Ninja: Handle depfiles with absolute paths to generated files

Ninja treats every (normalized) path as its own node.  It does not
recognize `/abs/path/to/file` in a depfile as matching `path/to/file`
even when `build.ninja` and the working directory are in `/abs/`.
See Ninja Issue 1251.  In cases where we pass absolute paths to the
compiler, it will write a depfile containing absolute paths.  If those
files are generated in the build tree by custom commands, `build.ninja`
references them by relative path in build statement outputs, so Ninja
does not hook up the dependency and rebuild the project correctly.

Add infrastructure to work around this problem by adding implicit
outputs to custom command build statements that reference the main
outputs by absolute path.  Use a `${cmake_ninja_workdir}` placeholder
to avoid repeating the base path.  For example:

    build out.txt | ${cmake_ninja_workdir}out.txt: CUSTOM_COMMAND ...

Ninja will create two nodes for the output file, one with a relative
path and one with an absolute path.  A depfile may then mention either
form of the path and Ninja will hook up the dependency.  Unfortunately
Ninja will also stat the file twice.

Issue: #13894
Fixes: #21865

1 files changed, 13 insertions, 0 deletions

diff --git a/Source/cmLocalNinjaGenerator.cxx b/Source/cmLocalNinjaGenerator.cxx
index 9eb3e46..fb6c730 100644
--- a/Source/cmLocalNinjaGenerator.cxx
+++ b/Source/cmLocalNinjaGenerator.cxx
@@ -263,6 +263,7 @@ void cmLocalNinjaGenerator::WriteBuildFileTop()
                                           this->GetConfigNames().front());
   }
   this->WriteNinjaFilesInclusionCommon(this->GetCommonFileStream());
+  this->WriteNinjaWorkDir(this->GetCommonFileStream());
 
   // For the rule file.
   this->WriteProjectHeader(this->GetRulesFileStream());
@@ -364,6 +365,17 @@ void cmLocalNinjaGenerator::WriteNinjaFilesInclusionCommon(std::ostream& os)
   os << "\n";
 }
 
+void cmLocalNinjaGenerator::WriteNinjaWorkDir(std::ostream& os)
+{
+  cmGlobalNinjaGenerator::WriteDivider(os);
+  cmGlobalNinjaGenerator::WriteComment(
+    os, "Logical path to working directory; prefix for absolute paths.");
+  cmGlobalNinjaGenerator* ng = this->GetGlobalNinjaGenerator();
+  std::string ninja_workdir = this->GetBinaryDirectory();
+  ng->StripNinjaOutputPathPrefixAsSuffix(ninja_workdir); // Also appends '/'.
+  os << "cmake_ninja_workdir = " << ng->EncodePath(ninja_workdir) << "\n";
+}
+
 void cmLocalNinjaGenerator::WriteProcessedMakefile(std::ostream& os)
 {
   cmGlobalNinjaGenerator::WriteDivider(os);
@@ -654,6 +666,7 @@ void cmLocalNinjaGenerator::WriteCustomCommandBuildStatement(
       cmNinjaBuild build("phony");
       build.Comment = cmStrCat("Phony custom command for ", mainOutput);
       build.Outputs = std::move(ccOutputs.ExplicitOuts);
+      build.WorkDirOuts = std::move(ccOutputs.WorkDirOuts);
       build.ExplicitDeps = std::move(ninjaDeps);
       build.OrderOnlyDeps = orderOnlyDeps;
       gg->WriteBuild(this->GetImplFileStream(fileConfig), build);