install: Allow absolute EXPORT destination with relative targets (#15258)

When install(EXPORT) is given an absolute destination we cannot compute
the install prefix relative to the installed export file location.
Previously we disallowed installation of targets in such exports with a
relative destination, but did not enforce this for target property
values besides the location of the main target file.  This could lead to
broken installations when the EXPORT is installed to an absolute path
but usage requirements are specified relative to the install prefix.

Since an EXPORT installed to an absolute destination cannot be relocated
we can just hard-code the value of CMAKE_INSTALL_PREFIX as the base for
relative paths.  This will allow absolute install(EXPORT) destinations
to work with relative destinations for targets and usage requirements.

Extend the ExportImport test with a case covering this behavior.

1 files changed, 20 insertions, 37 deletions

diff --git a/Source/cmExportInstallFileGenerator.cxx b/Source/cmExportInstallFileGenerator.cxx
index 23180f1..3f5866a 100644
--- a/Source/cmExportInstallFileGenerator.cxx
+++ b/Source/cmExportInstallFileGenerator.cxx
@@ -69,13 +69,24 @@ bool cmExportInstallFileGenerator::GenerateMainFile(std::ostream& os)
   this->GenerateExpectedTargetsCode(os, expectedTargets);
   }
 
-  // Add code to compute the installation prefix relative to the
-  // import file location.
+  // Set an _IMPORT_PREFIX variable for import location properties
+  // to reference if they are relative to the install prefix.
+  std::string installPrefix =
+    this->IEGen->GetMakefile()->GetSafeDefinition("CMAKE_INSTALL_PREFIX");
   const char* installDest = this->IEGen->GetDestination();
-  if(!cmSystemTools::FileIsFullPath(installDest))
+  if(cmSystemTools::FileIsFullPath(installDest))
     {
-    std::string installPrefix =
-      this->IEGen->GetMakefile()->GetSafeDefinition("CMAKE_INSTALL_PREFIX");
+    // The export file is being installed to an absolute path so the
+    // package is not relocatable.  Use the configured install prefix.
+    os <<
+      "# The installation prefix configured by this project.\n"
+      "set(_IMPORT_PREFIX \"" << installPrefix << "\")\n"
+      "\n";
+    }
+  else
+    {
+    // Add code to compute the installation prefix relative to the
+    // import file location.
     std::string absDest = installPrefix + "/" + installDest;
     std::string absDestS = absDest + "/";
     os << "# Compute the installation prefix relative to this file.\n"
@@ -106,9 +117,6 @@ bool cmExportInstallFileGenerator::GenerateMainFile(std::ostream& os)
       dest = cmSystemTools::GetFilenamePath(dest);
       }
     os << "\n";
-
-    // Import location properties may reference this variable.
-    this->ImportPrefix = "${_IMPORT_PREFIX}/";
     }
 
   std::vector<std::string> missingTargets;
@@ -209,12 +217,9 @@ bool cmExportInstallFileGenerator::GenerateMainFile(std::ostream& os)
      << "\n";
 
   // Cleanup the import prefix variable.
-  if(!this->ImportPrefix.empty())
-    {
-    os << "# Cleanup temporary variables.\n"
-       << "set(_IMPORT_PREFIX)\n"
-       << "\n";
-    }
+  os << "# Cleanup temporary variables.\n"
+     << "set(_IMPORT_PREFIX)\n"
+     << "\n";
   this->GenerateImportedFileCheckLoop(os);
 
   bool result = true;
@@ -394,11 +399,7 @@ cmExportInstallFileGenerator
   if(!cmSystemTools::FileIsFullPath(dest.c_str()))
     {
     // The target is installed relative to the installation prefix.
-    if(this->ImportPrefix.empty())
-      {
-      this->ComplainAboutImportPrefix(itgen);
-      }
-    value = this->ImportPrefix;
+    value = "${_IMPORT_PREFIX}/";
     }
   value += dest;
   value += "/";
@@ -508,24 +509,6 @@ cmExportInstallFileGenerator
   return namespaces;
 }
 
-
-//----------------------------------------------------------------------------
-void
-cmExportInstallFileGenerator
-::ComplainAboutImportPrefix(cmInstallTargetGenerator* itgen)
-{
-  const char* installDest = this->IEGen->GetDestination();
-  cmOStringStream e;
-  e << "install(EXPORT \""
-    << this->IEGen->GetExportSet()->GetName()
-    << "\") given absolute "
-    << "DESTINATION \"" << installDest << "\" but the export "
-    << "references an installation of target \""
-    << itgen->GetTarget()->GetName() << "\" which has relative "
-    << "DESTINATION \"" << itgen->GetDestination() << "\".";
-  cmSystemTools::Error(e.str().c_str());
-}
-
 //----------------------------------------------------------------------------
 void
 cmExportInstallFileGenerator