cmCacheManager: Fix warning comment for cache vars containing newlines

Reuse the logic from commit 637a0f7ed4 (cmCacheManager: Prevent
corruption from multiline help strings, 2025-02-04, v4.0.0-rc1~52^2)
for the warning message placed in the cache when a cache variable's
value contains one or more newline characters.

While we're at it, remove a superfluous early-exit in the help strings
method.

Fixes: #27807

Suggested-By: Yauheni Khnykin <yauheni.khnykin@here.com>

Source/cmCacheManager.cxx

@@ -410,9 +410,6 @@ void cmCacheManager::OutputHelpString(std::ostream& fout,
                                       std::string const& helpString)
 {
   std::string::size_type end = helpString.size();
-  if (end == 0) {
-    return;
-  }
   std::string oneLine;
   std::string::size_type pos = 0;
   for (std::string::size_type i = 1; i <= end; i++) {
@@ -437,7 +434,7 @@ void cmCacheManager::OutputWarningComment(std::ostream& fout,
   std::string::size_type end = message.size();
   std::string oneLine;
   std::string::size_type pos = 0;
-  for (std::string::size_type i = 0; i <= end; i++) {
+  for (std::string::size_type i = 1; i <= end; i++) {
     if ((i == end) || (message[i] == '\n') ||
         ((i - pos >= 60) && (message[i] == ' ') && wrapSpaces)) {
       fout << "# ";

Tests/RunCMake/CacheNewline/CacheSingleNewline-CMakeCacheRegex.txt

@@ -0,0 +1,5 @@
+//Offending entry
+NEWLINE_VARIABLE:STRING=
+# WARNING: Value of NEWLINE_VARIABLE contained a newline and was
+#  truncated\. Original value:
+# \\n

Tests/RunCMake/CacheNewline/CacheSingleNewline-stderr.txt

@@ -0,0 +1,2 @@
+CMake Warning:
+  Value of NEWLINE_VARIABLE contained a newline; truncating

Tests/RunCMake/CacheNewline/CacheSingleNewline.cmake

@@ -0,0 +1 @@
+set(NEWLINE_VARIABLE "\n" CACHE STRING "Offending entry")

Tests/RunCMake/CacheNewline/CacheStartsWithNewline-CMakeCacheRegex.txt

@@ -0,0 +1,6 @@
+//Offending entry
+NEWLINE_VARIABLE:STRING=
+# WARNING: Value of NEWLINE_VARIABLE contained a newline and was
+#  truncated\. Original value:
+# \\na
+# \\nb

Tests/RunCMake/CacheNewline/CacheStartsWithNewline-stderr.txt

@@ -0,0 +1,2 @@
+CMake Warning:
+  Value of NEWLINE_VARIABLE contained a newline; truncating

Tests/RunCMake/CacheNewline/CacheStartsWithNewline.cmake

@@ -0,0 +1 @@
+set(NEWLINE_VARIABLE "\na\nb" CACHE STRING "Offending entry")

Tests/RunCMake/CacheNewline/CheckCache.cmake

@@ -1,5 +1,9 @@
-set(CACHE_EXPECTED_FILE "${RunCMake_TEST_SOURCE_DIR}/cache-regex.txt")
-set(CACHE_ACTUAL_FILE "${RunCMake_BINARY_DIR}/CacheNewline-build/CMakeCache.txt")
+set(CACHE_EXPECTED_FILE "${RunCMake_TEST_SOURCE_DIR}/${case}-CMakeCacheRegex.txt")
+set(CACHE_ACTUAL_FILE "${RunCMake_TEST_BINARY_DIR}/CMakeCache.txt")
+
+if(NOT EXISTS ${CACHE_EXPECTED_FILE})
+  message(FATAL_ERROR "Could not find ${RunCMake_TEST_SOURCE_DIR}/${case}-CMakeCacheRegex.txt")
+endif()
 
 file(READ ${CACHE_EXPECTED_FILE} CACHE_EXPECTED)
 string(REGEX REPLACE "\r\n" "\n" CACHE_EXPECTED "${CACHE_EXPECTED}")

Tests/RunCMake/CacheNewline/RunCMakeTest.cmake

@@ -1,3 +1,16 @@
 include(RunCMake)
 
-run_cmake(CacheNewline)
+function(run_cmake_cache_newline_test case)
+  set(RunCMake-check-file CheckCache.cmake)
+  run_cmake(${case})
+
+  # Sanity check that the CMakeCache.txt is valid by trying to list variables.
+  unset(RunCMake-check-file)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/${case}-build)
+  run_cmake_command(${case}-List ${CMAKE_COMMAND} -L ${RunCMake_TEST_BINARY_DIR})
+endfunction()
+
+run_cmake_cache_newline_test(CacheNewlines)
+run_cmake_cache_newline_test(CacheStartsWithNewline)
+run_cmake_cache_newline_test(CacheSingleNewline)