ersatztv/ErsatzTV.Scanner/Core/Metadata/MediaServerReconciliationGuard.cs

using Microsoft.Extensions.Logging;

namespace ErsatzTV.Scanner.Core.Metadata;

// #477: a media-server library sweep computes "gone upstream" as existing.Except(incoming) and flags the
// result FileNotFound. If a successful fetch returns ZERO items (the server is up but mid-restore /
// mid-rebuild, or the library was emptied upstream) then existing.Except([]) is EVERY existing item, so
// the whole library is flagged FileNotFound in one pass. That is data-loss-adjacent: EmptyTrashHandler
// deletes state:FileNotFound rows permanently, and PlayoutSkipMissingItems empties every affected
// collection. An empty incoming set is indistinguishable at scan time from a transient error (both report
// zero), so the safe policy is to refuse the sweep — logged loudly — rather than nuke the library.
//
// This deliberately overrides the degenerate "last item removed => empty incoming => flag" case that a
// partial-deletion sweep would otherwise handle (see #476's cascade, which still fires for the common
// case where survivors are present and only some items are gone). The cost of not flagging a genuinely
// emptied library (stale rows persist until an item returns or the library is removed) is far smaller
// than a one-scan permanent wipe. Ratio-thresholds and projection-failure detection are deferred — see
// docs/decisions.md and the #477 follow-up.
internal static class MediaServerReconciliationGuard
{
    public static bool ShouldFlagMissing(
        ILogger logger,
        string libraryName,
        int incomingCount,
        int existingCount)
    {
        if (incomingCount == 0 && existingCount > 0)
        {
            logger.LogWarning(
                "Media server library {Library} returned zero items but {ExistingCount} exist locally; "
                + "skipping the file-not-found sweep to avoid flagging the entire library as missing "
                + "(expected if the server is mid-restore or the library was emptied upstream)",
                libraryName,
                existingCount);
            return false;
        }

        return true;
    }
}