Makefiles with symlink dependencies

I wasn't sure if my complex Makefile was following symlinks to the actual file dependencies. This minimal test seems to show that there are problems using symlinks in makefiles if other rules are needed to the files referenced by those symlinks.

Given a simple dependency of:

f-a1 <- f-a2 <- f-a3

This works as expected:

$ make init
rm f-* s-*
rm: cannot remove 'f-*': No such file or directory
rm: cannot remove 's-*': No such file or directory
make: [Makefile:12: clean] Error 1 (ignored)
ln -s -f f-b2 s-b2
$ make f-a3
touch f-a1
touch f-a2
touch f-a3
$ make f-a3
make: 'f-a3' is up to date.
$ make f-a1
make: 'f-a1' is up to date.
$ touch f-a1
$ make f-a3
touch f-a2
touch f-a3
$ make f-a3
make: 'f-a3' is up to date.

If we replace one of the later dependencies with a symlink to the dependency then things fall apart.

f-b1 <- f-b2 <- s-b2 (this is not a make rule but a symlink) <- f-b3 (this depends on the symlink)

$ make init
rm f-* s-*
ln -s -f f-b2 s-b2
$ make f-b3
make: *** No rule to make target 's-b2', needed by 'f-b3'.  Stop.

The above rule fails because the file referenced by the symlink does not exist. Make does not appear to trace through the symlink to the actual file which would allow it to find the rule that would allow it to build f-b2.

$ make f-b2
touch f-b1
touch f-b2
$ make f-b3
touch f-b3

If the file pointed to by the symlink is created then the rule appears to work.

$ touch f-b1
$ make f-b3
make: 'f-b3' is up to date.

...but it doesn't really work The above shows that that later rule is not linked to the earlier rules. Updating an ancestor does not fire the final rule and make the final target rebuild.

A response from stackoverflow suggested using ($realpath ...)

The internal make version of this function $(realpath) does not work for two reasons: (1) $(realpath) returns an absolute path and thus if a make target is specified as a relative path make will not match them. (2) $(realpath) expands to the empty string when the symlink target doesn't exist. The workaround is to call $(shell realpath --relative-to=. ...) This results in a huge number of process spawns that occur before any target can be run. You would think that this would cause abysmal performance but, in practice, it is very fast. A Makefile with hundreds of these dependencies can run in less than a second.

This works and is demonstrated via make f-c3