feat(auditlog): add AuditLogOptions + validator (#23)

src/ScadaLink.AuditLog/Configuration/AuditLogOptionsValidator.cs

@@ -0,0 +1,57 @@
+using Microsoft.Extensions.Options;
+
+namespace ScadaLink.AuditLog.Configuration;
+
+/// <summary>
+/// Validates <see cref="AuditLogOptions"/> on startup. The caps drive payload
+/// truncation in the M2+ writers, so an unset/zero cap would let arbitrarily
+/// large blobs into the central <c>AuditLog</c> table. <see cref="AuditLogOptions.ErrorCapBytes"/>
+/// must be at least as large as <see cref="AuditLogOptions.DefaultCapBytes"/>
+/// because the error cap is meant to capture <em>more</em> detail than the
+/// happy-path summary, not less. <see cref="AuditLogOptions.RetentionDays"/> is
+/// bounded to <c>[30, 3650]</c> to keep purge windows sane: too short would
+/// drop in-flight investigations, too long would defeat the partition-switch
+/// purge's purpose.
+/// </summary>
+public sealed class AuditLogOptionsValidator : IValidateOptions<AuditLogOptions>
+{
+    /// <summary>Inclusive lower bound for <see cref="AuditLogOptions.RetentionDays"/>.</summary>
+    public const int MinRetentionDays = 30;
+
+    /// <summary>Inclusive upper bound for <see cref="AuditLogOptions.RetentionDays"/>.</summary>
+    public const int MaxRetentionDays = 3650;
+
+    /// <inheritdoc />
+    public ValidateOptionsResult Validate(string? name, AuditLogOptions options)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+
+        var failures = new List<string>();
+
+        if (options.DefaultCapBytes <= 0)
+        {
+            failures.Add(
+                $"AuditLog:{nameof(AuditLogOptions.DefaultCapBytes)} ({options.DefaultCapBytes}) " +
+                "must be > 0; it drives payload-summary truncation in audit writers.");
+        }
+
+        if (options.ErrorCapBytes < options.DefaultCapBytes)
+        {
+            failures.Add(
+                $"AuditLog:{nameof(AuditLogOptions.ErrorCapBytes)} ({options.ErrorCapBytes}) " +
+                $"must be >= {nameof(AuditLogOptions.DefaultCapBytes)} ({options.DefaultCapBytes}); " +
+                "the error-row cap is intended to capture more detail than the happy-path summary.");
+        }
+
+        if (options.RetentionDays < MinRetentionDays || options.RetentionDays > MaxRetentionDays)
+        {
+            failures.Add(
+                $"AuditLog:{nameof(AuditLogOptions.RetentionDays)} ({options.RetentionDays}) " +
+                $"must be in [{MinRetentionDays}, {MaxRetentionDays}] days.");
+        }
+
+        return failures.Count == 0
+            ? ValidateOptionsResult.Success
+            : ValidateOptionsResult.Fail(failures);
+    }
+}