export(): raise an error on multiple calls with same FILE

Fixes: 20472

Source/cmExportCommand.cxx

@@ -24,6 +24,7 @@
 #include "cmMessageType.h"
 #include "cmPolicies.h"
 #include "cmStateTypes.h"
+#include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 #include "cmTarget.h"
 
@@ -183,6 +184,28 @@ bool cmExportCommand(std::vector<std::string> const& args,
     return false;
   }
 
+  // if cmExportBuildFileGenerator is already defined for the file
+  // and APPEND is not specified, if CMP0103 is OLD ignore previous definition
+  // else raise an error
+  if (gg->GetExportedTargetsFile(fname) != nullptr) {
+    switch (mf.GetPolicyStatus(cmPolicies::CMP0103)) {
+      case cmPolicies::WARN:
+        mf.IssueMessage(
+          MessageType::AUTHOR_WARNING,
+          cmStrCat(cmPolicies::GetPolicyWarning(cmPolicies::CMP0103), '\n',
+                   "export() command already specified for the file\n  ",
+                   arguments.Filename, "\nDid you miss 'APPEND' keyword?"));
+        CM_FALLTHROUGH;
+      case cmPolicies::OLD:
+        break;
+      default:
+        status.SetError(cmStrCat("command already specified for the file\n  ",
+                                 arguments.Filename,
+                                 "\nDid you miss 'APPEND' keyword?"));
+        return false;
+    }
+  }
+
   // Setup export file generation.
   std::unique_ptr<cmExportBuildFileGenerator> ebfg = nullptr;
   if (android) {

Source/cmPolicies.h

@@ -305,7 +305,10 @@ class cmMakefile;
          17, 0, cmPolicies::WARN)                                             \
   SELECT(POLICY, CMP0102,                                                     \
          "mark_as_advanced() does nothing if a cache entry does not exist.",  \
-         3, 17, 0, cmPolicies::WARN)
+         3, 17, 0, cmPolicies::WARN)                                          \
+  SELECT(POLICY, CMP0103,                                                     \
+         "multiple export() with same FILE without APPEND is not allowed.",   \
+         3, 18, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \