[core/zip] Add test for compression buffer sizes

This would have found any of the previous three commits.

core/zip/CMakeLists.txt

@@ -22,3 +22,5 @@ target_include_directories(Core PUBLIC
 )
 
 ROOT_INSTALL_HEADERS()
+
+ROOT_ADD_TEST_SUBDIRECTORY(test)

core/zip/test/CMakeLists.txt

@@ -0,0 +1,7 @@
+# Copyright (C) 1995-2024, Rene Brun and Fons Rademakers.
+# All rights reserved.
+#
+# For the licensing terms see $ROOTSYS/LICENSE.
+# For the list of contributors see $ROOTSYS/README/CREDITS.
+
+ROOT_ADD_GTEST(ZipTest ZipTest.cxx LIBRARIES Core)

core/zip/test/ZipTest.cxx

@@ -0,0 +1,70 @@
+#include <Compression.h>
+#include <RZip.h>
+
+#include <gtest/gtest.h>
+
+#include <memory>
+
+static void testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::EValues compressionAlgorithm)
+{
+   static constexpr size_t BufferSize = 256;
+   static constexpr size_t MaxBytes = 128;
+   static_assert(MaxBytes <= BufferSize, "MaxBytes must be smaller than BufferSize");
+   static constexpr size_t StartOffset = (BufferSize - MaxBytes) / 2;
+   // For extra "safety", allocate the buffers on the heap to avoid corrupting the stack should anything go wrong.
+   std::unique_ptr<char[]> source(new char[BufferSize]);
+   std::unique_ptr<char[]> target(new char[BufferSize]);
+
+   // Fill the buffers with monotonically increasing numbers. This is easy to compress, but that's fine because we scan
+   // through all possible sizes.
+   for (size_t i = 0; i < BufferSize; i++) {
+      source[i] = static_cast<char>(i);
+      target[i] = static_cast<char>(i);
+   }
+
+   // Now test all possible combinations of target and source sizes. The outer loop is for the target sizes because that
+   // allows us to check that nothing got overwritten.
+   for (size_t targetSize = 1; targetSize <= MaxBytes; targetSize++) {
+      for (size_t sourceSize = 1; sourceSize <= MaxBytes; sourceSize++) {
+         for (int cxlevel = 1; cxlevel <= 9; cxlevel++) {
+            int srcsize = static_cast<int>(sourceSize);
+            int tgtsize = static_cast<int>(targetSize);
+            int irep = -1;
+            R__zipMultipleAlgorithm(cxlevel, &srcsize, source.get(), &tgtsize, target.get() + StartOffset, &irep,
+                                    compressionAlgorithm);
+
+            for (size_t i = 0; i < StartOffset; i++) {
+               EXPECT_EQ(target[i], static_cast<char>(i));
+            }
+            for (size_t i = StartOffset + targetSize + 1; i < BufferSize; i++) {
+               EXPECT_EQ(target[i], static_cast<char>(i));
+            }
+         }
+      }
+   }
+}
+
+TEST(RZip, ZipBufferSizesOld)
+{
+   testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::kOldCompressionAlgo);
+}
+
+TEST(RZip, ZipBufferSizesZLIB)
+{
+   testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::kZLIB);
+}
+
+TEST(RZip, ZipBufferSizesLZMA)
+{
+   testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::kLZMA);
+}
+
+TEST(RZip, ZipBufferSizesLZ4)
+{
+   testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::kLZ4);
+}
+
+TEST(RZip, ZipBufferSizesZSTD)
+{
+   testZipBufferSizes(ROOT::RCompressionSetting::EAlgorithm::kZSTD);
+}