VS: Select and save a VS 2017 instance persistently

Visual Studio 2017 supports multiple instances installed on a single
machine.  We use the Visual Studio Installer tool to enumerate instances
and select one.  Once we select an instance for a given build tree, save
the result in `CMAKE_GENERATOR_INSTANCE` so we can re-configure the tree
with the same instance on future re-runs of CMake.

Fixes: #17268

4 files changed, 66 insertions, 7 deletions

diff --git a/Source/cmCMakeHostSystemInformationCommand.cxx b/Source/cmCMakeHostSystemInformationCommand.cxx
index 5106f52..662dd74 100644
--- a/Source/cmCMakeHostSystemInformationCommand.cxx
+++ b/Source/cmCMakeHostSystemInformationCommand.cxx
@@ -8,6 +8,9 @@
 #include "cmsys/SystemInformation.hxx"
 
 #if defined(_WIN32)
+#include "cmAlgorithms.h"
+#include "cmGlobalGenerator.h"
+#include "cmGlobalVisualStudio15Generator.h"
 #include "cmSystemTools.h"
 #include "cmVSSetupHelper.h"
 #define HAVE_VS_SETUP_HELPER
@@ -127,6 +130,17 @@ bool cmCMakeHostSystemInformationCommand::GetValue(
     value = this->ValueToString(info.GetOSPlatform());
 #ifdef HAVE_VS_SETUP_HELPER
   } else if (key == "VS_15_DIR") {
+    // If generating for the VS 15 IDE, use the same instance.
+    cmGlobalGenerator* gg = this->Makefile->GetGlobalGenerator();
+    if (cmHasLiteralPrefix(gg->GetName(), "Visual Studio 15 ")) {
+      cmGlobalVisualStudio15Generator* vs15gen =
+        static_cast<cmGlobalVisualStudio15Generator*>(gg);
+      if (vs15gen->GetVSInstance(value)) {
+        return true;
+      }
+    }
+
+    // Otherwise, find a VS 15 instance ourselves.
     cmVSSetupAPIHelper vsSetupAPIHelper;
     if (vsSetupAPIHelper.GetVSInstanceInfo(value)) {
       cmSystemTools::ConvertToUnixSlashes(value);
diff --git a/Source/cmGlobalVisualStudio15Generator.cxx b/Source/cmGlobalVisualStudio15Generator.cxx
index d2bf7cc..014d93d 100644
--- a/Source/cmGlobalVisualStudio15Generator.cxx
+++ b/Source/cmGlobalVisualStudio15Generator.cxx
@@ -111,6 +111,53 @@ void cmGlobalVisualStudio15Generator::WriteSLNHeader(std::ostream& fout)
   }
 }
 
+bool cmGlobalVisualStudio15Generator::SetGeneratorInstance(
+  std::string const& i, cmMakefile* mf)
+{
+  if (!i.empty()) {
+    if (!this->vsSetupAPIHelper.SetVSInstance(i)) {
+      std::ostringstream e;
+      /* clang-format off */
+      e <<
+        "Generator\n"
+        "  " << this->GetName() << "\n"
+        "could not find specified instance of Visual Studio:\n"
+        "  " << i;
+      /* clang-format on */
+      mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+      return false;
+    }
+  }
+
+  std::string vsInstance;
+  if (!this->vsSetupAPIHelper.GetVSInstanceInfo(vsInstance)) {
+    std::ostringstream e;
+    /* clang-format off */
+    e <<
+      "Generator\n"
+      "  " << this->GetName() << "\n"
+      "could not find any instance of Visual Studio.\n";
+    /* clang-format on */
+    mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+    return false;
+  }
+
+  // Save the selected instance persistently.
+  std::string genInstance = mf->GetSafeDefinition("CMAKE_GENERATOR_INSTANCE");
+  if (vsInstance != genInstance) {
+    this->CMakeInstance->AddCacheEntry(
+      "CMAKE_GENERATOR_INSTANCE", vsInstance.c_str(),
+      "Generator instance identifier.", cmStateEnums::INTERNAL);
+  }
+
+  return true;
+}
+
+bool cmGlobalVisualStudio15Generator::GetVSInstance(std::string& dir) const
+{
+  return vsSetupAPIHelper.GetVSInstanceInfo(dir);
+}
+
 bool cmGlobalVisualStudio15Generator::InitializeWindows(cmMakefile* mf)
 {
   // If the Win 8.1 SDK is installed then we can select a SDK matching
diff --git a/Source/cmGlobalVisualStudio15Generator.h b/Source/cmGlobalVisualStudio15Generator.h
index e934882..852a4e7 100644
--- a/Source/cmGlobalVisualStudio15Generator.h
+++ b/Source/cmGlobalVisualStudio15Generator.h
@@ -27,6 +27,11 @@ public:
   virtual void WriteSLNHeader(std::ostream& fout);
 
   virtual const char* GetToolsVersion() { return "15.0"; }
+
+  bool SetGeneratorInstance(std::string const& i, cmMakefile* mf) override;
+
+  bool GetVSInstance(std::string& dir) const;
+
 protected:
   bool InitializeWindows(cmMakefile* mf) override;
   virtual bool SelectWindowsStoreToolset(std::string& toolset) const;
diff --git a/Source/cmVSSetupHelper.cxx b/Source/cmVSSetupHelper.cxx
index 4f5f4e8..c2f8deb 100644
--- a/Source/cmVSSetupHelper.cxx
+++ b/Source/cmVSSetupHelper.cxx
@@ -273,13 +273,6 @@ bool cmVSSetupAPIHelper::EnumerateAndChooseVSInstance()
   if (cmSystemTools::GetEnv("VS150COMNTOOLS", envVSCommonToolsDir)) {
     cmSystemTools::ConvertToUnixSlashes(envVSCommonToolsDir);
   }
-  // FIXME: If the environment variable value changes between runs
-  // of CMake within a given build tree the results are not defined.
-  // Instead we should save a CMAKE_GENERATOR_INSTANCE value in the cache
-  // (similar to CMAKE_GENERATOR_TOOLSET) to hold it persistently.
-  // Unfortunately doing so will require refactoring elsewhere in
-  // order to make sure the value is available in time to create
-  // the generator.
 
   std::vector<VSInstanceInfo> vecVSInstances;
   SmartCOMPtr<IEnumSetupInstances> enumInstances = NULL;