Support Bazel's TEST_PREMATURE_EXIT_FILE and add it to CLI as well

This tells Catch2 to create an empty file at specified path before
the tests start, and delete it after the tests finish. This allows
callers to catch cases where the test binary silently exits before
finishing (e.g. via call to `exit(0)` inside the code under test),
by looking whether the file still exists.

Closes #3020

src/catch2/catch_session.cpp

@@ -26,6 +26,8 @@
 #include <catch2/internal/catch_istream.hpp>
 
 #include <cassert>
+#include <cstdio>
+#include <cstdlib>
 #include <exception>
 #include <iomanip>
 #include <set>
@@ -140,6 +142,50 @@ namespace Catch {
             }
         }
 
+        // Creates empty file at path. The path must be writable, we do not
+        // try to create directories in path because that's hard in C++14.
+        void setUpGuardFile( std::string const& guardFilePath ) {
+            if ( !guardFilePath.empty() ) {
+#if defined( _MSC_VER )
+                std::FILE* file = nullptr;
+                if ( fopen_s( &file, guardFilePath.c_str(), "w" ) ) {
+                    char msgBuffer[100];
+                    const auto err = errno;
+                    std::string errMsg;
+                    if ( !strerror_s( msgBuffer, err ) ) {
+                        errMsg = msgBuffer;
+                    } else {
+                        errMsg = "Could not translate errno to a string";
+                    }
+
+#else
+                std::FILE* file = std::fopen( guardFilePath.c_str(), "w" );
+                if ( !file ) {
+                    const auto err = errno;
+                    const char* errMsg = std::strerror( err );
+#endif
+
+                    CATCH_RUNTIME_ERROR( "Could not open the exit guard file '"
+                                         << guardFilePath << "' because '"
+                                         << errMsg << "' (" << err << ')' );
+                }
+                const int ret = std::fclose( file );
+                CATCH_ENFORCE(
+                    ret == 0,
+                    "Error when closing the exit guard file: " << ret );
+            }
+        }
+
+        // Removes file at path. Assuming we created it in setUpGuardFile.
+        void tearDownGuardFile( std::string const& guardFilePath ) {
+            if ( !guardFilePath.empty() ) {
+                const int ret = std::remove( guardFilePath.c_str() );
+                CATCH_ENFORCE(
+                    ret == 0,
+                    "Error when removing the exit guard file: " << ret );
+            }
+        }
+
     } // anon namespace
 
     Session::Session() {
@@ -258,6 +304,7 @@ namespace Catch {
             static_cast<void>(std::getchar());
         }
         int exitCode = runInternal();
+
         if( ( m_configData.waitForKeypress & WaitForKeypress::BeforeExit ) != 0 ) {
             Catch::cout() << "...waiting for enter/ return before exiting, with code: " << exitCode << '\n' << std::flush;
             static_cast<void>(std::getchar());
@@ -298,6 +345,10 @@ namespace Catch {
         CATCH_TRY {
             config(); // Force config to be constructed
 
+            // We need to retrieve potential Bazel config with the full Config
+            // constructor, so we have to create the guard file after it is created.
+            setUpGuardFile( m_config->getExitGuardFilePath() );
+
             seedRng( *m_config );
 
             if (m_configData.filenamesAsTags) {
@@ -327,9 +378,12 @@ namespace Catch {
             TestGroup tests { CATCH_MOVE(reporter), m_config.get() };
             auto const totals = tests.execute();
 
+            // If we got here, running the tests finished normally-enough.
+            // They might've failed, but that would've been reported elsewhere.
+            tearDownGuardFile( m_config->getExitGuardFilePath() );
+
             if ( tests.hadUnmatchedTestSpecs()
                 && m_config->warnAboutUnmatchedTestSpecs() ) {
-                // UnmatchedTestSpecExitCode
                 return UnmatchedTestSpecExitCode;
             }