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

[Keith Prussing]

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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-- 
Keith Prussing