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

[Tal Dayan]

A more readable version of the example files and log is available at

https://github.com/FPGAwars/apio/issues/676

On Wed, Jul 16, 2025 at 9:35 AM 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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