Image build doesn't use caches

[moneytoo]

When I repeatedly attempt to build an image, it always assembles it without using any cached layer. I tried various options (nocache=False, layers=True etc.) but was unsuccessful.

My Dockerfile/Containerfile looks for example like this:

FROM docker.io/library/fedora:latest

RUN dnf update -y

RUN echo 'Hey'

Building the image using this library always re-executes all the steps:

import podman

client = podman.PodmanClient(base_url="unix:///tmp/podman.sock")

print(client.version())

image, logs = client.images.build(
	dockerfile="Dockerfile",
	tag="localhost/test-podman",
	path="src/",
	rm=False,
)
for log in logs:
	print(log)

However if I use the docker library instead, it uses caches:

import docker

client = docker.DockerClient(base_url='unix:///tmp/podman.sock')

print(client.version())

image, logs = client.images.build(
	dockerfile="Dockerfile",
	tag="localhost/test-docker",
	path="src/",
	rm=False,
)

for log in logs:
	print(log)

I'm experiencing this issue with podman version 5.2.2 (on Alma Linux 9), version 4.9.3 (Ubuntu 24.04 in WSL), version 5.4.1 (on mac OS).

$ time python run-podman.py

real    0m11.543s
user    0m0.118s
sys     0m0.027s
$ time python run-podman.py

real    0m12.186s
user    0m0.109s
sys     0m0.033s
$ time python run-docker.py

real    0m12.512s
user    0m0.147s
sys     0m0.011s
$ time python run-docker.py

real    0m0.323s
user    0m0.132s
sys     0m0.000s

[inknos]

definitely a bug. can reproduce on Fedora 42, thanks for the report.

[fulminemizzega]

If it helps, I've just encountered the same on fedora 42, it looks like docker-py is using the compat API (/build) and podman-py uses /libpod/build.

[fulminemizzega]

I compared how podman-remote works to this library, by defaults it sends many more things. Using this command:

podman-remote --build-arg base=quay.io/fedora-ostree-desktops/base-atomic:42 -v $(pwd)/dnf-cache:/var/cache/libdnf5:z -t sidecar  .

I see all these parameters sent in the http request

{'buildargs': '{"base":"quay.io/fedora-ostree-desktops/base-atomic:42"}',
          'compatvolumes': '0',
          'dockerfile': '["Containerfile"]',
          'forcerm': '1',
          'httpproxy': '1',
          'identitylabel': '1',
          'idmappingoptions': '{"HostUIDMapping":true,"HostGIDMapping":true,"UIDMap":[],"GIDMap":[],"AutoUserNs":false,"AutoUserNsOpts":{"Size":0,"InitialSize":0,"PasswdFile":"","GroupFile":"","AdditionalUIDMappings":null,"AdditionalGIDMappings":null}}',
          'inheritlabels': '1',
          'isolation': '0',
          'jobs': '1',
          'layers': '1',
          'networkmode': '0',
          'nsoptions': '[{"Name":"user","Host":true,"Path":""}]',
          'omithistory': '0',
          'output': 'sidecar',
          'outputformat': 'application/vnd.oci.image.manifest.v1+json',
          't': 'sidecar',
          'volume': '[omitted]dnf-cache:/var/cache/libdnf5:z'}

By trial and error I found that setting layers and outputformat as above allows podman to use cached layers. Of these two, the outputformat parameter is not documented in the podman API and I believe it corresponds to the --format flat in podman build docs (see https://docs.podman.io/en/latest/markdown/podman-build.1.html#format). I do not know go, but if I have understood how the code works, these are all the parameters supported and quite a few are not documented: identitylabel, idmappingoptions, isolation, jobs, networkmode (only wrong type), nsoptions, omithistory, output (there is outputs "TBD"), outputformat.

As a possible fix, maybe podman-py could default both layers and outputformat to those values, mirroring how the podman cli works.

[inknos]

Thanks for the detailed report

As a possible fix, maybe podman-py could default both layers and outputformat to those values, mirroring how the podman cli works.

I fully agree.

Are you interested in opening a PR to fix it? otherwise I'll put this in my stack of things to work on :)

[fulminemizzega]

Sure, I'll make a PR.

[inknos]

thanks @fulminemizzega , don't hesitate to ping me for help or to review your PR