nil pointer exception when discarding llbBridgeForwarder

[sipsma]

Getting this exception:

time="2022-09-26T16:03:40Z" level=error msg="/moby.buildkit.v1.frontend.LLBBridge/Solve returned error: rpc error: code = Unknown desc = process \"/_shim uname -m\" did not complete successfully: exit code: 1\n"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb0 pc=0x93778c]

goroutine 249258 [running]:
github.com/moby/buildkit/frontend/gateway.(*llbBridgeForwarder).Discard(0x4000644f20)
        /src/frontend/gateway/gateway.go:356 +0x1ac
github.com/moby/buildkit/solver/llbsolver.(*Solver).Solve(0x400054c4e0, {0x112d028, 0x40019c4330}, {0x4001384000, 0x19}, {0x4001384040, 0x19}, {0x0, 0x0, {0x0, ...}, ...}, ...)
        /src/solver/llbsolver/solver.go:324 +0x1380
github.com/moby/buildkit/control.(*Controller).Solve(0x400055ec80, {0x112d028, 0x40019c4330}, 0x400101c120)
        /src/control/control.go:302 +0x910
github.com/moby/buildkit/api/services/control._Control_Solve_Handler.func1({0x112d028, 0x40019c4330}, {0xf22500, 0x400101c120})
        /src/api/services/control/control.pb.go:1611 +0x7c
github.com/moby/buildkit/util/grpcerrors.UnaryServerInterceptor({0x112d028, 0x40019c4330}, {0xf22500, 0x400101c120}, 0x4001180000, 0x4004a56048)
        /src/util/grpcerrors/intercept.go:14 +0x48
github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1({0x112d028, 0x40019c4330}, {0xf22500, 0x400101c120})
        /src/vendor/github.com/grpc-ecosystem/go-grpc-middleware/chain.go:25 +0x58
go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc.UnaryServerInterceptor.func1({0x112cf80, 0x4003a60140}, {0xf22500, 0x400101c120}, 0x4001180000, 0x4001180020)
        /src/vendor/go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc/interceptor.go:325 +0x578
main.unaryInterceptor.func1({0x112d028, 0x40019c4120}, {0xf22500, 0x400101c120}, 0x4001180000, 0x4001180020)
        /src/cmd/buildkitd/main.go:566 +0x134
github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1({0x112d028, 0x40019c4120}, {0xf22500, 0x400101c120})
        /src/vendor/github.com/grpc-ecosystem/go-grpc-middleware/chain.go:25 +0x58
github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1({0x112d028, 0x40019c4120}, {0xf22500, 0x400101c120}, 0x4001180000, 0x4004a56048)
        /src/vendor/github.com/grpc-ecosystem/go-grpc-middleware/chain.go:34 +0xc4
github.com/moby/buildkit/api/services/control._Control_Solve_Handler({0xeb7a40, 0x400055ec80}, {0x112d028, 0x40019c4120}, 0x4000c5a000, 0x4000a00a20)
        /src/api/services/control/control.pb.go:1613 +0x14c
google.golang.org/grpc.(*Server).processUnaryRPC(0x400060c8c0, {0x11433b0, 0x400338e9c0}, 0x4000f78240, 0x400055b1a0, 0x1a2d078, 0x0)
        /src/vendor/google.golang.org/grpc/server.go:1282 +0xc38
google.golang.org/grpc.(*Server).handleStream(0x400060c8c0, {0x11433b0, 0x400338e9c0}, 0x4000f78240, 0x0)
        /src/vendor/google.golang.org/grpc/server.go:1619 +0xa54
google.golang.org/grpc.(*Server).serveStreams.func1.2(0x40028c04d0, 0x400060c8c0, {0x11433b0, 0x400338e9c0}, 0x4000f78240)
        /src/vendor/google.golang.org/grpc/server.go:921 +0x94
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /src/vendor/google.golang.org/grpc/server.go:919 +0x1f0

When doing the following:

1. Pull alpine image for a few platforms
2. Run the same execop on each of the platform images
3. Build and push a multiplatform image with the results of those execops
4. Pull that new multiplatform image
5. Run the same execops on each pulled platform image

I consistently get that nil pointer exception on step 5. This is all in the context of experimental Dagger code, so I don't have a pure LLB reproducer yet, but can update one when I have a a free min.

Not immediately sure what's happening, it seems like this result must be a typed nil: https://github.com/sipsma/buildkit/blob/61307b8f2e73d681721b51856c10cb8606e5ea70/frontend/gateway/gateway.go#L601-L601

But not sure if that's an expected but unhandled case or a totally unexpected case.

[tonistiigi]

I think I fixed similar cases in #3025 and @jedevc did the same for refs.