Skip commit pack download during clone when prefetch packs exist

When gvfs clone runs with a shared cache that already has prefetch
packs (e.g., from a previous clone), defer the commit+trees pack
download. If the commit is already present in the prefetch packs,
the download is skipped entirely. If it is not, the fallback path
downloads on demand when branch creation, gitattributes lookup, or
checkout requires it.

This avoids redundantly re-downloading trees that are already in
prefetch packs when the tip commit happens to be covered by the
existing cache.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv

GVFS/GVFS/CommandLine/CloneVerb.cs

@@ -361,6 +361,29 @@ private static bool IsForceCheckoutErrorCloneFailure(string checkoutError)
             return true;
         }
 
+        private static bool HasUsablePrefetchPacks(
+            GVFSGitObjects gitObjects,
+            GVFSEnlistment enlistment,
+            PhysicalFileSystem fileSystem)
+        {
+            string[] prefetchPacks = gitObjects.ReadPackFileNames(
+                enlistment.GitPackRoot,
+                GVFSConstants.PrefetchPackPrefix);
+
+            foreach (string packPath in prefetchPacks)
+            {
+                string idxPath = Path.ChangeExtension(packPath, ".idx");
+                string incompletePath = Path.ChangeExtension(packPath, GVFSConstants.InProgressPrefetchMarkerExtension);
+
+                if (fileSystem.FileExists(idxPath) && !fileSystem.FileExists(incompletePath))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         private Result TryCreateEnlistment(
             string fullEnlistmentRootPathParameter,
             string normalizedEnlistementRootPath,
@@ -617,15 +640,30 @@ private Result CreateClone(
             GVFSContext context = new GVFSContext(tracer, fileSystem, gitRepo, enlistment);
             GVFSGitObjects gitObjects = new GVFSGitObjects(context, objectRequestor);
 
-            if (!this.TryDownloadCommit(
-                refs.GetTipCommitId(branch),
-                enlistment,
-                objectRequestor,
-                gitObjects,
-                gitRepo,
-                out errorMessage))
+            string commitId = refs.GetTipCommitId(branch);
+            bool skippedCommitDownload = false;
+
+            if (gitRepo.CommitAndRootTreeExists(commitId, out _))
             {
-                return new Result(errorMessage);
+                tracer.RelatedInfo("Commit {0} already exists locally, skipping download", commitId);
+            }
+            else if (HasUsablePrefetchPacks(gitObjects, enlistment, fileSystem))
+            {
+                tracer.RelatedInfo("Prefetch packs found in shared cache but commit {0} is not present; deferring download", commitId);
+                skippedCommitDownload = true;
+            }
+            else
+            {
+                if (!this.TryDownloadCommit(
+                    commitId,
+                    enlistment,
+                    objectRequestor,
+                    gitObjects,
+                    gitRepo,
+                    out errorMessage))
+                {
+                    return new Result(errorMessage);
+                }
             }
 
             if (!GVFSVerb.TrySetRequiredGitConfigSettings(enlistment) ||
@@ -643,6 +681,24 @@ private Result CreateClone(
             GitProcess git = new GitProcess(enlistment);
             string originBranchName = "origin/" + branch;
             GitProcess.Result createBranchResult = git.CreateBranchWithUpstream(branch, originBranchName);
+            if (createBranchResult.ExitCodeIsFailure && skippedCommitDownload)
+            {
+                tracer.RelatedInfo("Branch creation failed after deferring commit download, downloading commit now");
+                if (!this.TryDownloadCommit(
+                    commitId,
+                    enlistment,
+                    objectRequestor,
+                    gitObjects,
+                    gitRepo,
+                    out errorMessage))
+                {
+                    return new Result(errorMessage);
+                }
+
+                skippedCommitDownload = false;
+                createBranchResult = git.CreateBranchWithUpstream(branch, originBranchName);
+            }
+
             if (createBranchResult.ExitCodeIsFailure)
             {
                 return new Result("Unable to create branch '" + originBranchName + "': " + createBranchResult.Errors + "\r\n" + createBranchResult.Output);
@@ -654,7 +710,31 @@ private Result CreateClone(
 
             if (!this.TryDownloadRootGitAttributes(enlistment, gitObjects, gitRepo, out errorMessage))
             {
-                return new Result(errorMessage);
+                if (skippedCommitDownload)
+                {
+                    tracer.RelatedInfo("Root .gitattributes download failed after deferring commit download, downloading commit now");
+                    if (!this.TryDownloadCommit(
+                        commitId,
+                        enlistment,
+                        objectRequestor,
+                        gitObjects,
+                        gitRepo,
+                        out errorMessage))
+                    {
+                        return new Result(errorMessage);
+                    }
+
+                    skippedCommitDownload = false;
+
+                    if (!this.TryDownloadRootGitAttributes(enlistment, gitObjects, gitRepo, out errorMessage))
+                    {
+                        return new Result(errorMessage);
+                    }
+                }
+                else
+                {
+                    return new Result(errorMessage);
+                }
             }
 
             this.CreateGitScript(enlistment);
@@ -677,7 +757,7 @@ private Result CreateClone(
                 // again and retry the checkout.
 
                 if (!this.TryDownloadCommit(
-                    refs.GetTipCommitId(branch),
+                    commitId,
                     enlistment,
                     objectRequestor,
                     gitObjects,