Emit warning when using --shard-count (> 1) with --order rand

4 files changed, 20 insertions, 0 deletions

diff --git a/src/catch2/catch_config.cpp b/src/catch2/catch_config.cpp
index 86b9b8dd..e71ee353 100644
--- a/src/catch2/catch_config.cpp
+++ b/src/catch2/catch_config.cpp
@@ -209,6 +209,7 @@ namespace Catch {
     double Config::minDuration() const                 { return m_data.minDuration; }
     TestRunOrder Config::runOrder() const              { return m_data.runOrder; }
     uint32_t Config::rngSeed() const                   { return m_data.rngSeed; }
+    bool Config::rngSeedWasFixed() const               { return m_data.rngSeedWasFixed; }
     unsigned int Config::shardCount() const            { return m_data.shardCount; }
     unsigned int Config::shardIndex() const            { return m_data.shardIndex; }
     ColourMode Config::defaultColourMode() const       { return m_data.defaultColourMode; }
@@ -273,6 +274,7 @@ namespace Catch {
                     << bazelRandomSeed << "') as proper seed.\n";
             } else {
                 m_data.rngSeed = *parsedSeed;
+                m_data.rngSeedWasFixed = true;
             }
         }
     }
diff --git a/src/catch2/catch_config.hpp b/src/catch2/catch_config.hpp
index 429c7156..3d0c1e96 100644
--- a/src/catch2/catch_config.hpp
+++ b/src/catch2/catch_config.hpp
@@ -63,6 +63,7 @@ namespace Catch {
 
         int abortAfter = -1;
         uint32_t rngSeed = generateRandomSeed(GenerateFrom::Default);
+        bool rngSeedWasFixed = false;
 
         unsigned int shardCount = 1;
         unsigned int shardIndex = 0;
@@ -134,6 +135,7 @@ namespace Catch {
         double minDuration() const override;
         TestRunOrder runOrder() const override;
         uint32_t rngSeed() const override;
+        bool rngSeedWasFixed() const;
         unsigned int shardCount() const override;
         unsigned int shardIndex() const override;
         ColourMode defaultColourMode() const override;
diff --git a/src/catch2/catch_session.cpp b/src/catch2/catch_session.cpp
index 97def8dc..5e9c5c19 100644
--- a/src/catch2/catch_session.cpp
+++ b/src/catch2/catch_session.cpp
@@ -348,6 +348,19 @@ namespace Catch {
         CATCH_TRY {
             config(); // Force config to be constructed
 
+            if ( m_config->shardCount() > 1 &&
+                 m_config->runOrder() == TestRunOrder::Randomized &&
+                 !m_config->rngSeedWasFixed() ) {
+                Catch::cerr()
+                    << "Warning: using sharding (--shard-count) with random "
+                       "order (--order rand, the default) and without a fixed "
+                       "numeric --rng-seed does not guarantee disjoint coverage "
+                       "between shard invocations. Pass the same numeric "
+                       "--rng-seed to every shard, or use --order decl or "
+                       "--order lex instead.\n"
+                    << std::flush;
+            }
+
             // 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() );
diff --git a/src/catch2/internal/catch_commandline.cpp b/src/catch2/internal/catch_commandline.cpp
index 4ce42397..22948c27 100644
--- a/src/catch2/internal/catch_commandline.cpp
+++ b/src/catch2/internal/catch_commandline.cpp
@@ -75,9 +75,11 @@ namespace Catch {
         auto const setRngSeed = [&]( std::string const& seed ) {
                 if( seed == "time" ) {
                     config.rngSeed = generateRandomSeed(GenerateFrom::Time);
+                    config.rngSeedWasFixed = false;
                     return ParserResult::ok(ParseResultType::Matched);
                 } else if (seed == "random-device") {
                     config.rngSeed = generateRandomSeed(GenerateFrom::RandomDevice);
+                    config.rngSeedWasFixed = false;
                     return ParserResult::ok(ParseResultType::Matched);
                 }
 
@@ -88,6 +90,7 @@ namespace Catch {
                     return ParserResult::runtimeError( "Could not parse '" + seed + "' as seed" );
                 }
                 config.rngSeed = *parsedSeed;
+                config.rngSeedWasFixed = true;
                 return ParserResult::ok( ParseResultType::Matched );
             };
         auto const setDefaultColourMode = [&]( std::string const& colourMode ) {