Fix tag normalization for Azure Pipelines

Author: ChristopherMann

src/GitVersion.BuildAgents.Tests/Agents/AzurePipelinesTests.cs

@@ -77,4 +77,43 @@ public void AzurePipelinesBuildNumberWithSemVer(string buildNumberFormat, string
         var logMessage = this.buildServer.SetBuildNumber(vars);
         logMessage.ShouldBe(logPrefix + expectedBuildNumber);
     }
+
+    [Test]
+    public void GetCurrentBranchShouldHandleBranches()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable("BUILD_SOURCEBRANCH", $"refs/heads/{MainBranch}");
+
+        // Act
+        var result = this.buildServer.GetCurrentBranch(false);
+
+        // Assert
+        result.ShouldBe($"refs/heads/{MainBranch}");
+    }
+
+    [Test]
+    public void GetCurrentBranchShouldHandleTags()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable("BUILD_SOURCEBRANCH", "refs/tags/1.0.0");
+
+        // Act
+        var result = this.buildServer.GetCurrentBranch(false);
+
+        // Assert
+        result.ShouldBeNull();
+    }
+
+    [Test]
+    public void GetCurrentBranchShouldHandlePullRequests()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable("BUILD_SOURCEBRANCH", "refs/pull/1/merge");
+
+        // Act
+        var result = this.buildServer.GetCurrentBranch(false);
+
+        // Assert
+        result.ShouldBe("refs/pull/1/merge");
+    }
 }

src/GitVersion.BuildAgents/Agents/AzurePipelines.cs

@@ -17,8 +17,24 @@ public override string[] SetOutputVariables(string name, string? value) =>
         $"##vso[task.setvariable variable=GitVersion.{name};isOutput=true]{value}"
     ];
 
-    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("GIT_BRANCH")
-        ?? Environment.GetEnvironmentVariable("BUILD_SOURCEBRANCH");
+    public override string? GetCurrentBranch(bool usingDynamicRepos)
+    {
+        var gitBranch = Environment.GetEnvironmentVariable("GIT_BRANCH");
+        if (gitBranch is not null)
+            return gitBranch;
+
+        var sourceBranch = Environment.GetEnvironmentVariable("BUILD_SOURCEBRANCH");
+
+        // https://learn.microsoft.com/en-us/azure/devops/pipelines/build/variables?view=azure-devops&tabs=yaml
+        // BUILD_SOURCEBRANCH must be used only for "real" branches, not for tags.
+        // Azure Pipelines sets BUILD_SOURCEBRANCH to refs/tags/<tag> when the pipeline is triggered for a tag.
+        if (sourceBranch?.StartsWith("refs/tags", StringComparison.OrdinalIgnoreCase) == true)
+        {
+            return null;
+        }
+
+        return sourceBranch;
+    }
 
     public override bool PreventFetch() => true;