Diagnose recursive project/enable_language without crashing (#15999)

Calling `project()` or `enable_language()` from a toolchain file will
infinitely recurse since those commands load the toolchain file.
Diagnose and reject this case with an error message instead of crashing
when the stack eventually overflows.

Source/cmGlobalGenerator.cxx

@@ -398,6 +398,21 @@ cmGlobalGenerator::EnableLanguage(std::vector<std::string>const& languages,
     return;
     }
 
+  std::set<std::string> cur_languages(languages.begin(), languages.end());
+  for (std::set<std::string>::iterator li = cur_languages.begin();
+       li != cur_languages.end(); ++li)
+    {
+    if (!this->LanguagesInProgress.insert(*li).second)
+      {
+      std::ostringstream e;
+      e << "Language '" << *li << "' is currently being enabled.  "
+        "Recursive call not allowed.";
+      mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+      cmSystemTools::SetFatalErrorOccured();
+      return;
+      }
+    }
+
   if(this->TryCompileOuterMakefile)
     {
     // In a try-compile we can only enable languages provided by caller.
@@ -823,6 +838,12 @@ cmGlobalGenerator::EnableLanguage(std::vector<std::string>const& languages,
     {
     cmSystemTools::SetFatalErrorOccured();
     }
+
+  for (std::set<std::string>::iterator li = cur_languages.begin();
+       li != cur_languages.end(); ++li)
+    {
+    this->LanguagesInProgress.erase(*li);
+    }
 }
 
 //----------------------------------------------------------------------------

Source/cmGlobalGenerator.h

@@ -457,6 +457,7 @@ private:
   // in EnableLanguagesFromGenerator
   std::map<std::string, bool> IgnoreExtensions;
   std::set<std::string> LanguagesReady; // Ready for try_compile
+  std::set<std::string> LanguagesInProgress;
   std::map<std::string, std::string> OutputExtensions;
   std::map<std::string, std::string> LanguageToOutputExtension;
   std::map<std::string, std::string> ExtensionToLanguage;

Tests/RunCMake/CMakeLists.txt

@@ -152,6 +152,7 @@ add_RunCMake_test(ObjectLibrary)
 add_RunCMake_test(Swift)
 add_RunCMake_test(TargetObjects)
 add_RunCMake_test(TargetSources)
+add_RunCMake_test(ToolchainFile)
 add_RunCMake_test(find_dependency)
 add_RunCMake_test(CompileDefinitions)
 add_RunCMake_test(CompileFeatures)

Tests/RunCMake/ToolchainFile/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.5)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/ToolchainFile/CallEnableLanguage-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/ToolchainFile/CallEnableLanguage-stderr.txt

@@ -0,0 +1,5 @@
+^CMake Error at CallEnableLanguage-toolchain.cmake:[0-9]+ \(enable_language\):
+  Language 'NONE' is currently being enabled.  Recursive call not allowed.
+Call Stack \(most recent call first\):
+  .*/Modules/CMakeDetermineSystem.cmake:[0-9]+ \(include\)
+  CMakeLists.txt:[0-9]+ \(project\)$

Tests/RunCMake/ToolchainFile/CallEnableLanguage-toolchain.cmake

@@ -0,0 +1 @@
+enable_language(NONE)

Tests/RunCMake/ToolchainFile/CallProject-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/ToolchainFile/CallProject-stderr.txt

@@ -0,0 +1,5 @@
+^CMake Error at CallProject-toolchain.cmake:[0-9]+ \(project\):
+  Language 'NONE' is currently being enabled.  Recursive call not allowed.
+Call Stack \(most recent call first\):
+  .*/Modules/CMakeDetermineSystem.cmake:[0-9]+ \(include\)
+  CMakeLists.txt:[0-9]+ \(project\)$

Tests/RunCMake/ToolchainFile/CallProject-toolchain.cmake

@@ -0,0 +1 @@
+project(Bad NONE)

Tests/RunCMake/ToolchainFile/RunCMakeTest.cmake

@@ -0,0 +1,9 @@
+include(RunCMake)
+
+function(run_cmake_toolchain t)
+  set(RunCMake_TEST_OPTIONS -DCMAKE_TOOLCHAIN_FILE=${RunCMake_SOURCE_DIR}/${t}-toolchain.cmake)
+  run_cmake(${t})
+endfunction()
+
+run_cmake_toolchain(CallEnableLanguage)
+run_cmake_toolchain(CallProject)