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

[Tal Dayan]

Looking at the end state of scons after invocation #3, the actual md5 of
file2 doesn't match its md5 in the dblite.

https://i.imgur.com/NGco3yQ.png

On Wed, Jul 16, 2025 at 10:33 AM Tal Dayan <tal at zapta.com> wrote:

> 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
>> >
>> > ----------------------------------------------
>> >
>> > _______________________________________________
>> > Scons-users mailing list
>> > Scons-users at scons.org
>> > https://pairlist4.pair.net/mailman/listinfo/scons-users
>>
>>
>>
>> --
>> Keith Prussing
>> _______________________________________________
>> Scons-users mailing list
>> Scons-users at scons.org
>> https://pairlist4.pair.net/mailman/listinfo/scons-users
>>
>
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