package uninstalls when adding a new dependency? - bash

Question

In the entrypoint script of a docker -based on alpine linux-, I have the following lines:
#!/bin/sh
echo "============== START ============"
echo $#
NOLOAD=0
FILE=""
RET=1
if [ ! -f /initialized ]; then
echo "not initialized"
apk add --virtual .init-deps bash
echo "bash installed"
echo "Building from server"
apk add --virtual .init-deps git
echo "git installed"
bash load_git.sh "${GIT_SERVER}" "${GIT_USERNAME}" "${GIT_PASSWORD}" "${GIT_BRANCH}"
RET=$?
echo cloning done
fi
echo "just before purging all dependencies"
apk --purge del .init-deps
I expect bash to install, as well as git and the run the load_git.sh script using bash. I am literary logging every other line, but am getting some strange results -before the load_git.sh script is even run:
============== START ============
not initialized
(1/6) Installing ncurses-terminfo-base (6.1_p20191130-r0)
(2/6) Installing ncurses-terminfo (6.1_p20191130-r0)
(3/6) Installing ncurses-libs (6.1_p20191130-r0)
(4/6) Installing readline (8.0.1-r0)
(5/6) Installing bash (5.0.11-r1)
Executing bash-5.0.11-r1.post-install
(6/6) Installing .init-deps (20200109.202215)
Executing busybox-1.31.1-r8.trigger
OK: 18 MiB in 24 packages
bash installed
Building from server
(1/12) Installing ca-certificates (20191127-r0)
(2/12) Installing nghttp2-libs (1.40.0-r0)
(3/12) Installing libcurl (7.67.0-r0)
(4/12) Installing expat (2.2.9-r1)
(5/12) Installing pcre2 (10.34-r1)
(6/12) Installing git (2.24.1-r0)
(7/12) Upgrading .init-deps (20200109.202215 -> 20200109.202216)
(8/12) Purging bash (5.0.11-r1)
Executing bash-5.0.11-r1.pre-deinstall
(9/12) Purging readline (8.0.1-r0)
(10/12) Purging ncurses-libs (6.1_p20191130-r0)
(11/12) Purging ncurses-terminfo (6.1_p20191130-r0)
(12/12) Purging ncurses-terminfo-base (6.1_p20191130-r0)
Executing busybox-1.31.1-r8.trigger
Executing ca-certificates-20191127-r0.trigger
OK: 25 MiB in 25 packages
git installed
/init.sh: line 17: bash: not found
cloning done
outside if statement to get source
just before purging all dependencies
Build failed, starting shell
I can't wrap my head around the fact that upon installing git it also purging bash, this makes no sense at all.
EDIT: this only happens when starting the docker over an ssh connection in a synology nas, when using a local docker image and starting it locally it works perfectly fine.

Answer 1

It looks what's happening here is that the apk add --virtual .init-deps option creates a virtual package and makes it depend on the other packages that just got installed. When you do it a second time, it creates a new virtual package that depends (only) on the second set of packages, and upgrades the virtual package to the new version; when you do that, the first set of packages gets automatically uninstalled.
There are two easy workarounds here: either remove this --virtual .init-deps option, from both lines, or combine all of your installation into a single apk add line.
(It's usually not great practice to download packages on container startup, particularly since deleting and recreating containers is a fairly routine operation. Better practice would be to do this once in the image's Dockerfile
RUN apk add bash git
but also to consider whether you actually need either of these tools to run the application packaged in your image.)

Answer 2

I tried reproducing this and managed with the script below on a remote machine:
#!/bin/sh
if [ ! -f /initialized ]; then
apk add --virtual .init-deps bash
apk add --virtual .init-deps git
bash -c "bash works"
fi
But consecutive runs have varying results:
ubuntu#dev:~$ docker run c81a2d3a5f6b
fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/community/x86_64/APKINDEX.tar.gz
(1/6) Installing ncurses-terminfo-base (6.1_p20191130-r0)
(2/6) Installing ncurses-terminfo (6.1_p20191130-r0)
(3/6) Installing ncurses-libs (6.1_p20191130-r0)
(4/6) Installing readline (8.0.1-r0)
(5/6) Installing bash (5.0.11-r1)
Executing bash-5.0.11-r1.post-install
(6/6) Installing .init-deps (20200109.224208)
Executing busybox-1.31.1-r8.trigger
OK: 15 MiB in 20 packages
(1/12) Installing ca-certificates (20191127-r0)
(2/12) Installing nghttp2-libs (1.40.0-r0)
(3/12) Installing libcurl (7.67.0-r0)
(4/12) Installing expat (2.2.9-r1)
(5/12) Installing pcre2 (10.34-r1)
(6/12) Installing git (2.24.1-r0)
(7/12) Upgrading .init-deps (20200109.224208 -> 20200109.224209)
(8/12) Purging bash (5.0.11-r1)
Executing bash-5.0.11-r1.pre-deinstall
(9/12) Purging readline (8.0.1-r0)
(10/12) Purging ncurses-libs (6.1_p20191130-r0)
(11/12) Purging ncurses-terminfo (6.1_p20191130-r0)
(12/12) Purging ncurses-terminfo-base (6.1_p20191130-r0)
Executing busybox-1.31.1-r8.trigger
Executing ca-certificates-20191127-r0.trigger
OK: 22 MiB in 21 packages
/test.sh: line 5: bash: not found
ubuntu#dev:~$ docker run c81a2d3a5f6b
fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/community/x86_64/APKINDEX.tar.gz
(1/6) Installing ncurses-terminfo-base (6.1_p20191130-r0)
(2/6) Installing ncurses-terminfo (6.1_p20191130-r0)
(3/6) Installing ncurses-libs (6.1_p20191130-r0)
(4/6) Installing readline (8.0.1-r0)
(5/6) Installing bash (5.0.11-r1)
Executing bash-5.0.11-r1.post-install
(6/6) Installing .init-deps (20200109.224207)
Executing busybox-1.31.1-r8.trigger
OK: 15 MiB in 20 packages
OK: 15 MiB in 20 packages
bash works
So I think this is a timing problem in apk