Make previous flow recreation faster

Author: premun

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrBackflower.cs

@@ -505,15 +505,35 @@ private NativePath GetPatchName(SourceMapping mapping, LastFlows lastFlows, Code
         LastFlows previousFlows,
         string branchToCreate,
         string targetBranch,
+        (Codeflow PreviousFlow, LastFlows LastFlows)? lastRewind,
         CancellationToken cancellationToken)
     {
-        await targetRepo.ResetWorkingTree();
-        await targetRepo.ForceCheckoutAsync(targetBranch);
+        Backflow previousFlow;
+
+        int i = 1;
+        if (lastRewind.HasValue)
+        {
+            i = depth - 1;
+            previousFlow = (Backflow)lastRewind.Value.PreviousFlow;
+            previousFlows = lastRewind.Value.LastFlows;
 
-        Backflow previousFlow = previousFlows.LastBackFlow
-            ?? throw new DarcException("No more backflows found to recreate");
+            await targetRepo.ForceCheckoutAsync(previousFlow.RepoSha);
+            await _vmrCloneManager.PrepareVmrAsync(
+                [_vmrInfo.VmrUri],
+                [previousFlow.VmrSha],
+                previousFlow.VmrSha,
+                resetToRemote: false,
+                cancellationToken);
+        }
+        else
+        {
+            previousFlow = previousFlows.LastBackFlow
+                ?? throw new DarcException("No more backflows found to recreate");
+            await targetRepo.ResetWorkingTree();
+            await targetRepo.ForceCheckoutAsync(targetBranch);
+        }
 
-        for (int i = 1; i < depth; i++)
+        while (i++ < depth)
         {
             var previousFlowSha = await _localGitClient.BlameLineAsync(
                 targetRepo.Path / VersionFiles.VersionDetailsXml,

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrCodeflower.cs

@@ -401,20 +401,26 @@ private async Task<CodeFlowResult> RecreatePreviousFlowsAndApplyChanges(
             AzureDevOpsRepository = codeflowOptions.Build.AzureDevOpsRepository
         };
 
+        Codeflow? previousFlow = null;
+        LastFlows? previousFlows = null;
+
         // We recursively try to re-create previous flows until we find the one that introduced the conflict with the current flown
         int flowsToRecreate = 1;
         while (flowsToRecreate < 50)
         {
             _logger.LogInformation("Trying to recreate {count} previous flow(s)..", flowsToRecreate);
 
             // We rewing to the previous flow and create a branch there
-            (Codeflow previousFlow, LastFlows previousFlows) = await RewindToPreviousFlowAsync(
+            (previousFlow, previousFlows) = await RewindToPreviousFlowAsync(
                 codeflowOptions.Mapping,
                 repo,
                 flowsToRecreate,
                 lastFlows,
                 codeflowOptions.HeadBranch,
                 codeflowOptions.TargetBranch,
+                previousFlow != null && previousFlows != null
+                    ? (previousFlow, previousFlows)
+                    : null,
                 cancellationToken);
 
             // We store the SHA where the head branch originates from so that later we can diff it
@@ -430,8 +436,8 @@ codeflowOptions with
                     {
                         Build = previouslyAppliedBuild,
                         CurrentFlow = currentIsBackflow
-                            ? new Backflow(previouslyAppliedBuild.Commit, previousFlow.RepoSha)
-                            : new ForwardFlow(previouslyAppliedBuild.Commit, previousFlow.VmrSha),
+                            ? new Backflow(previouslyAppliedBuild.Commit, previousFlow!.RepoSha)
+                            : new ForwardFlow(previouslyAppliedBuild.Commit, previousFlow!.VmrSha),
                         EnableRebase = false,
                         ForceUpdate = true,
                     },
@@ -498,6 +504,7 @@ await HandleRevertedFiles(
         LastFlows previousFlows,
         string branchToCreate,
         string targetBranch,
+        (Codeflow PreviousFlow, LastFlows LastFlows)? lastRewind,
         CancellationToken cancellationToken);
 
     /// <summary>

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrForwardFlower.cs

@@ -441,14 +441,35 @@ private async Task CommentIncludedPRs(
         LastFlows previousFlows,
         string branchToCreate,
         string targetBranch,
+        (Codeflow PreviousFlow, LastFlows LastFlows)? lastRewind,
         CancellationToken cancellationToken)
     {
-        var previousFlow = previousFlows.LastForwardFlow;
         var vmr = _localGitRepoFactory.Create(_vmrInfo.VmrPath);
-        await vmr.ResetWorkingTree();
-        await vmr.ForceCheckoutAsync(targetBranch);
 
-        for (int i = 1; i < depth; i++)
+        ForwardFlow previousFlow;
+        int i = 1;
+        if (lastRewind.HasValue)
+        {
+            i = depth - 1;
+            previousFlow = (ForwardFlow)lastRewind.Value.PreviousFlow;
+            previousFlows = lastRewind.Value.LastFlows;
+            await _vmrCloneManager.PrepareVmrAsync(
+                [_vmrInfo.VmrUri],
+                [previousFlow.VmrSha],
+                previousFlow.VmrSha,
+                resetToRemote: false,
+                cancellationToken);
+
+            await sourceRepo.ForceCheckoutAsync(previousFlow.RepoSha);
+        }
+        else
+        {
+            previousFlow = previousFlows.LastForwardFlow;
+            await vmr.ResetWorkingTree();
+            await vmr.ForceCheckoutAsync(targetBranch);
+        }
+
+        while (i++ < depth)
         {
             var previousFlowSha = await _localGitClient.BlameLineAsync(
                 _vmrInfo.SourceManifestPath,