ExternalData: Tolerate files duplicated across multiple targets

If multiple ExternalData_Target_Add calls generate the same output file
then we need to avoid calling add_custom_command multiple times with
that output.  This was already done within a single target by setting a
variable in the local function scope.  This will not be visible in other
calls though so we need to use a directory property instead to prevent
adding a custom command multiple times for one output in a directory.

Normally it is not safe to have multiple custom commands that produce
the same output file across multiple independent targets, but since we
use atomic replacement of outputs the resulting races should not be a
problem.  For the convenience of projects, tolerate this instead of
diagnosing it.  In particular, we previously allowed up to two copies
of the custom command in one directory because CMake has a fallback
from MAIN_DEPENDENCY to an `<output>.rule` file.

While at it, add a note to the documentation that typically only one
external data target should be needed for a project.

Reported-by: David Manthey <david.manthey@kitware.com>

3 files changed, 27 insertions, 0 deletions

diff --git a/Tests/Module/ExternalData/CMakeLists.txt b/Tests/Module/ExternalData/CMakeLists.txt
index f07ab71..1018dd8 100644
--- a/Tests/Module/ExternalData/CMakeLists.txt
+++ b/Tests/Module/ExternalData/CMakeLists.txt
@@ -53,4 +53,5 @@ ExternalData_Add_Target(Data1)
 add_subdirectory(Data2)
 add_subdirectory(Data3)
 add_subdirectory(Data4)
+add_subdirectory(Data5)
 add_subdirectory(DataNoSymlinks)
diff --git a/Tests/Module/ExternalData/Data5/CMakeLists.txt b/Tests/Module/ExternalData/Data5/CMakeLists.txt
new file mode 100644
index 0000000..13c7fab
--- /dev/null
+++ b/Tests/Module/ExternalData/Data5/CMakeLists.txt
@@ -0,0 +1,22 @@
+# Test adding the same data file in multiple tests
+ExternalData_Add_Test(Data5.A
+  NAME Data5Check.A
+  COMMAND ${CMAKE_COMMAND}
+    -D Data5=DATA{../Data.dat}
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/Data5Check.cmake
+  )
+ExternalData_Add_Target(Data5.A)
+ExternalData_Add_Test(Data5.B
+  NAME Data5Check.B
+  COMMAND ${CMAKE_COMMAND}
+    -D Data5=DATA{../Data.dat}
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/Data5Check.cmake
+  )
+ExternalData_Add_Target(Data5.B)
+ExternalData_Add_Test(Data5.C
+  NAME Data5Check.C
+  COMMAND ${CMAKE_COMMAND}
+    -D Data5=DATA{../Data.dat}
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/Data5Check.cmake
+  )
+ExternalData_Add_Target(Data5.C)
diff --git a/Tests/Module/ExternalData/Data5/Data5Check.cmake b/Tests/Module/ExternalData/Data5/Data5Check.cmake
new file mode 100644
index 0000000..4dea9a4
--- /dev/null
+++ b/Tests/Module/ExternalData/Data5/Data5Check.cmake
@@ -0,0 +1,4 @@
+file(STRINGS "${Data5}" lines LIMIT_INPUT 1024)
+if(NOT "x${lines}" STREQUAL "xInput file already transformed.")
+  message(SEND_ERROR "Input file:\n  ${Data5}\ndoes not have expected content, but [[${lines}]]")
+endif()