[Scons-users] SCons misses a dependency which results in incorrect output.

[Tal Dayan]

Hi Keith, I updated the example files here
https://github.com/FPGAwars/apio/issues/676

They now include the md5 of the files and a dump of .sconsign.dblite

On Wed, Jul 16, 2025 at 10:08 AM Keith Prussing <kprussing74 at gmail.com>
wrote:

> I suspect it's because `file1` has the same hash in the
> .sconsign.dblite as the last "good" build (i.e. the first one). Thus
> you get the line "scons: `file2' is up to date." on the third run.
> However, I am not an expert in the specifics of SCons' hashing
> methods.
>
> On Wed, Jul 16, 2025 at 12:38 PM Tal Dayan <tal at zapta.com> wrote:
> >
> > Hi all,
> >
> > We encountered this problem with the nextpnr tool and created here a
> small and independent example that demonstrates it.
> >
> > In the example below, a shell script 'command.sh' reads the source file
> 'file1' and writes it to the target file 'file2'. However, if the
> > source file starts with 'bad' it exits with an error code, *after*
> creating the target file.
> >
> > The script `run.sh', runs scons three times with these values of the
> source file file1 'good', 'bad', and 'good'.  The expectation is that after
> the third scons run, file2 should contain the value 'good' but it contains
> the value 'bad'.
> >
> > Do we miss anything or is it simply a bug?
> >
> > SConstruct:
> >
> > ----------------------------------------------
> >
> > # SCons environment
> >
> > env = Environment()
> >
> >
> > # Copy file1 → file2 using command.sh
> >
> > # Inject an error if file1 starts with 'bad"
> >
> > file2 = env.Command(
> >
> >     target='file2',
> >
> >     source='file1',
> >
> >     action='./command.sh $SOURCE > $TARGET'
> >
> > )
> >
> >
> > # Make 'file2' the default target
> >
> > Default(file2)
> >
> > ----------------------------------------------
> >
> >
> > command.sh:
> >
> > ----------------------------------------------
> >
> > #!/bin/bash
> >
> > # Usage: ./command input > output
> >
> >
> > # Read from the first argument and copy to stdout
> >
> > cat "$1"
> >
> >
> > # If the input file starts with 'bad', inject an error AFTER creating
> the output file.
> >
> >
> > first_line=$(head -n 1 "$1")
> >
> >
> > if [[ "$first_line" == bad* ]]; then
> >
> >   exit 1
> >
> > fi
> >
> >
> > exit 0
> >
> > ----------------------------------------------
> >
> >
> >
> > run.sh
> >
> > ----------------------------------------------
> >
> > #!/bin/bash
> >
> >
> > # Clean up.
> >
> > rm -f .sconsign.dblite
> >
> > rm -f file[12]
> >
> >
> > echo
> >
> > echo "---- Iteration 1: file1 = 'good'"
> >
> > echo "good" > file1
> >
> > scons
> >
> > echo
> >
> >
> > echo "File1"
> >
> > cat -n file1
> >
> >
> > echo "File2"
> >
> > cat -n file2
> >
> >
> >
> > echo
> >
> > echo "---- Iteration 2: file1 = 'bad'"
> >
> > echo "bad" > file1
> >
> > cat -n file1
> >
> > scons
> >
> > echo
> >
> >
> > echo "File1"
> >
> > cat -n file1
> >
> >
> > echo "File2"
> >
> > cat -n file2
> >
> >
> > echo
> >
> > echo "---- Iteration 3: file1 = 'good'"
> >
> > echo "good" > file1
> >
> > cat -n file1
> >
> > scons
> >
> > echo
> >
> >
> > echo "File1"
> >
> > cat -n file1
> >
> >
> > echo "File2"
> >
> > cat -n file2
> >
> > ----------------------------------------------
> >
> >
> >
> > Run log:
> >
> > ----------------------------------------------
> >
> > $ ./run.sh
> >
> >
> > ---- Iteration 1: file1 = 'good'
> >
> > scons: Reading SConscript files ...
> >
> > scons: done reading SConscript files.
> >
> > scons: Building targets ...
> >
> > ./command.sh file1 > file2
> >
> > scons: done building targets.
> >
> >
> > File1
> >
> >      1 good
> >
> > File2
> >
> >      1 good
> >
> >
> > ---- Iteration 2: file1 = 'bad'
> >
> >      1 bad
> >
> > scons: Reading SConscript files ...
> >
> > scons: done reading SConscript files.
> >
> > scons: Building targets ...
> >
> > ./command.sh file1 > file2
> >
> > scons: *** [file2] Error 1
> >
> > scons: building terminated because of errors.
> >
> >
> > File1
> >
> >      1 bad
> >
> > File2
> >
> >      1 bad
> >
> >
> > ---- Iteration 1: file1 = 'good'
> >
> >      1 good
> >
> > scons: Reading SConscript files ...
> >
> > scons: done reading SConscript files.
> >
> > scons: Building targets ...
> >
> > scons: `file2' is up to date.
> >
> > scons: done building targets.
> >
> >
> > File1
> >
> >      1 good
> >
> > File2
> >
> >      1 bad
> >
> > ----------------------------------------------
> >
> > _______________________________________________
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> > Scons-users at scons.org
> > https://pairlist4.pair.net/mailman/listinfo/scons-users
>
>
>
> --
> Keith Prussing
> _______________________________________________
> Scons-users mailing list
> Scons-users at scons.org
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>
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