[Scons-users] Making SCons deal more robustly with incremental linking

[Evan Driscoll]

OK, so this is a bit of a weird request maybe.

I am trying to add an SCons flag that will cause builds with GCC to use 
incremental linking. This boils down to adding "-fuse-ld=gold 
-fno-use-linker-plugin -Wl,--incremental" to LINKFLAGS, but I don't 
think that's so relevant.

When this flag is present, I would like to make two enhancements:
1. Every file on which the linker is invoked should be marked as 
Precious
2. Every file on which the linker is invoked should have itself marked 
as a dependency

For my purposes, a "mostly-there" solution that only is called for 
programs and not shared libraries would be fine. I also am not averse to 
modifying the scons source; we have a local version with some reasonably 
heavy patches anyway.


In case it's not clear, what I mean by #2 is the following: the output 
of the linker depends not only on the .o files listed on the link line, 
but on the initial state of the exe, so the has of the exe before the 
link should contribute to the signature of the build step. Without some 
explicit action that I'm not sure what to do, the following sequence 
will cause a mis-build:
  1. Build under scons
  2. Modify an object file outside of scons
  3. Relink outside of scons
  4. Revert the object file to what it was
  5. Rebuild with scons -- it will not relink because it thinks nothing 
has changed
This sequence of actions would presumably be rare, but is serious if it 
happens: the result could easily be extremely hard to diagnose, and, 
because we share an scons cache between different working copies and 
users, a bad build could poison multiple builds. (This is enough of a 
concern we would quite possibly disable caching for linked files.)

I tried setting PROGEMITTER to something simple that adds the resulting 
exe to the input list, but SCons complained about a circular dependency. 
The main other thing that comes to mind is somewhat of a hack:
  1. For each linked file foo, make something like
       Command('foo.hash', 'foo', 'md5sum $SOURCE > $TARGET')
  2. During SConscript running, read foo.hash, put its contents into a 
Value, and make foo depend on that Value
but that seems a bit ugly... and I'd still have to know how to 
automatically do that for each linked file.

I can also imagine that a scanner could come in handy here, but that's 
an area I'm basically unfamiliar with. And the man page says that it 
should return a list of "File Nodes", meaning that I don't know how to 
get it to return something not in the file system.


(Finally, just as a note: I think I haven't completely solved the 
problem even with the above. Gold will embed a Build ID in the resulting 
exe that will change each time, which means that with a solution to #2 
SCons would always consider it out-of-date. You can specify how it 
generates the Build ID, but I haven't gotten it to work quite right 
yet.)

Evan