bud: layer cache is broken with --build-args

[terceiro]

Description

Building images with the same build args does not use the cache. This worked in v0.19.2, and has been broken by 9b29958. #2967 fixes it for builds with no build args, but having build args makes the cache not be used..

Steps to reproduce the issue:

I'm using the following files:

bisect.sh:

set -eu
if [ -f Makefile ]; then
    make bin/buildah
    basedir=$(pwd)
    export PATH=$(pwd)/bin:$PATH
fi
cd $(dirname $0)
set -x
buildah bud --layers=true --build-arg=N=7
buildah bud --layers=true --build-arg=N=7 . | tee /tmp/log
! grep '^7' /tmp/log

Containerfile:

FROM debian
ARG N
RUN seq 1 ${N}

Then from the buildah source tree, I run the bisect script. First, on the master branch from today (0508fba):

buildah (master) $ sh /tmp/foobar/bisect.sh 
GO111MODULE=on go build -mod=vendor -ldflags '-X main.GitCommit=0508fba0 -X main.buildInfo=1612918285 -X main.cniVersion=v0.8.1 ' -gcflags "" -o bin/buildah -tags "seccomp apparmor btrfs_noversion exclude_graphdriver_btrfs " ./cmd/buildah
+ buildah bud --layers=true --build-arg=N=7
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache c352def690165a2c9a87a59bee469509afcdedf4869d66ac1f100d04667f42ad
--> c352def6901
STEP 3: RUN seq 1 ${N}
1
2
3
4
5
6
7
--> Using cache eec66aa2b27ca0195ff64ab5d7d49081e20a1b62e04311bd64dcf875e1e39505
--> eec66aa2b27
eec66aa2b27ca0195ff64ab5d7d49081e20a1b62e04311bd64dcf875e1e39505
+ buildah bud --layers=true --build-arg=N=7 .
+ tee /tmp/log
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache c352def690165a2c9a87a59bee469509afcdedf4869d66ac1f100d04667f42ad
--> c352def6901
STEP 3: RUN seq 1 ${N}
1
2
3
4
5
6
7
--> Using cache eec66aa2b27ca0195ff64ab5d7d49081e20a1b62e04311bd64dcf875e1e39505
--> eec66aa2b27
eec66aa2b27ca0195ff64ab5d7d49081e20a1b62e04311bd64dcf875e1e39505
+ set +x
7
FAIL

Now, against the latest release where this worked (v0.19.2):

buildah (good) $ git describe --tags
v1.19.2
buildah (good) $ sh /tmp/foobar/bisect.sh 
GO111MODULE=on go build -mod=vendor -ldflags '-X main.GitCommit=7b83c4be -X main.buildInfo=1612918406 -X main.cniVersion=v0.7.2-0.20190904153231-83439463f784 ' -gcflags "" -o bin/buildah -tags "seccomp apparmor btrfs_noversion exclude_graphdriver_btrfs " ./cmd/buildah
+ buildah bud --layers=true --build-arg=N=7
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache abaed72f15975cdfc19f9050cdee3aa0c1b653a1e55843a40614f6b9977a3781
--> abaed72f159
STEP 3: RUN seq 1 ${N}
--> Using cache de565e075cbbf490df04094afcf0e612f863ccf819b7b44035870afdbd31b789
--> de565e075cb
de565e075cbbf490df04094afcf0e612f863ccf819b7b44035870afdbd31b789
+ buildah bud --layers=true --build-arg=N=7 .
+ tee /tmp/log
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache abaed72f15975cdfc19f9050cdee3aa0c1b653a1e55843a40614f6b9977a3781
--> abaed72f159
STEP 3: RUN seq 1 ${N}
--> Using cache de565e075cbbf490df04094afcf0e612f863ccf819b7b44035870afdbd31b789
--> de565e075cb
de565e075cbbf490df04094afcf0e612f863ccf819b7b44035870afdbd31b789
+ set +x

(the later run reuses the layers created by the one from master and cached, so even the first build already does not execute the RUN instruction). But if I change the 7 in the script to something else, say 8, it works as intended:

buildah (good) $ git describe --tags
v1.19.2
buildah (good) $ sh /tmp/foobar/bisect.sh 
GO111MODULE=on go build -mod=vendor -ldflags '-X main.GitCommit=7b83c4be -X main.buildInfo=1612918557 -X main.cniVersion=v0.7.2-0.20190904153231-83439463f784 ' -gcflags "" -o bin/buildah -tags "seccomp apparmor btrfs_noversion exclude_graphdriver_btrfs " ./cmd/buildah
+ buildah bud --layers=true --build-arg=N=8
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache abaed72f15975cdfc19f9050cdee3aa0c1b653a1e55843a40614f6b9977a3781
--> abaed72f159
STEP 3: RUN seq 1 ${N}
1
2
3
4
5
6
7
8
STEP 4: COMMIT
--> 19592c29913
19592c29913abb10d85d8c1086d060520bdfc415eed64272b33a4c425698dcd1
+ buildah bud --layers=true --build-arg=N=8 .
+ tee /tmp/log
STEP 1: FROM debian
STEP 2: ARG N
--> Using cache abaed72f15975cdfc19f9050cdee3aa0c1b653a1e55843a40614f6b9977a3781
--> abaed72f159
STEP 3: RUN seq 1 ${N}
--> Using cache 19592c29913abb10d85d8c1086d060520bdfc415eed64272b33a4c425698dcd1
--> 19592c29913
19592c29913abb10d85d8c1086d060520bdfc415eed64272b33a4c425698dcd1
+ grep ^8 /tmp/log

cc @umohnani8