From 12a225d56d0e3220f1c55235c59600e7e895d735 Mon Sep 17 00:00:00 2001
From: Ben Boeckel <ben.boeckel@kitware.com>
Date: Mon, 6 Oct 2025 09:51:55 -0400
Subject: [PATCH] Ninja: handle trailing newlines in variable stanzas

When a variable ends with a newline, the Ninja generator escapes it as
expected with `$\n`. However, when writing it out, whitespace is
trimmed. This leaves a trailing `$` instead which consumes the newline
and masks the following stanza. In a custom command, this hides the
`restat = 1` setting.

Detect this case and restore the newline as necessary.

See: #25983
---
 Source/cmGlobalNinjaGenerator.cxx                     |  7 +++++++
 .../RunCMake/Ninja/CommentsWithTrailingNewlines.cmake | 11 +++++++++++
 Tests/RunCMake/Ninja/RunCMakeTest.cmake               |  1 +
 3 files changed, 19 insertions(+)
 create mode 100644 Tests/RunCMake/Ninja/CommentsWithTrailingNewlines.cmake

diff --git a/Source/cmGlobalNinjaGenerator.cxx b/Source/cmGlobalNinjaGenerator.cxx
index 8527f4fcdf..b443d26069 100644
--- a/Source/cmGlobalNinjaGenerator.cxx
+++ b/Source/cmGlobalNinjaGenerator.cxx
@@ -521,6 +521,13 @@ void cmGlobalNinjaGenerator::WriteVariable(std::ostream& os,
   if (variablesShouldNotBeTrimmed.find(name) ==
       variablesShouldNotBeTrimmed.end()) {
     val = cmTrimWhitespace(value);
+    // If the value ends with `\n` and a `$` was left at the end of the trimmed
+    // value, put the newline back. Otherwise the next stanza is hidden by the
+    // trailing `$` escaping the newline.
+    if (cmSystemTools::StringEndsWith(value, "\n") &&
+        cmSystemTools::StringEndsWith(val, "$")) {
+      val += '\n';
+    }
   } else {
     val = value;
   }
diff --git a/Tests/RunCMake/Ninja/CommentsWithTrailingNewlines.cmake b/Tests/RunCMake/Ninja/CommentsWithTrailingNewlines.cmake
new file mode 100644
index 0000000000..f4c7e6afdb
--- /dev/null
+++ b/Tests/RunCMake/Ninja/CommentsWithTrailingNewlines.cmake
@@ -0,0 +1,11 @@
+enable_language(C)
+
+add_executable(comments_with_newlines hello.c)
+add_custom_command(TARGET comments_with_newlines PRE_LINK
+        COMMAND "${CMAKE_COMMAND}" -E echo prelink
+        COMMENT "prelink with trailing newline
+")
+add_custom_command(TARGET comments_with_newlines POST_BUILD
+        COMMAND "${CMAKE_COMMAND}" -E echo postbuild
+        COMMENT "postbuild with trailing newline
+")
diff --git a/Tests/RunCMake/Ninja/RunCMakeTest.cmake b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
index 9e9caf11f5..737bcdb478 100644
--- a/Tests/RunCMake/Ninja/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
@@ -81,6 +81,7 @@ function(run_WithBuild name)
 endfunction()
 run_WithBuild(CommentsWithDollars)
 run_WithBuild(CommentsWithNewlines)
+run_WithBuild(CommentsWithTrailingNewlines)
 
 function(run_VerboseBuild)
   run_cmake(VerboseBuild)