CMake Policies Design Discussion: Difference between revisions

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This page describes a proposal for a formal backwards/forwards compatibility feature.

=Motivating Examples=
This page has moved [ here].
Consider code such as
  add_definitions("-DFOO=\\\"hello\\ world\\\"")
which tries to add the option
  -DFOO=\"hello\ world\"
to the compile command line.  The code works in CMake 2.4's <code>Unix Makefiles</code> generator and produces a definition as if
  #define FOO "hello world"
appeared in the source code.  It works only because of the way the makefile generator happens to place the definition string in the command line.  It may not work with the VS IDE generators.
In CMake HEAD we provide the <code>COMPILE_DEFINITIONS</code> directory property so that one may write
  set_property(DIRECTORY PROPERTY COMPILE_DEFINITIONS "FOO=\"hello world\"")
to get the correct behavior on all generators.  Since CMake HEAD contains the appropriate escaping support it is desirable to allow the user to write
  add_definitions("-DFOO=\"hello world\"")
and get the expected behavior.  Unfortunately if we were to start escaping the definitions given to <code>add_definitions</code> we would break compatibility with projects that are already adding their own escapes.  In hindsight we should have either supported escapes from the beginning or make the command give an error that the definition value is not supported, but it is too late now.
A similar problem appears in the <code>add_custom_command</code> command where support for properly escaping all arguments was added late.  The solution currently used by that command is to require the user to add a <code>VERBATIM</code> argument to the command to get proper escaping.  Using that solution for <code>add_definitions</code> would make the user write
  add_definitions(VERBATIM "-DFOO=\"hello world\"")
just to get CMake to do the right thing.  For compatibility CMake would have to implement the wrong behavior by default forever.
==Missing Link Directories==
Projects used to be able to write this (wrong) code and it would work by accident:
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/ B)
where "<code>B</code>" is meant to link "<code>/path/to/</code>".  This code is incorrect
because it asks CMake to link to <code>B</code> but does not provide the proper
linker search path for it.  It used to work by accident because the
<code>-L/path/to</code> would get added as part of the implementation of linking to
A.  The correct code would be
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/ B)
or even better
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/ /path/to/
Currently we provide the <code>CMAKE_OLD_LINK_PATHS</code> variable to allow projects or users to quickly work around the problem.  Full compatibility would require us to support thie behavior by default forever.  That would allow new projects to be written with the same bug.
An alternative is to require all libraries to be linked via full path (where target names are expanded automatically).  Whenever a non-full-path is given we could produce a warning that tells the user to start using <code>find_library</code> or something like that but then implement the old linker search path computation for compatibility.  It is desirable to let projects that have been updated for newer CMake versions tell CMake that they know what they are doing and to not warn or use the compatibility behavior.
=Proposed Solution=
We propose the following solution to this problem.
Each change that introduces a compatibility issue is assigned a new identification number (like CMP_00001).  Then we try to detect cases in user code where it '''might''' be an issue and deal with it.  We can maintain in the implementation of CMake a mapping from policy id to a rule to deal with the issue when encountered.  The possible means of dealing with such cases are:
{| border="1"
|- bgcolor="#abcdef"
! Rule !! Behavior !! Meaning !! Default?
| OLD || old || Project suppresses the diagnostic || never
| NEW || new || Project declares the case has been resolved || never
| WARN || old || Emit warning when case is encountered || most cases
| REQUIRED_IF_USED || new || Must use the new behavior || future CMake release, compatibility can be dropped but still checked for
| REQUIRED_ALWAYS || new || Must use the new behavior || future CMake release, compatibility is dropped as well as the code to check if the file uses this issue
Several releases after a compatibility issue has been introduced we can remove implementation of support for the old behavior and set it to "REQUIRED" internally that requires a project to declare the issue NEW or to be of a CMake version such that we know it would have generated warning. See the feature lifespan example.
