add_custom_command: Diagnose MAIN_DEPENDENCY limitation.

The new policy CMP0057 diagnoses reuse of the same MAIN_DEPENDENCY across
multiple custom commands.

Help/manual/cmake-policies.7.rst

@@ -114,3 +114,4 @@ All Policies
    /policy/CMP0054
    /policy/CMP0055
    /policy/CMP0056
+   /policy/CMP0057

Help/policy/CMP0057.rst

@@ -0,0 +1,21 @@
+CMP0057
+-------
+
+Disallow multiple ``MAIN_DEPENDENCY`` specifications for the same file.
+
+CMake 3.3 and above no longer allow the same input file to be used
+as a ``MAIN_DEPENDENCY`` in more than one custom command.
+
+Listing the same input file more than once in this context has not been
+supported by earlier versions either and would lead to build time issues
+but was not diagnosed.
+
+The ``OLD`` behavior for this policy is to allow using the same input file
+in a ``MAIN_DEPENDENCY`` specfication more than once.
+The ``NEW`` behavior is to disallow using the same input file in a
+``MAIN_DEPENDENCY`` specification more than once.
+
+This policy was introduced in CMake version 3.3.
+CMake version |release| warns when the policy is not set and uses
+``OLD`` behavior.  Use the :command:`cmake_policy` command to set
+it to ``OLD`` or ``NEW`` explicitly.

Help/release/dev/main_dependency_diagnostic.rst

@@ -0,0 +1,6 @@
+main_dependency_diagnostic
+--------------------------
+
+* Listing the same input file as a MAIN_DEPENDENCY of a custom command
+  can lead to broken build time behavior.  This is now diagnosed.
+  See policy :policy:`CMP0057`.

Source/cmMakefile.cxx

@@ -985,6 +985,33 @@ cmMakefile::AddCustomCommandToOutput(const std::vector<std::string>& outputs,
         }
       else
         {
+        std::ostringstream e;
+        cmake::MessageType messageType = cmake::AUTHOR_WARNING;
+        bool issueMessage = false;
+
+        switch(this->GetPolicyStatus(cmPolicies::CMP0057))
+          {
+          case cmPolicies::WARN:
+            e << (this->GetPolicies()->
+              GetPolicyWarning(cmPolicies::CMP0057)) << "\n";
+            issueMessage = true;
+          case cmPolicies::OLD:
+            break;
+          case cmPolicies::NEW:
+          case cmPolicies::REQUIRED_IF_USED:
+          case cmPolicies::REQUIRED_ALWAYS:
+            issueMessage = true;
+            messageType = cmake::FATAL_ERROR;
+            break;
+          }
+
+        if(issueMessage)
+          {
+          e << "\"" << main_dependency << "\" can only be specified as a "
+            "custom command MAIN_DEPENDENCY once.";
+          IssueMessage(messageType, e.str());
+          }
+
         // The existing custom command is different.  We need to
         // generate a rule file for this new command.
         file = 0;

Source/cmPolicies.cxx

@@ -374,6 +374,11 @@ cmPolicies::cmPolicies()
     CMP0056, "CMP0056",
     "Honor link flags in try_compile() source-file signature.",
     3,2,0, cmPolicies::WARN);
+
+  this->DefinePolicy(
+    CMP0057, "CMP0057",
+    "Disallow multiple MAIN_DEPENDENCY specifications for the same file.",
+    3,3,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()

Source/cmPolicies.h

@@ -113,6 +113,8 @@ public:
     /// or keywords when unquoted.
     CMP0055, ///< Strict checking for break() command.
     CMP0056, ///< Honor link flags in try_compile() source-file signature.
+    CMP0057, ///< Disallow multiple MAIN_DEPENDENCY specifications
+    /// for the same file.
 
     /** \brief Always the last entry.
      *

Tests/RunCMake/CMP0057/CMP0057-NEW-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CMP0057/CMP0057-NEW-stderr.txt

@@ -0,0 +1,4 @@
+CMake Error at CMP0057-NEW.cmake:8 \(add_custom_command\):
+  "input.txt" can only be specified as a custom command MAIN_DEPENDENCY once.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)

Tests/RunCMake/CMP0057/CMP0057-NEW.cmake

@@ -0,0 +1,13 @@
+cmake_policy(SET CMP0057 NEW)
+
+add_custom_command(OUTPUT out1
+  COMMAND ${CMAKE_COMMAND} -E echo out1
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_command(OUTPUT out2
+  COMMAND ${CMAKE_COMMAND} -E echo out2
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_target(mytarget1 ALL DEPENDS out1 out2)

Tests/RunCMake/CMP0057/CMP0057-OLD.cmake

@@ -0,0 +1,13 @@
+cmake_policy(SET CMP0057 OLD)
+
+add_custom_command(OUTPUT out1
+  COMMAND ${CMAKE_COMMAND} -E echo out1
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_command(OUTPUT out2
+  COMMAND ${CMAKE_COMMAND} -E echo out2
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_target(mytarget1 ALL DEPENDS out1 out2)

Tests/RunCMake/CMP0057/CMP0057-WARN-stderr.txt

@@ -0,0 +1,9 @@
+CMake Warning \(dev\) at CMP0057-WARN.cmake:6 \(add_custom_command\):
+  Policy CMP0057 is not set: Disallow multiple MAIN_DEPENDENCY specifications
+  for the same file.  Run "cmake --help-policy CMP0057" for policy details.
+  Use the cmake_policy command to set the policy and suppress this warning.
+
+  "input.txt" can only be specified as a custom command MAIN_DEPENDENCY once.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/CMP0057/CMP0057-WARN.cmake

@@ -0,0 +1,11 @@
+add_custom_command(OUTPUT out1
+ COMMAND ${CMAKE_COMMAND} -E echo out1
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_command(OUTPUT out2
+  COMMAND ${CMAKE_COMMAND} -E echo out2
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_target(mytarget1 ALL DEPENDS out1 out2)

Tests/RunCMake/CMP0057/CMP0057-once_is_ok.cmake

@@ -0,0 +1,8 @@
+cmake_policy(SET CMP0057 NEW)
+
+add_custom_command(OUTPUT out1
+  COMMAND ${CMAKE_COMMAND} -E echo out1
+  MAIN_DEPENDENCY input.txt
+)
+
+add_custom_target(mytarget1 ALL DEPENDS out1)

Tests/RunCMake/CMP0057/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.1)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/CMP0057/RunCMakeTest.cmake

@@ -0,0 +1,7 @@
+include(RunCMake)
+
+run_cmake(CMP0057-OLD)
+run_cmake(CMP0057-NEW)
+run_cmake(CMP0057-WARN)
+
+run_cmake(CMP0057-once_is_ok)

Tests/RunCMake/CMakeLists.txt

@@ -63,6 +63,7 @@ add_RunCMake_test(CMP0051)
 add_RunCMake_test(CMP0053)
 add_RunCMake_test(CMP0054)
 add_RunCMake_test(CMP0055)
+add_RunCMake_test(CMP0057)
 add_RunCMake_test(CTest)
 
 if(NOT CMake_TEST_EXTERNAL_CMAKE)