Use GITHUB_REF only for CI builds on branches

Author: ThomasPiskol

src/GitVersion.Core.Tests/BuildAgents/GitHubActionsTests.cs

@@ -83,7 +83,7 @@ public void GetCurrentBranchShouldHandleTags()
         var result = this.buildServer.GetCurrentBranch(false);
 
         // Assert
-        result.ShouldBe("refs/tags/1.0.0");
+        result.ShouldBeNull();
     }
 
     [Test]
@@ -96,7 +96,7 @@ public void GetCurrentBranchShouldHandlePullRequests()
         var result = this.buildServer.GetCurrentBranch(false);
 
         // Assert
-        result.ShouldBe("refs/pull/1/merge");
+        result.ShouldBeNull();
     }
 
     [Test]

src/GitVersion.Core/BuildAgents/GitHubActions.cs

@@ -50,7 +50,18 @@ public override void WriteIntegration(Action<string?> writer, VersionVariables v
         }
     }
 
-    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("GITHUB_REF");
+    public override string? GetCurrentBranch(bool usingDynamicRepos)
+    {
+        // https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables
+        // GITHUB_REF must be used only for "real" branches, not for tags and pull requests.
+        // Bug fix for https://github.com/GitTools/GitVersion/issues/2838
+        string? githubRef = Environment.GetEnvironmentVariable("GITHUB_REF");
+        if (githubRef != null && githubRef.StartsWith("refs/heads/"))
+        {
+            return githubRef;
+        }
+        return null;
+    }
 
     public override bool PreventFetch() => true;
 }