bake: panic when using undefined typed variables as null

### Contributing guidelines

- [x] I've read the [contributing guidelines](https://github.com/docker/buildx/blob/master/.github/CONTRIBUTING.md) and wholeheartedly agree

### I've found a bug and checked that ...

- [x] ... the documentation does not mention anything about my problem
- [x] ... there are no open or closed issues that are related to my problem

### Description

Using a typed variable which has no default value and no provided value will panic in at least two scenarios when being used as build args.  My original scenario was using it in a ternary operation, e.g. `FOO = FOO == null ? "bar" : FOO`.  While coming up with a minimal reproduction, I found that a simple `FOO = FOO` will panic as well, though with a slightly different call stack.  This *only* applies to typed variables.

Given that this is specific to typed variables, I'm assuming the issue is directly related to my previous contributions in this area and will look into it (feel free to assign to me).  Thus, I'm going to skimp on areas of this report as they are likely irrelevant.  (If it ends up being something totally different, I'll add more details at that point.)

### Expected behaviour

A panic should not occur.  Even if there's some technicality behind why my examples are actually invalid, it should result in a user-facing error message.

### Actual behaviour

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x7fcba1]
```

### Buildx version

github.com/docker/buildx v0.29.1 a32761aeb3debd39be1eca514af3693af0db334b

### Docker info

```text

```

### Builders list

```text
NAME/NODE           DRIVER/ENDPOINT                   STATUS     BUILDKIT   PLATFORMS
buildkit-dev        docker-container                                        
 \_ buildkit-dev0    \_ unix:///var/run/docker.sock   inactive              
foo                 docker-container                                        
 \_ foo0             \_ unix:///var/run/docker.sock   running    v0.24.0    linux/amd64 (+4), linux/arm64, linux/arm (+2), linux/ppc64le, (7 more)
jd                  docker-container                                        
 \_ jd0              \_ unix:///var/run/docker.sock   running    v0.22.0    linux/amd64 (+4), linux/arm64, linux/arm (+2), linux/ppc64le, (7 more)
temp                docker-container                                        
 \_ temp0            \_ unix:///var/run/docker.sock   inactive              
default*            docker                                                  
 \_ default          \_ default                       running    v0.25.1    linux/amd64 (+4), linux/arm64, linux/arm (+2), linux/ppc64le, (6 more)
teamx               docker                                                  
 \_ teamx            \_ teamx                         running    v0.25.1    linux/amd64 (+4), linux/arm64, linux/arm (+2), linux/ppc64le, (6 more)
worker              docker                                                  
 \_ worker           \_ worker                        running    v0.24.0    linux/amd64 (+2), linux/arm64, linux/arm (+2), linux/ppc64le, (5 more)
```

### Configuration

Minimal example of my original issue:
```hcl
variable "FOO" {
  type = string
}

target "default" {
  args = {
    ARG = FOO == null ? "yo" : FOO
  }
}
```
Note that `ARG = FOO == null` actually works... it results in `"ARG": "true"`, so it's being evaluated as `null`, which suggests the false expression is the one causing the panic:
```sh
$ docker buildx bake --print
[+] Building 0.0s (1/1) FINISHED                                                                                                                                                                                                                   
 => [internal] load local bake definitions                                                                                                                                                                                                    0.0s
 => => reading docker-bake.hcl 107B / 107B                                                                                                                                                                                                    0.0s
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x7fcba1]

