fix: should ignore nil client

[morlay]

maybe fix #675

I have another issue

With multi-node builder, but single platform build buildx build --platform=linux/amd64, will cause the nil issue too

[crazy-max]

LGTM, ptal @AkihiroSuda

[thaJeztah]

Fix looks ok to address the issue, but perhaps we should also look into why this slice contains nil values in the first place, @crazy-max ?

[crazy-max]

Multi-node build support and the underlying clients iteration was first introduced while resolving drivers (#37). Client nil check was already there

buildx/build/build.go

Lines 283 to 285 in 7d312ea

	if c == nil {
	continue
	}

but was left out to be included in #535.

but perhaps we should also look into why this slice contains nil values in the first place,

Yes agree, might be an issue with the context dialer while creating the buildkit client for docker-container driver. Will take a look.

[thaJeztah]

Was initially thinking, perhaps the slice is initialized with a length/size (not taking into account that filtering might happen). Haven't looked at the code though (was reading from my phone), but though it wouldn't hurt to leave a comment ☺️🤗

[crazy-max]

Yes you did well :)

[tonistiigi]

This does not look correct and iiuc can return cleanly on error cases as skip over the builds. We should first figure out why this happens. Probably the correct solution is to return an error. If skipping over a node is determined to be better it should be in resolveDriversBase and pick a replacement node instead of skipping over a (part of) build.