Fix #19423: Add VSCMD_VER guard to conanvcvars.bat

conan/tools/microsoft/visual.py

@@ -157,6 +157,10 @@ def generate(self, scope="build"):
             @echo off
             set __VSCMD_ARG_NO_LOGO=1
             set VSCMD_SKIP_SENDTELEMETRY=1
+            if defined VSCMD_VER (
+                echo conanvcvars.bat: Visual Studio environment already active, skipping activation
+                goto :eof
+            )
             echo conanvcvars.bat: Activating environment Visual Studio {vs_version} - {vcvarsarch} - winsdk_version={winsdk_version} - vcvars_ver={vcvars_ver}
             {vcvars}
             """)
@@ -168,7 +172,9 @@ def generate(self, scope="build"):
         is_ps1 = self._conanfile.conf.get("tools.env.virtualenv:powershell", check_type=str)
         if is_ps1:
             content_ps1 = textwrap.dedent(rf"""
-            if (-not $env:VSCMD_ARG_VCVARS_VER){{
+            if ($env:VSCMD_VER) {{
+                Write-Host "conanvcvars.ps1: Visual Studio environment already active, skipping activation"
+            }} elseif (-not $env:VSCMD_ARG_VCVARS_VER) {{
                 Push-Location "$PSScriptRoot"
                 cmd /c "conanvcvars.bat&set" |
                 foreach {{

test/integration/toolchains/microsoft/test_vcvars_guard.py

@@ -0,0 +1,96 @@
+"""
+Test for GitHub issue #19423:
+conanvcvars.bat should activate only once, since it can't be deactivated.
+The fix adds a VSCMD_VER guard so repeated calls to conanbuild.bat don't
+keep appending vcvars paths to %PATH%.
+"""
+import os
+import textwrap
+
+import pytest
+
+from conan.test.utils.tools import TestClient
+
+
+@pytest.mark.skipif(not (os.name == "nt"), reason="Windows-only test")
+def test_vcvars_guard_in_generated_bat():
+    """
+    The generated conanvcvars.bat must contain an 'if defined VSCMD_VER' guard
+    so that vcvarsall.bat is only called once, preventing PATH accumulation.
+    Regression test for https://github.com/conan-io/conan/issues/19423
+    """
+    client = TestClient(path_with_spaces=False)
+    client.save({"conanfile.txt": "[generators]\nVCVars"})
+    client.run(
+        'install . -s os=Windows -s compiler=msvc -s compiler.version=193 '
+        '-s compiler.runtime=dynamic '
+        '-c tools.microsoft.msbuild:installation_path=C:/'  # fake path, avoids vswhere lookup
+    )
+
+    bat_content = client.load("conanvcvars.bat")
+
+    # The guard must be present
+    assert "if defined VSCMD_VER" in bat_content, (
+        "conanvcvars.bat must guard against re-activation using 'if defined VSCMD_VER'"
+    )
+    assert "goto :eof" in bat_content, (
+        "conanvcvars.bat must skip activation with 'goto :eof' when VS env is already active"
+    )
+    assert "already active, skipping activation" in bat_content, (
+        "conanvcvars.bat must print a message when skipping re-activation"
+    )
+
+    # The actual vcvars call must still be present (for first-time activation)
+    assert "vcvarsall.bat" in bat_content, (
+        "conanvcvars.bat must still call vcvarsall.bat for first activation"
+    )
+
+
+@pytest.mark.skipif(not (os.name == "nt"), reason="Windows-only test")
+def test_vcvars_guard_order_correct():
+    """
+    The guard must come BEFORE the vcvarsall.bat call in conanvcvars.bat.
+    """
+    client = TestClient(path_with_spaces=False)
+    client.save({"conanfile.txt": "[generators]\nVCVars"})
+    client.run(
+        'install . -s os=Windows -s compiler=msvc -s compiler.version=193 '
+        '-s compiler.runtime=dynamic '
+        '-c tools.microsoft.msbuild:installation_path=C:/'
+    )
+
+    bat_content = client.load("conanvcvars.bat")
+
+    guard_pos = bat_content.find("if defined VSCMD_VER")
+    vcvars_pos = bat_content.find("vcvarsall.bat")
+
+    assert guard_pos != -1, "Guard 'if defined VSCMD_VER' not found in conanvcvars.bat"
+    assert vcvars_pos != -1, "vcvarsall.bat call not found in conanvcvars.bat"
+    assert guard_pos < vcvars_pos, (
+        "The VSCMD_VER guard must appear BEFORE the vcvarsall.bat call"
+    )
+
+
+@pytest.mark.skipif(not (os.name == "nt"), reason="Windows-only test")
+def test_vcvars_powershell_guard_in_generated_ps1():
+    """
+    The generated conanvcvars.ps1 must also guard against re-activation
+    using $env:VSCMD_VER.
+    """
+    client = TestClient(path_with_spaces=False)
+    client.save({"conanfile.txt": "[generators]\nVCVars"})
+    client.run(
+        'install . -s os=Windows -s compiler=msvc -s compiler.version=193 '
+        '-s compiler.runtime=dynamic '
+        '-c tools.microsoft.msbuild:installation_path=C:/ '
+        '-c tools.env.virtualenv:powershell=True'
+    )
+
+    ps1_content = client.load("conanvcvars.ps1")
+
+    assert "$env:VSCMD_VER" in ps1_content, (
+        "conanvcvars.ps1 must guard against re-activation using '$env:VSCMD_VER'"
+    )
+    assert "already active, skipping activation" in ps1_content, (
+        "conanvcvars.ps1 must print a message when skipping re-activation"
+    )