goroutine 1 [running]:
github.com/zclconf/go-cty/cty.Type.FriendlyName(...)
	github.com/zclconf/go-cty@v1.17.0/cty/type.go:47
github.com/hashicorp/hcl/v2/hclsyntax.describeConditionalTypeMismatch({{0x2f03390?, 0xc0000108e5?}}, {{0x0?, 0x0?}})
	github.com/hashicorp/hcl/v2@v2.24.0/hclsyntax/expression.go:1069 +0x401
github.com/hashicorp/hcl/v2/hclsyntax.(*ConditionalExpr).Value(0xc0003aa540, 0xc00000e9a8)
	github.com/hashicorp/hcl/v2@v2.24.0/hclsyntax/expression.go:767 +0x68a
github.com/hashicorp/hcl/v2/hclsyntax.(*ObjectConsExpr).Value(0xc0006946e0, 0xc00000e9a8)
	github.com/hashicorp/hcl/v2@v2.24.0/hclsyntax/expression.go:1211 +0x27d
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.DecodeExpression({0x2c506c0?, 0x2c51858?}, {0x74931a5151d8, 0xc0006946e0}, 0x2f34b00?, {0xc00053ae00, 0xc00036c088})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:288 +0x9b
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.decodeBodyToStruct({0x2c506c0?, 0x2c51858?}, {0x2f03278, 0xc0006872e0}, 0xc00000e9a8, {0x2936ec0?, 0xc00036c000?, 0x4166df?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:138 +0xd67
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.decodeBodyToValue({0x2c506c0?, 0x2c51858?}, {0x2f03278, 0xc0006872e0}, 0xc00000e9a8, {0x2936ec0?, 0xc00036c000?, 0x50d76d?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:57 +0xee
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.DecodeBody({0x2c506c0?, 0x0?}, {0x2f03278, 0xc0006872e0}, 0xc00000e9a8, {0x272d3a0?, 0xc00036c000?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:30 +0x11b
github.com/docker/buildx/bake/hclparser.decodeBody(...)
	github.com/docker/buildx/bake/hclparser/hclparser.go:1239
github.com/docker/buildx/bake/hclparser.(*parser).resolveBlock(0xc000075400, 0xc00036b6c0, 0x0)
	github.com/docker/buildx/bake/hclparser/hclparser.go:525 +0xebd
github.com/docker/buildx/bake/hclparser.Parse({0x2f03240, 0xc00000e6f0}, {0x2c50158?, 0xc000434540?, 0x2c506b8?}, {0x276fa20, 0xc000434570})
	github.com/docker/buildx/bake/hclparser/hclparser.go:929 +0x1f1a
github.com/docker/buildx/bake.ParseFiles({0xc000434510, 0x1, 0x1?}, 0xc000434540)
	github.com/docker/buildx/bake/bake.go:380 +0xa4f
github.com/docker/buildx/bake.ReadTargets({0xc00022af20?, 0x0?}, {0xc000434510?, 0xc0004075f0?, 0x0?}, {0xc0003a2800, 0x1, 0x0?}, {0x0, 0x0, ...}, ...)
	github.com/docker/buildx/bake/bake.go:199 +0x46
github.com/docker/buildx/commands.runBake({_, _}, {_, _}, {_, _, _}, {{0x454d780, 0x0, 0x0}, ...}, ...)
	github.com/docker/buildx/commands/bake.go:231 +0x1f3e
github.com/docker/buildx/commands.bakeCmd.func1(0xc000035208, {0xc0003a2430, 0x0, 0x1})
	github.com/docker/buildx/commands/bake.go:493 +0x372
github.com/docker/cli/cli-plugins/plugin.RunPlugin.func1.1.2(0xc000035208, {0xc0003a2430, 0x0, 0x1})
	github.com/docker/cli@v28.4.0+incompatible/cli-plugins/plugin/plugin.go:65 +0x64
github.com/spf13/cobra.(*Command).execute(0xc000035208, {0xc000406f20, 0x1, 0x1})
	github.com/spf13/cobra@v1.10.1/command.go:1015 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004ad508)
	github.com/spf13/cobra@v1.10.1/command.go:1148 +0x46f
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.10.1/command.go:1071
github.com/docker/cli/cli-plugins/plugin.RunPlugin(0xc0002e8140, 0xc000034c08, {{0x298177a, 0x5}, {0x298be5f, 0xb}, {0x2ec9ad0, 0x7}, {0x0, 0x0}, ...})
	github.com/docker/cli@v28.4.0+incompatible/cli-plugins/plugin/plugin.go:80 +0x145
main.runPlugin(0xc0002e8140)
	github.com/docker/buildx/cmd/buildx/main.go:64 +0xca
main.run(0xc0002e8140)
	github.com/docker/buildx/cmd/buildx/main.go:78 +0xe5
main.main()
	github.com/docker/buildx/cmd/buildx/main.go:88 +0x8c
```

Similarly, this panics as well... `null` is a legitimate value, and it's reasonable to expect that an undefined variable is equivalent to `null` in this usage:
```hcl
variable "FOO" {
  type = string
}

target "default" {
  args = {
    ARG = FOO
  }
}
```
```sh
$ docker buildx bake --print
[+] Building 0.0s (1/1) FINISHED                                                                                                                                                                                                                   
 => [internal] load local bake definitions                                                                                                                                                                                                    0.0s
 => => reading docker-bake.hcl 86B / 86B                                                                                                                                                                                                      0.0s
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x7a4d1e]

