Tar: file changed as we read it

[4ilo]

Issue

When using tar to create/modify an archive inside a sandboxfs filesystem, sometimes tar will fail with the file changed as we read it error message.

When using sandboxfs this issue is observed regularly but not consistent. Without sandboxfs we never see the issue.

Minimal example

./BUILD
genrule(
    name = "test",
    tools = ["script.sh"],
    outs = ["test.tar"],
    cmd = "$(location script.sh) $@",
)

./script.sh
#!/bin/sh -e

for x in {0..1000}; do
    mkdir -p workdir
    touch workdir/$x.txt
done

tar -C workdir -cf $1 .

for x in {0..100}; do
    rm -rf workdir/*
    tar -C workdir -xf $1 .
    tar -C workdir -cf $1 .
done

Command: bazel clean && bazel build :test --experimental_use_sandboxfs --genrule_strategy=sandboxed --spawn_strategy=sandboxed

[thekyz]

You can fix that by having your first genrule output a folder then depend on it with a second genrule and tar that. Something like this (untested):

./BUILD
genrule(
    name = "test",
    tools = ["script.sh"],
    outs = ["test.out"],
    cmd = """
mkdir -p $(location test.out)
for x in {0..1000}; do
    touch $(location test.out)/$x.txt
done
    """.
)

genrule(
    name = "tar",
    tools = ["script.sh"],
    srcs = [":test.out"],
    outs = ["test.tar"],
    cmd = """
tar cf $(location test.tar) -C $(location test.out) .
    """.
)

[blockjon-dd]

I'm not sure I understand why the solution proposed by @thekyz is reasonable or valid. Can someone explain?