export: Add policy CMP0024 to disallow include() of export files

Currently, export() is executed at configure-time.

One problem with this is that certain exported properties like
the link interface may not be complete at the point the export() is
encountered leading to an incorrect or incomplete exported
representation. Additionally, the generated IMPORTED_LOCATION
property may even be incorrect if commands following the export()
have an effect on it.

Another problem is that it requires the C++ implementation of cmake
to be capable of computing the exported information at configure time.
This is a limitation on the cleanup and maintenance of the code. At
some point in the future, this limitation will be dropped and more
implementation will be moved from cmTarget to cmGeneratorTarget.

5 files changed, 74 insertions, 1 deletions

diff --git a/Source/cmExportCommand.cxx b/Source/cmExportCommand.cxx
index f059ceb..4558fb6 100644
--- a/Source/cmExportCommand.cxx
+++ b/Source/cmExportCommand.cxx
@@ -170,6 +170,8 @@ bool cmExportCommand
   ebfg.SetCommand(this);
   ebfg.SetExportOld(this->ExportOld.IsEnabled());
 
+  this->Makefile->AddExportedTargetsFile(fname);
+
   // Compute the set of configurations exported.
   std::vector<std::string> configurationTypes;
   this->Makefile->GetConfigurations(configurationTypes);
diff --git a/Source/cmIncludeCommand.cxx b/Source/cmIncludeCommand.cxx
index bb891d6..a3d8b83 100644
--- a/Source/cmIncludeCommand.cxx
+++ b/Source/cmIncludeCommand.cxx
@@ -88,6 +88,45 @@ bool cmIncludeCommand
       fname = mfile.c_str();
       }
     }
+
+  std::string fname_abs =
+      cmSystemTools::CollapseFullPath(fname.c_str(),
+                                      this->Makefile->GetStartDirectory());
+
+  if (this->Makefile->IsExportedTargetsFile(fname_abs))
+    {
+    const char *modal = 0;
+    cmake::MessageType messageType = cmake::AUTHOR_WARNING;
+
+    switch(this->Makefile->GetPolicyStatus(cmPolicies::CMP0024))
+      {
+      case cmPolicies::WARN:
+        modal = "should";
+      case cmPolicies::OLD:
+        break;
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS:
+      case cmPolicies::NEW:
+        modal = "may";
+        messageType = cmake::FATAL_ERROR;
+      }
+    if (modal)
+      {
+      cmOStringStream e;
+      e << (this->Makefile->GetPolicies()
+            ->GetPolicyWarning(cmPolicies::CMP0024)) << "\n";
+      e << "The file\n  " << fname_abs << "\nwas generated by the export() "
+        "command.  It " << modal << " not be used as the argument to the "
+        "include() command.  Use ALIAS targets instead to refer to targets "
+        "by alternative names.\n";
+      this->Makefile->IssueMessage(messageType, e.str().c_str());
+      if (messageType == cmake::FATAL_ERROR)
+        {
+        return false;
+        }
+      }
+    }
+
   std::string fullFilePath;
   bool readit =
     this->Makefile->ReadListFile( this->Makefile->GetCurrentListFile(),
diff --git a/Source/cmMakefile.h b/Source/cmMakefile.h
index 8bce9fd..362b066 100644
--- a/Source/cmMakefile.h
+++ b/Source/cmMakefile.h
@@ -489,6 +489,18 @@ public:
       return this->cmCurrentListFile.c_str();
     }
 
+  void AddExportedTargetsFile(const std::string &filename)
+  {
+    this->ExportedTargetsFiles.insert(filename);
+  }
+
+  bool IsExportedTargetsFile(const std::string &filename) const
+  {
+    const std::set<std::string>::const_iterator it
+                                  = this->ExportedTargetsFiles.find(filename);
+    return it != this->ExportedTargetsFiles.end();
+  }
+
   //@}
 
   /**
@@ -1041,7 +1053,7 @@ private:
   void EnforceDirectoryLevelRules();
 
   bool GeneratingBuildSystem;
-
+  std::set<std::string> ExportedTargetsFiles;
   /**
    * Old version of GetSourceFileWithOutput(const char*) kept for
    * backward-compatibility. It implements a linear search and support
diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index a823f05..45670b3 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -601,6 +601,25 @@ cmPolicies::cmPolicies()
     "The NEW behavior for this policy is to not to allow mixing of the "
     "keyword and plain signatures.",
     2,8,12,0, cmPolicies::WARN);
+
+  this->DefinePolicy(
+    CMP0024, "CMP0024",
+    "Disallow include export result.",
+    "CMake 2.8.12 and lower allowed use of the include() command with the "
+    "result of the export() command.  This relies on the assumption that "
+    "the export() command has an immediate effect at configure-time during a "
+    "cmake run.  Certain properties of targets are not fully determined "
+    "until later at generate-time, such as the link language and complete "
+    "list of link libraries.  Future refactoring will change the effect of "
+    "the export() command to be executed at generate-time.  Use ALIAS "
+    "targets instead in cases where the goal is to refer to targets by "
+    "another name"
+    "\n"
+    "The OLD behavior for this policy is to allow including the result "
+    "of an export() command.  "
+    "The NEW behavior for this policy is to not to allow including the "
+    "result of an export() command.",
+    2,8,13,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()
diff --git a/Source/cmPolicies.h b/Source/cmPolicies.h
index 5b843a9..bafe5b2 100644
--- a/Source/cmPolicies.h
+++ b/Source/cmPolicies.h
@@ -74,6 +74,7 @@ public:
     /// target property
     CMP0022, ///< INTERFACE_LINK_LIBRARIES defines the link interface
     CMP0023, ///< Disallow mixing keyword and plain tll signatures
+    CMP0024, ///< Disallow including export() result.
 
     /** \brief Always the last entry.
      *