goroutine 1 [running]:
github.com/zclconf/go-cty/cty.Type.FriendlyName(...)
	github.com/zclconf/go-cty@v1.17.0/cty/type.go:47
github.com/zclconf/go-cty/cty/convert.MismatchMessage({{0x0?, 0x0?}}, {{0x2f03390?, 0xc0000108e5?}})
	github.com/zclconf/go-cty@v1.17.0/cty/convert/mismatch_msg.go:57 +0x2de
github.com/zclconf/go-cty/cty/convert.mismatchMessageCollectionsFromStructural({{0x2f031d0?, 0xc000069ae0?}}, {{0x2f03198?, 0xc000069b00?}})
	github.com/zclconf/go-cty@v1.17.0/cty/convert/mismatch_msg.go:191 +0x38d
github.com/zclconf/go-cty/cty/convert.MismatchMessage({{0x2f031d0?, 0xc000069ae0?}}, {{0x2f03198?, 0xc000069b00?}})
	github.com/zclconf/go-cty@v1.17.0/cty/convert/mismatch_msg.go:51 +0x45a
github.com/zclconf/go-cty/cty/convert.Convert({{{0x2f031d0?, 0xc000069ae0?}}, {0x259e020?, 0xc00033d680?}}, {{0x2f03198?, 0xc000069b00?}})
	github.com/zclconf/go-cty@v1.17.0/cty/convert/public.go:49 +0x168
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.DecodeExpression({0x2c506c0?, 0x2c51858?}, {0x751c18f50178, 0xc000204640}, 0x2f34b00?, {0xc000310b40, 0xc0003ce088})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:295 +0x114
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.decodeBodyToStruct({0x2c506c0?, 0x2c51858?}, {0x2f03278, 0xc0000c7ec0}, 0xc000780318, {0x2936ec0?, 0xc0003ce000?, 0x4166df?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:138 +0xd67
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.decodeBodyToValue({0x2c506c0?, 0x2c51858?}, {0x2f03278, 0xc0000c7ec0}, 0xc000780318, {0x2936ec0?, 0xc0003ce000?, 0x50d76d?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:57 +0xee
github.com/docker/buildx/bake/hclparser/gohcl.DecodeOptions.DecodeBody({0x2c506c0?, 0x0?}, {0x2f03278, 0xc0000c7ec0}, 0xc000780318, {0x272d3a0?, 0xc0003ce000?})
	github.com/docker/buildx/bake/hclparser/gohcl/decode.go:30 +0x11b
github.com/docker/buildx/bake/hclparser.decodeBody(...)
	github.com/docker/buildx/bake/hclparser/hclparser.go:1239
github.com/docker/buildx/bake/hclparser.(*parser).resolveBlock(0xc000074280, 0xc000357930, 0x0)
	github.com/docker/buildx/bake/hclparser/hclparser.go:525 +0xebd
github.com/docker/buildx/bake/hclparser.Parse({0x2f03240, 0xc000780138}, {0x2c50158?, 0xc000261050?, 0x2c506b8?}, {0x276fa20, 0xc000261080})
	github.com/docker/buildx/bake/hclparser/hclparser.go:929 +0x1f1a
github.com/docker/buildx/bake.ParseFiles({0xc000260ff0, 0x1, 0x1?}, 0xc000261050)
	github.com/docker/buildx/bake/bake.go:380 +0xa4f
github.com/docker/buildx/bake.ReadTargets({0xc0001aaf20?, 0x0?}, {0xc000260ff0?, 0xc000376cc0?, 0x0?}, {0xc000502570, 0x1, 0x0?}, {0x0, 0x0, ...}, ...)
	github.com/docker/buildx/bake/bake.go:199 +0x46
github.com/docker/buildx/commands.runBake({_, _}, {_, _}, {_, _, _}, {{0x454d780, 0x0, 0x0}, ...}, ...)
	github.com/docker/buildx/commands/bake.go:231 +0x1f3e
github.com/docker/buildx/commands.bakeCmd.func1(0xc0004fd208, {0xc0001fe110, 0x0, 0x1})
	github.com/docker/buildx/commands/bake.go:493 +0x372
github.com/docker/cli/cli-plugins/plugin.RunPlugin.func1.1.2(0xc0004fd208, {0xc0001fe110, 0x0, 0x1})
	github.com/docker/cli@v28.4.0+incompatible/cli-plugins/plugin/plugin.go:65 +0x64
github.com/spf13/cobra.(*Command).execute(0xc0004fd208, {0xc000376020, 0x1, 0x1})
	github.com/spf13/cobra@v1.10.1/command.go:1015 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004f5508)
	github.com/spf13/cobra@v1.10.1/command.go:1148 +0x46f
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.10.1/command.go:1071
github.com/docker/cli/cli-plugins/plugin.RunPlugin(0xc00042a3c0, 0xc0004fcc08, {{0x298177a, 0x5}, {0x298be5f, 0xb}, {0x2ec9ad0, 0x7}, {0x0, 0x0}, ...})
	github.com/docker/cli@v28.4.0+incompatible/cli-plugins/plugin/plugin.go:80 +0x145
main.runPlugin(0xc00042a3c0)
	github.com/docker/buildx/cmd/buildx/main.go:64 +0xca
main.run(0xc00042a3c0)
	github.com/docker/buildx/cmd/buildx/main.go:78 +0xe5
main.main()
	github.com/docker/buildx/cmd/buildx/main.go:88 +0x8c
```


### Build logs

```text

```

### Additional info

There are at least two potential workarounds:
1. remove the typing (if it's only used as documentation and not actually relied on)
2. add an explicit default; `default = null` avoids the panic in each of those scenarios

I'm assuming the fix will be effectively implementing the second behind the scenes.

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Uh oh!

bake: panic when using undefined typed variables as null #3463

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bake: panic when using undefined typed variables as null #3463

Description

Contributing guidelines

I've found a bug and checked that ...

Description

Expected behaviour

Actual behaviour

Buildx version

Docker info

Builders list

Configuration

Build logs

Additional info

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions