The cache export step hangs

Hi,

first of all, sorry if this is a double post, but since the other reports I found are a kite old and without recent activity, I decided/try to sum it all here:

Looks like the "preparing build cache for the export" step is hanging pointing to some kind of bug here:

In my last tries, it had more than 1h, no CPU usage, just stuck. Meaning that beside inline cache type, the others are unusable.

How to reproduce:

• dockerfile with multistage build
• cache-to with local or registry type with mode default/max
• first export will work well, however in the second run (maybe due cache-from) it will hang up infinitely

notes:

• I also used a multi-platform build but I don't think it's related
• tested in a GitLab CI pipeline

Samples

Here is the build commands that i used with just some redacted commands:

local type:

    - docker buildx build
      --cache-from=type=local,src=docker_cache/
      --cache-to=type=local,dest=docker_cache/
      --ssh default=...
      --output type=image,name=registry.dev:foo,push=true
      --platform=linux/amd64,linux/arm,linux/arm64
      .

registry type:

    - docker buildx build
      --cache-from=type=registry,ref=registry.dev/cache
      --cache-to=type=registry,ref=registry.dev/cache
      --ssh default=...
      --output type=image,name=registry:foo,push=true
      --platform=linux/amd64,linux/arm,linux/arm64
      .

Other reports that might be related:

• https://github.com/moby/buildkit/issues/1704
• https://github.com/docker/buildx/issues/237

Start by reproducing the hang with a multistage Dockerfile and the local or registry cache-to/cache-from commands shown in the issue, comparing the first and second exports. Read the related reports in moby/buildkit#1704 and docker/buildx#237; done means the second export completes instead of hanging indefinitely.