==Proposed CMAKE_POLICY Command==
We will introduce a new command for projects to use for setting the rule for each policy.
The signature
  cmake_policy(<OLD|NEW> [policy-id1 [policy-id2 [...]]])
sets a list of policies to use OLD or NEW behavior.  It is an error to specify a policy id that does not exist (because it might refer to a policy id introduced in a future version of CMake).
The signature
  cmake_policy(VERSION <major[.minor[.patch]]>)
sets all rules for policies introduced in the specified version or below to NEW and all other rules to WARN.  It also requires the version of CMake running to be at least that high.
The <code>cmake_minimum_required(VERSION)</code> command can do something similar (but use of new commands versus fixing other issues are distinct cases so the defaults may need to be different).
The signature
pushes or pops the current policy state on a stack.  The stack is automatically pushed/popped when entering a new directory (under <code>add_subdirectory</code> for example).  Within a directory the number of PUSH and POP calls must match or it is an error.  This signature is useful in a .cmake script meant for inclusion in other projects.  It could write
  cmake_policy(NEW CMP_00001)
  ... code using CMP_00001 ...
The command could also provide an alias for each policy id that is more human readable:
Let's define a few example policy ids
{| border="1"
|- bgcolor="#abcdef"
! Id !! Alias !! Description
| CMP_00000 || CMP_POLICY_SPECIFICATION || Projects should specify a cmake_policy command at their top level.
| CMP_00001 || CMP_ESCAPE_DEFINITIONS || Enable proper escaping of definitions added with ADD_DEFINITIONS
| CMP_00002 || CMP_ESCAPE_CUSTOM_COMMANDS || Enable proper escaping of custom command lines with ADD_CUSTOM_COMMAND, make VERBATIM argument an error
| CMP_00003 || CMP_NO_AUTOMATIC_LINK_PATHS || Disable generation of compatibility -L options on link lines
The code
  add_definitions("-DFOO=\\\"hello\\ world\\\"")
can now produce a warning CMP_00001 that CMake does not know whether to escape the definition (only when a non-trivial value is given).
The project may suppress the warning in their release branch:
  add_definitions("-DFOO=\\\"hello\\ world\\\"")
or fix the code:
  add_definitions("-DFOO=\"hello world\"")
Either way CMake knows exactly how to handle the code and does not need to warn.
Similarly, the code
  add_custom_command(OUTPUT foo.txt COMMAND mycmd -with -some -arguments -out foo.txt)
may now produce a warning CMP_00002 that CMake does not know whether to escape the command.
The project may write
  add_custom_command(OUTPUT foo.txt COMMAND mycmd -with -some -arguments -out foo.txt)
to get rid of the warning and use proper escaping.
The code
  target_link_libraries(mytarget /path/to/ B)
may now produce a warning CMP_00003 that library B may not have the correct link directory specified but then generate -L/path/to anyway.  The project may write
  target_link_libraries(mytarget /path/to/ B)
Once the project has fixed all issues related to upgrading to CMake 2.6 it can just write
  cmake_policy(VERSION 2.6)
to get all the updated behavior.  Future versions of CMake (> 2.6) may then warn on their new policies.
==Feature Lifespan Example==
Using the CMP_00001 || CMP_ESCAPE_DEFINITIONS example above let's examine how it would work. In CMake 2.4 definitions had to be escaped by the user which probably does not work on MSVC etc. So in 2.6 we want to start switching over to having CMake handle the escaping.
* CMake 2.4: old behavior, issue CMP_0001 does not even exist
* CMake 2.6: issue CMP_0001 is created and set to WARN and version number 2.6. By default old projects will still work but will receive a warning if they use this feature. The policy can be either made OLD <code>cmake_policy(OLD CMP_0001)</code> with no other changes to the CMakeLists file or the issue can be fixed by changing their code and setting it to NEW <code>cmake_policy(NEW CMP_0001)</code>. Or they can ignore it and just live with the warnings.
