one fragment made the whole folder unsyncable (JsonException on every read), and the next append was written onto the same line as the fragment, corrupting it too.

 ReadAllCommitsAsync(file)
   for each line
-    deserialize (throws on a truncated last line)
+    if last line and file doesn't end in '\n'
+      deserialize, or skip with a warning if it fails
+    else
+      deserialize (malformed lines mid-file still throw)

 AddRangeFromSync(commits)
   lock client
+    RepairTrailingFragmentAsync(file)
+      bytes after last '\n' parse as a commit → append "\n"
+      otherwise                              → truncate to last '\n'
     read existing ids, append new commits

Dropping the fragment loses nothing: the device that wrote it still has the commit and re-sends it on the next sync, which deduplicates by commit Id. A line counts as terminated only by \n, so \r\n works and a bare trailing \r counts as unterminated.

Evidence

New tests in JsonSyncableTests, run against the old JsonSyncable.cs:

• Before: 5 of 11 fail

  failed ReadingIgnoresTruncatedLastLine
  failed AppendingRemovesTruncatedLastLine(newline: "\n")
  failed AppendingRemovesTruncatedLastLine(newline: "\r\n")
  failed AppendingRemovesFragmentWhenFileHasNoCompleteLines
  failed UnterminatedCompleteLastLineIsKept(trailing: "")
  

  After: 11 of 11 pass. The Syncable backend suite (53 tests) also passes, and dotnet format --verify-no-changes is clean.

MalformedLineInTheMiddleThrows and TerminatedMalformedLastLineThrows pass both before and after; they guard against mid-file corruption being silently skipped.

Merge Danger

Door: two-way

Reverting restores the old behaviour. The only write-side effect is that a trailing fragment is truncated from a client file, and that fragment was unreadable anyway.

Blast Radius: contained

Only JsonSyncable changes. A file ending in a truncated line that is actually the only copy of a commit would lose that commit, but the writer appends commits it already holds, so that shouldn't happen.

Test plan

• dotnet test src/SIL.Harmony.Tests -- --filter-class '*JsonSyncableTests'
• dotnet test src/SIL.Harmony.Tests -- --filter-namespace 'SIL.Harmony.Tests.Syncable'
• CI

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