From 86d3a026adaabca97ca1486591343e4200fe1be8 Mon Sep 17 00:00:00 2001
From: "claude[bot]" <41898282+claude[bot]@users.noreply.github.com>
Date: Fri, 31 Oct 2025 12:48:10 +0000
Subject: [PATCH] Restore-DbaDbSnapshot - Add retry logic for deadlock errors

This fixes issue #9233 where Restore-DbaDbSnapshot occasionally fails with
deadlock errors when restoring multiple databases in parallel.

The deadlock occurs when:
1. One process executes RESTORE DATABASE (holds locks on master)
2. Another process calls Get-DbaDbSnapshot (queries system tables in master)

The fix implements retry logic with exponential backoff:
- Detects deadlock errors (SQL error 1205)
- Retries up to 3 times with 2^n second delays (2s, 4s, 8s)
- Logs retry attempts at verbose level
- Preserves original error handling for non-deadlock errors

Co-authored-by: Chrissy LeMaire <potatoqualitee@users.noreply.github.com>
---
 public/Restore-DbaDbSnapshot.ps1 | 36 ++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/public/Restore-DbaDbSnapshot.ps1 b/public/Restore-DbaDbSnapshot.ps1
index 1c4ce11e610c..568e0f897a9e 100644
--- a/public/Restore-DbaDbSnapshot.ps1
+++ b/public/Restore-DbaDbSnapshot.ps1
@@ -166,14 +166,38 @@ function Restore-DbaDbSnapshot {
 
                 # Need a proper restore now
                 if ($Pscmdlet.ShouldProcess($server, "Restore db $db from $snap")) {
-                    try {
-                        if ($Force) {
-                            $null = Stop-DbaProcess -SqlInstance $server -Database $db.Name, $snap.Name -WarningAction SilentlyContinue
+                    $maxRetries = 3
+                    $retryCount = 0
+                    $restoreSuccess = $false
+
+                    while (-not $restoreSuccess -and $retryCount -lt $maxRetries) {
+                        try {
+                            if ($Force) {
+                                $null = Stop-DbaProcess -SqlInstance $server -Database $db.Name, $snap.Name -WarningAction SilentlyContinue
+                            }
+
+                            $null = $server.Query("USE master; RESTORE DATABASE [$($db.Name)] FROM DATABASE_SNAPSHOT='$($snap.Name)'")
+                            $restoreSuccess = $true
+                        } catch {
+                            # Check if this is a deadlock error (error 1205)
+                            if ($_.Exception.InnerException.Number -eq 1205) {
+                                $retryCount++
+                                if ($retryCount -lt $maxRetries) {
+                                    $waitSeconds = [Math]::Pow(2, $retryCount)
+                                    Write-Message -Level Verbose -Message "Deadlock detected during restore of $db on $server. Retrying in $waitSeconds seconds (attempt $retryCount of $maxRetries)"
+                                    Start-Sleep -Seconds $waitSeconds
+                                } else {
+                                    Stop-Function -Message "Failiure attempting to restore $db on $server after $maxRetries attempts due to deadlock" -ErrorRecord $_ -Continue
+                                }
+                            } else {
+                                Stop-Function -Message "Failiure attempting to restore $db on $server" -ErrorRecord $_ -Continue
+                                break
+                            }
                         }
+                    }
 
-                        $null = $server.Query("USE master; RESTORE DATABASE [$($db.Name)] FROM DATABASE_SNAPSHOT='$($snap.Name)'")
-                    } catch {
-                        Stop-Function -Message "Failiure attempting to restore $db on $server" -ErrorRecord $_ -Continue
+                    if (-not $restoreSuccess) {
+                        continue
                     }
                 }