Explicitly Appending Environment State to a SCons Builder Dependency MD5 Fingerprint

Question

I have modified my SCons Builder action to use subprocess.Popen() instead of env.Execute(). But I still need to add some md5-sum dependency check to the contents of env so that it rebuilds and caches new target files. How do I append to the SCons internal MD5 hash?

Answer 1

Suppose you have a builder that uses an environment variable:

# Just an example. The point is that the output of this function
# depends upon an environment variable
def bld_func(target, source, env):
    with open(str(target[0]), 'w') as f:
        f.write(' '.join(str(t) for t in target) + '\n')
        f.write(' '.join(str(s) for s in source) + '\n')
        f.write(env.subst('$MY_VAR') + '\n')
bld = Builder(action=bld_func)

and a SConstruct that invokes it:

env = Environment()
env['BUILDERS']['BLD'] = bld
env.BLD('output.txt', 'input.txt', MY_VAR=ARGUMENTS['MY_VAR'])

This builder won't be re-executed if you change the value of MY_VAR:

$ scons -Q MY_VAR=cc output.txt
bld_func(["output.txt"], ["input.txt"])
$ scons -Q MY_VAR=gcc output.txt
scons: `output.txt' is up to date.

But, you can add a dependency of the value of a string with env.Value:

env.Depends('output.txt', env.Value(env.subst('$MY_VAR')))

The logical place to put such a dependency is in an emitter:

def bld_emitter(target, source, env):
    env.Depends(target, env.Value(env.subst('$MY_VAR')))
    return target, source
bld = Builder(action=bld_func, emitter=bld_emitter)

Complete SConstruct:

# Create a builder that depends upon an environment variable
def bld_func(target, source, env):
    with open(str(target[0]), 'w') as f:
        f.write(' '.join(str(t) for t in target) + '\n')
        f.write(' '.join(str(s) for s in source) + '\n')
        f.write(env.subst('$MY_VAR') + '\n')
def bld_emitter(target, source, env):
    env.Depends(target, env.Value(env.subst('$MY_VAR')))
    return target, source
bld = Builder(action=bld_func, emitter=bld_emitter)

# Attach it to an environment
env = Environment()
env['BUILDERS']['BLD'] = bld

# Invoke the builder
env.BLD('output.txt', 'input.txt', MY_VAR=ARGUMENTS['MY_VAR'])

Sample output:

$ scons -Q MY_VAR=cc output.txt
bld_func(["output.txt"], ["input.txt"])
$ scons -Q MY_VAR=gcc output.txt
bld_func(["output.txt"], ["input.txt"])
$ scons -Q MY_VAR=gcc output.txt
scons: `output.txt' is up to date.