Add FetchByCommitForFullClone knob to enable fetch by commit for full clones (#5276)

* Add FetchByCommitForFullClone knob to enable fetch by commit for full clones

* Log shouldFetchByCommit

---------

Co-authored-by: Semih Okur <seokur@microsoft.com>

Author: tarunramsinghani

src/Agent.Plugins/GitSourceProvider.cs

@@ -937,11 +937,18 @@ public async Task GetSourceAsync(
             executionContext.Debug($"sourceVersion : {sourceVersion}");
             executionContext.Debug($"fetchTags : {fetchTags}");
 
+            // Determine if we should use fetch by commit based on shallow vs full clone scenarios
+            bool shouldFetchByCommit = fetchByCommit && !string.IsNullOrEmpty(sourceVersion) &&
+                (fetchDepth > 0 || AgentKnobs.FetchByCommitForFullClone.GetValue(executionContext).AsBoolean());
+
+            executionContext.Debug($"shouldFetchByCommit : {shouldFetchByCommit}");
+
             if (IsPullRequest(sourceBranch))
             {
                 // Build a 'fetch-by-commit' refspec iff the server allows us to do so in the shallow fetch scenario
+                // or if it's a full clone and the FetchByCommitForFullClone knob is enabled
                 // Otherwise, fall back to fetch all branches and pull request ref
-                if (fetchDepth > 0 && fetchByCommit && !string.IsNullOrEmpty(sourceVersion))
+                if (shouldFetchByCommit)
                 {
                     refFetchedByCommit = $"{_remoteRefsPrefix}{sourceVersion}";
                     additionalFetchSpecs.Add($"+{sourceVersion}:{refFetchedByCommit}");
@@ -955,8 +962,9 @@ public async Task GetSourceAsync(
             else
             {
                 // Build a refspec iff the server allows us to fetch a specific commit in the shallow fetch scenario
+                // or if it's a full clone and the FetchByCommitForFullClone knob is enabled
                 // Otherwise, use the default fetch behavior (i.e. with no refspecs)
-                if (fetchDepth > 0 && fetchByCommit && !string.IsNullOrEmpty(sourceVersion))
+                if (shouldFetchByCommit)
                 {
                     refFetchedByCommit = $"{_remoteRefsPrefix}{sourceVersion}";
                     additionalFetchSpecs.Add($"+{sourceVersion}:{refFetchedByCommit}");

src/Agent.Sdk/Knob/AgentKnobs.cs

@@ -200,6 +200,13 @@ public class AgentKnobs
             new EnvironmentKnobSource("AGENT_USE_NODE20_IN_UNSUPPORTED_SYSTEM"),
             new BuiltInDefaultKnobSource("false"));
 
+        public static readonly Knob FetchByCommitForFullClone = new Knob(
+            nameof(FetchByCommitForFullClone),
+            "If true, allow fetch by commit when doing a full clone (depth=0).",
+            new RuntimeKnobSource("VSTS.FetchByCommitForFullClone"),
+            new EnvironmentKnobSource("VSTS_FETCHBYCOMMITFORFULLCLONE"),
+            new BuiltInDefaultKnobSource("false"));
+
         // Agent logging
         public static readonly Knob AgentPerflog = new Knob(
             nameof(AgentPerflog),