Ignore non-JSON stdout preamble in catch_discover_tests (#3162)

extras/CatchAddTests.cmake

@@ -134,6 +134,19 @@ function(catch_discover_tests_impl)
     endforeach()
   endif()
 
+  # The test executable might print to stdout before Catch2 emits its JSON,
+  # e.g. a third-party library logging from a static initializer. Such text
+  # would break the JSON parser, so we strip everything before the first '{',
+  # which is where Catch2's JSON output starts.
+  string(FIND "${listing_output}" "{" json_start)
+  if(json_start EQUAL -1)
+    message(FATAL_ERROR
+      "Could not find the start of JSON output when listing tests from executable '${_TEST_EXECUTABLE}':\n"
+      "  Output: ${listing_output}\n"
+    )
+  endif()
+  string(SUBSTRING "${listing_output}" ${json_start} -1 listing_output)
+
   # Parse JSON output for list of tests/class names/tags
   string(JSON version GET "${listing_output}" "version")
   if(NOT version STREQUAL "1")

tests/TestScripts/DiscoverTests/VerifyRegistration.py

@@ -79,7 +79,12 @@ def get_test_names(build_path: str) -> List[TestInfo]:
                             check = True,
                             text = True)
 
-    test_listing = json.loads(result.stdout)
+    # The executable might print to stdout before Catch2's JSON output, e.g.
+    # from a third-party library's static initializer, so we skip everything
+    # before the first '{', just like CatchAddTests.cmake does.
+    json_start = result.stdout.find('{')
+    assert json_start != -1, f"Could not find JSON output in:\n{result.stdout}"
+    test_listing = json.loads(result.stdout[json_start:])
 
     assert test_listing['version'] == 1
 

tests/TestScripts/DiscoverTests/register-tests.cpp

@@ -8,6 +8,20 @@
 
 #include <catch2/catch_test_macros.hpp>
 
+#include <iostream>
+
+namespace {
+    // Emulate a third-party library that writes to stdout from a static
+    // initializer. This text precedes Catch2's JSON output during test
+    // discovery, and `catch_discover_tests` has to ignore it. See #3162.
+    struct PrintsDuringStaticInit {
+        PrintsDuringStaticInit() {
+            std::cout << "Some third-party library started up successfully\n";
+        }
+    };
+    const PrintsDuringStaticInit printsDuringStaticInit{};
+}
+
 TEST_CASE("@Script[C:\\EPM1A]=x;\"SCALA_ZERO:\"", "[script regressions]"){}
 TEST_CASE("Some test") {}
 TEST_CASE( "Let's have a test case with a long name. Longer. No, even longer. "