Fix: IP Scanner: Error reading the neighbor table ended the scan (R15)

An exception of GetIpNetTable2 (Win32Exception of NeighborTable.GetTable)
was not caught in GetNeighborEntryAsync. ARP / NDP is on by default and
every local host reads the table before its probes, so an IP helper error
aborted the whole scan. The error is logged once per scan and an empty
snapshot is stored (no entry: hosts are only found by their probes, no MAC
address from the table); the table is read again on the next miss.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: BornToBeRoot

Source/NETworkManager.Models/Network/IPScanner.cs

@@ -7,6 +7,7 @@
 using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
+using log4net;
 using NETworkManager.Models.Lookup;
 using NETworkManager.Utilities;
 
@@ -20,6 +21,8 @@ public sealed class IPScanner(IPScannerOptions options)
 {
     #region Variables
 
+    private static readonly ILog Log = LogManager.GetLogger(typeof(IPScanner));
+
     /// <summary>
     ///     Time between the start of two hosts during the ramp-up, see <see cref="StartScheduler" />. With the
     ///     default settings the first 128 hosts are started within ~1.3 s instead of at the same moment (~11 connection
@@ -56,6 +59,9 @@ private sealed record NeighborTableSnapshot(Dictionary<IPAddress, NeighborInfo>
     private readonly SemaphoreSlim _neighborTableSemaphore = new(1, 1);
     private volatile NeighborTableSnapshot _neighborTable = new([], DateTime.MinValue);
 
+    // The neighbor table could not be read (logged once per scan), only changed while holding the semaphore
+    private bool _neighborTableReadFailed;
+
     private readonly SemaphoreSlim _neighborVerificationSemaphore =
         new(MaxConcurrentNeighborVerifications, MaxConcurrentNeighborVerifications);
 
@@ -758,7 +764,27 @@ private async Task<NeighborInfo> GetNeighborEntryAsync(IPAddress ipAddress, Date
             // Timestamp before reading the table, so entries created while reading are covered by the next read
             var timestamp = DateTime.UtcNow;
 
-            _neighborTable = new NeighborTableSnapshot(NeighborTable.GetNeighborMap(), timestamp);
+            Dictionary<IPAddress, NeighborInfo> entries;
+
+            try
+            {
+                entries = NeighborTable.GetNeighborMap();
+            }
+            catch (Exception ex)
+            {
+                // E.g. an error of the IP helper API: no entry instead of ending the scan (hosts are only found by their
+                // probes and get no MAC address from the table), read again on the next miss
+                if (!_neighborTableReadFailed)
+                {
+                    _neighborTableReadFailed = true;
+
+                    Log.Error("GetNeighborEntryAsync - Could not read the neighbor table (ARP / NDP).", ex);
+                }
+
+                entries = [];
+            }
+
+            _neighborTable = new NeighborTableSnapshot(entries, timestamp);
 
             return _neighborTable.Entries.GetValueOrDefault(ipAddress);
         }