Standardize exit codes for various failures

The main reason for this is to be able to distinguish between
different errors (or "errors") based on the return code. Before
this change, it was impossible to use the exit code to figure out
whether a test binary failed because all tests were skipped or
because exactly 4 assertions have failed.

This meant that using `catch_discover_tests` and telling it to
check for exit code == 4 to determine skipped tests could lead to
false negatives.

src/catch2/catch_session.cpp

@@ -34,7 +34,13 @@
 namespace Catch {
 
     namespace {
-        const int MaxExitCode = 255;
+        static constexpr int TestFailureExitCode = 42;
+        static constexpr int UnspecifiedErrorExitCode = 1;
+        static constexpr int AllTestsSkippedExitCode = 4;
+        static constexpr int NoTestsRunExitCode = 2;
+        static constexpr int UnmatchedTestSpecExitCode = 3;
+        static constexpr int InvalidTestSpecExitCode = 5;
+
 
         IEventListenerPtr createReporter(std::string const& reporterName, ReporterConfig&& config) {
             auto reporter = Catch::getRegistryHub().getReporterRegistry().create(reporterName, CATCH_MOVE(config));
@@ -198,8 +204,7 @@ namespace Catch {
     }
 
     int Session::applyCommandLine( int argc, char const * const * argv ) {
-        if( m_startupExceptions )
-            return 1;
+        if ( m_startupExceptions ) { return UnspecifiedErrorExitCode; }
 
         auto result = m_cli.parse( Clara::Args( argc, argv ) );
 
@@ -215,7 +220,7 @@ namespace Catch {
                 << TextFlow::Column( result.errorMessage() ).indent( 2 )
                 << "\n\n";
             errStream->stream() << "Run with -? for usage\n\n" << std::flush;
-            return MaxExitCode;
+            return UnspecifiedErrorExitCode;
         }
 
         if( m_configData.showHelp )
@@ -285,8 +290,7 @@ namespace Catch {
     }
 
     int Session::runInternal() {
-        if( m_startupExceptions )
-            return 1;
+        if ( m_startupExceptions ) { return UnspecifiedErrorExitCode; }
 
         if (m_configData.showHelp || m_configData.libIdentify) {
             return 0;
@@ -297,7 +301,7 @@ namespace Catch {
                           << ") must be greater than the shard index ("
                           << m_configData.shardIndex << ")\n"
                           << std::flush;
-            return 1;
+            return UnspecifiedErrorExitCode;
         }
 
         CATCH_TRY {
@@ -320,7 +324,7 @@ namespace Catch {
                 for ( auto const& spec : invalidSpecs ) {
                     reporter->reportInvalidTestSpec( spec );
                 }
-                return 1;
+                return InvalidTestSpecExitCode;
             }
 
 
@@ -334,29 +338,29 @@ namespace Catch {
 
             if ( tests.hadUnmatchedTestSpecs()
                 && m_config->warnAboutUnmatchedTestSpecs() ) {
-                return 3;
+                // UnmatchedTestSpecExitCode
+                return UnmatchedTestSpecExitCode;
             }
 
             if ( totals.testCases.total() == 0
                 && !m_config->zeroTestsCountAsSuccess() ) {
-                return 2;
+                return NoTestsRunExitCode;
             }
 
             if ( totals.testCases.total() > 0 &&
                  totals.testCases.total() == totals.testCases.skipped
                 && !m_config->zeroTestsCountAsSuccess() ) {
-                return 4;
+                return AllTestsSkippedExitCode;
             }
 
-            // Note that on unices only the lower 8 bits are usually used, clamping
-            // the return value to 255 prevents false negative when some multiple
-            // of 256 tests has failed
-            return (std::min) (MaxExitCode, static_cast<int>(totals.assertions.failed));
+            if ( totals.assertions.failed ) { return TestFailureExitCode; }
+            return 0;
+
         }
 #if !defined(CATCH_CONFIG_DISABLE_EXCEPTIONS)
         catch( std::exception& ex ) {
             Catch::cerr() << ex.what() << '\n' << std::flush;
-            return MaxExitCode;
+            return UnspecifiedErrorExitCode;
         }
 #endif
     }

tests/TestScripts/testBazelReporter.py

@@ -52,7 +52,7 @@ try:
     )
     stdout = ret.stdout
 except subprocess.SubprocessError as ex:
-    if ex.returncode == 1:
+    if ex.returncode == 42:
         # The test cases are allowed to fail.
         test_passing = False
         stdout = ex.stdout