(stdlib) Fix memory leak in tests

This may seem minor and petty, but it causes a real problem now that we
start running the stdlib tests through Valgrind too. In other words, we
will do our very best to prevent *any* form of memory problems from
slipping into this code base. That's the theory, at least. :-)

https://gitlab.perlang.org/perlang/perlang/-/merge_requests/558

release-notes/v0.6.0.md

@@ -45,6 +45,7 @@
 - Return non-`const` strings [[!544][544]]
 - Support dynamically resizing `StringBuilder` buffer [[!550][550]]
 - Fix buffer overrun in dynamic resizing of `StringBuilder` [[!551][551]]
+- Fix memory leak in tests [[!558][558]]
 
 #### CI
 - Add `dotnet test` CI job [[!499][499]]
@@ -145,3 +146,4 @@
 [554]: https://gitlab.perlang.org/perlang/perlang/merge_requests/554
 [555]: https://gitlab.perlang.org/perlang/perlang/merge_requests/555
 [556]: https://gitlab.perlang.org/perlang/perlang/merge_requests/556
+[558]: https://gitlab.perlang.org/perlang/perlang/merge_requests/558

src/stdlib/test/print.cc

@@ -9,26 +9,24 @@
 #include "perlang_stdlib.h"
 
 bool fwrite_mocked = false;
-std::string* captured_output = nullptr;
+std::string captured_output;
 
 extern "C" void __real_fwrite(const void *__restrict __ptr, size_t __size, size_t __n, FILE *__restrict __s);
 
 extern "C" void __wrap_fwrite(const void *__restrict __ptr, size_t __size, size_t __n, FILE *__restrict __s)
 {
     if (fwrite_mocked) {
-        // TODO: handle multiple lines of output. Possibly append to previous list?
-        if (captured_output != nullptr) {
-            delete captured_output;
-        }
-
         // __ptr is not necessarily NUL-terminated at this point, so we must be careful to only copy the data that would
-        // have been printed to the file handle and then add a NUL terminator manually.
+        // have been printed to the file handle and then add a NUL terminator manually. Note that the use of a stack-
+        // allocated buffer here means that we would hypothetically fail on very long input.
         unsigned long string_length = (__size * __n );
-        char* captured_output_buffer = (char *)malloc(string_length + 1);
+        char captured_output_buffer[string_length + 1];
         strncpy(captured_output_buffer, (const char *)__ptr, string_length);
         captured_output_buffer[string_length] = '\0';
 
-        captured_output = new std::string(captured_output_buffer);
+        // TODO: handle multiple lines of output. Possibly append to previous list?
+        captured_output.erase();
+        captured_output.append(captured_output_buffer);
     }
     else {
         __real_fwrite(__ptr, __size, __n, __s);
@@ -41,7 +39,7 @@ TEST_CASE( "perlang::print float, 103.1f" )
     perlang::print(103.1f);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "103.1\n");
+    REQUIRE(captured_output == "103.1\n");
 }
 
 TEST_CASE( "perlang::print float, positive infinity" )
@@ -52,7 +50,7 @@ TEST_CASE( "perlang::print float, positive infinity" )
     perlang::print(f);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "Infinity\n");
+    REQUIRE(captured_output == "Infinity\n");
 }
 
 TEST_CASE( "perlang::print double, 123.45" )
@@ -61,7 +59,7 @@ TEST_CASE( "perlang::print double, 123.45" )
     perlang::print(123.45);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "123.45\n");
+    REQUIRE(captured_output == "123.45\n");
 }
 
 TEST_CASE( "perlang::print double, -46.0" )
@@ -70,7 +68,7 @@ TEST_CASE( "perlang::print double, -46.0" )
     perlang::print(-46.0);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "-46\n");
+    REQUIRE(captured_output == "-46\n");
 }
 
 TEST_CASE( "perlang::print double, 4294967296.123" )
@@ -79,7 +77,7 @@ TEST_CASE( "perlang::print double, 4294967296.123" )
     perlang::print(4294967296.123);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "4294967296.123\n");
+    REQUIRE(captured_output == "4294967296.123\n");
 }
 
 TEST_CASE( "perlang::print double, 4294967283" )
@@ -88,7 +86,7 @@ TEST_CASE( "perlang::print double, 4294967283" )
     perlang::print(4294967283.0);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "4294967283\n");
+    REQUIRE(captured_output == "4294967283\n");
 }
 
 TEST_CASE( "perlang::print double, 9223372036854775807" )
@@ -97,7 +95,7 @@ TEST_CASE( "perlang::print double, 9223372036854775807" )
     perlang::print(9223372036854775807.0);
     fwrite_mocked = false;
 
-    REQUIRE(*captured_output == "9.22337203685478E+18\n");
+    REQUIRE(captured_output == "9.22337203685478E+18\n");
 }
 
 // TODO: Make this test work. We need to (linker-)wrap puts to make it happen.