Ninja: Avoid empty phony edges for target ordering

Since commit v3.9.0-rc1~230^2~2 (ninja: break unnecessary target
dependencies, 2017-04-17) we unconditionally generate a phony edge for
target ordering.  It is needed in case a later target depends on it.
However, if the phony edge has no inputs then `ninja -d explain` prints:

    ninja explain: output ... of phony edge with no inputs doesn't exist

Furthermore the phony edge's output is considered dirty and can cause
dependents to be incorrectly considered dirty.  Avoid this by always
generating at least one input to the target ordering phony edges.
If we have no real dependencies just use a path that always exists.

Fixes: #17942

4 files changed, 25 insertions, 0 deletions

diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index 4ce3cef..7dfce57 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -821,6 +821,19 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
   orderOnlyDeps.erase(std::unique(orderOnlyDeps.begin(), orderOnlyDeps.end()),
                       orderOnlyDeps.end());
 
+  // The phony target must depend on at least one input or ninja will explain
+  // that "output ... of phony edge with no inputs doesn't exist" and consider
+  // the phony output "dirty".
+  if (orderOnlyDeps.empty()) {
+    // Any path that always exists will work here.  It would be nice to
+    // use just "." but that is not supported by Ninja < 1.7.
+    std::string tgtDir;
+    tgtDir += this->LocalGenerator->GetCurrentBinaryDirectory();
+    tgtDir += "/";
+    tgtDir += this->LocalGenerator->GetTargetDirectory(this->GeneratorTarget);
+    orderOnlyDeps.push_back(this->ConvertToNinjaPath(tgtDir));
+  }
+
   {
     cmNinjaDeps orderOnlyTarget;
     orderOnlyTarget.push_back(this->OrderDependsTargetForTarget());
diff --git a/Tests/RunCMake/Ninja/NoWorkToDo-nowork-stdout.txt b/Tests/RunCMake/Ninja/NoWorkToDo-nowork-stdout.txt
new file mode 100644
index 0000000..60a9228
--- /dev/null
+++ b/Tests/RunCMake/Ninja/NoWorkToDo-nowork-stdout.txt
@@ -0,0 +1 @@
+^ninja: no work to do
diff --git a/Tests/RunCMake/Ninja/NoWorkToDo.cmake b/Tests/RunCMake/Ninja/NoWorkToDo.cmake
new file mode 100644
index 0000000..a2fa0cb
--- /dev/null
+++ b/Tests/RunCMake/Ninja/NoWorkToDo.cmake
@@ -0,0 +1,2 @@
+enable_language(C)
+add_executable(hello hello.c)
diff --git a/Tests/RunCMake/Ninja/RunCMakeTest.cmake b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
index 3bb2b6b..b6e6cd4 100644
--- a/Tests/RunCMake/Ninja/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
@@ -21,6 +21,15 @@ function(run_NinjaToolMissing)
 endfunction()
 run_NinjaToolMissing()
 
+function(run_NoWorkToDo)
+  run_cmake(NoWorkToDo)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/NoWorkToDo-build)
+  run_cmake_command(NoWorkToDo-build ${CMAKE_COMMAND} --build .)
+  run_cmake_command(NoWorkToDo-nowork ${CMAKE_COMMAND} --build . -- -d explain)
+endfunction()
+run_NoWorkToDo()
+
 function(run_CMP0058 case)
   # Use a single build tree for a few tests without cleaning.
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CMP0058-${case}-build)