* CMake 2.8: Nothing changes, stays as a warning
* CMake 2.10: We change the default value of issue CMP_0001 to REQUIRED_IF_USED.  Implementation of the old behavior is removed, but the check for its need is still present.  If the user code causes CMake to hit the check then it must also have specified <code>cmake_policy(NEW CMP_0001)</code> or <code>cmake_policy(VERSION 2.6)</code> (or higher).  Otherwise an error is produced.
* CMake 2.12: We change the default value of issue CMP_0001 to REQUIRED_ALWAYS.  Implementation of the old behavior '''and''' the check for it are removed.  The user code '''must''' specify <code>cmake_policy(NEW CMP_0001)</code> or <code>cmake_policy(VERSION 2.6)</code> (or higher) whether or not the feature is used.  Otherwise an error is produced.
This solution has the following advantages:
* All compatibility issues are documented and given unique identifiers
** Documentation can include association with CMake version ranges (when introduced, when REQUIRED, etc.)
* We can be more aggressive about detecting cases without worrying about false positives because it is just a warning
* When generating a warning or error message we can reference the feature id and tell the user what to do
* Projects can be converted to newer CMake features incrementally
* Project authors will be motivated by the warnings to update their code, but it will build for users without extra work
* Code written that does not hit one of these issues will work without any special declarations
** The hello-world example does not need to show version specification
* We have an exit-path to remove support for a compatibility feature eventually
** Make projects set the rule to NEW explicitly or via VERSION specification
* Any compatibility bug introduced accidentally and reported by a user can be fixed in CMake by converting it to one of these issues
Note <code>CMP_00000</code> in the above example.  This should be the first (lowest numbered) policy we introduce.  We want all projects to specify a policy version level at the top:
  cmake_policy(VERSION 2.6)
This should somehow interface with <code>cmake_minimum_required</code> since many projects have it already:
  cmake_minimum_required(VERSION 2.4)
We could also consider making it an optional argument to the project command for brevity:
  project(FOO CMAKE 2.6)
It should be a warning to encounter a command other than <code>project</code>, <code>cmake_policy</code> or <code>cmake_minimum_required</code> before a policy version has been set.
==Interaction with Previous Compatibility Features==
CMake 2.4 currently presents all users with the cache entry <code>CMAKE_BACKWARDS_COMPATIBILITY</code>.  The policy mechanism replaces it, but we may still need to present it for old projects and honor it for cases supported by 2.4 in case projects set it.  I propose the following behavior
* CMake 2.6 does not present <code>CMAKE_BACKWARDS_COMPATIBILITY</code> by default
* If the project does not set the policy version (which produces the CMP_00000 warning) or sets the policy version to 2.4 or lower then we add <code>CMAKE_BACKWARDS_COMPATIBILITY</code> to the cache with a starting value of the policy version level.
* It is an error for anything (project or user) to set <code>CMAKE_BACKWARDS_COMPATIBILITY</code> to a value of higher than 2.4.
* Any check of the value performed by CMake 2.4 (for values 2.2 and lower of course) is left in place.
* All other checks (for value 2.4) we have in CMake HEAD right now get converted to policies.
There are a few other variables/properties checked by CMake HEAD that approximate the policy feature.
* The variable <code>CMAKE_OLD_LINK_PATHS</code> is used for CMP_00003 above.
* The global property <code>ALLOW_DUPLICATE_CUSTOM_TARGETS</code> is used to allow multiple <code>ADD_CUSTOM_TARGET</code> commands to specify the same target.
These (and others?) need to be converted to policies appropriately.
CMake 2.4 provides the <code>VERBATIM</code> option to <code>ADD_CUSTOM_COMMAND</code> to enable correct escaping.  This needs to be converted to a policy as follows
* Old behavior is to accept VERBATIM or not and escape accordingly
* New behavior is to escape correctly and reject the VERBATIM option

Latest revision as of 15:41, 30 April 2018

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