🧹 chore: Add env var support to go module

go/README.md

@@ -13,3 +13,26 @@ Run:
 ```
 dagger check -l
 ```
+
+## Environment variables
+
+Pass `--env-vars` to set environment variables (in `KEY=VALUE` format) in the
+Go container, e.g. for `GOEXPERIMENT` flags. As a toolchain, set it via a
+customization in `dagger.json`:
+
+```json
+{
+  "toolchains": [
+    {
+      "name": "go",
+      "source": "github.com/papercomputeco/daggerverse/go@main",
+      "customizations": [
+        {
+          "argument": "envVars",
+          "default": "GOEXPERIMENT=jsonv2"
+        }
+      ]
+    }
+  ]
+}
+```

go/main.go

@@ -5,37 +5,67 @@ import (
 	"dagger/go/internal/dagger"
 	"errors"
 	"fmt"
+	"strings"
 )
 
 type Go struct {
 	// +private
 	Source *dagger.Directory
+
+	// EnvVars is an optional list of environment variables to set in the
+	// Go container. Each entry must be in "KEY=VALUE" format.
+	// This is useful for build-time variables like GOEXPERIMENT.
+	//
+	// +private
+	EnvVars []string
 }
 
 func New(
 	// +defaultPath="/"
 	source *dagger.Directory,
+
+	// Optional environment variables to set in the Go container.
+	// Each entry must be in "KEY=VALUE" format (e.g. "GOEXPERIMENT=jsonv2").
+	// +optional
+	envVars []string,
 ) *Go {
 	return &Go{
-		Source: source,
+		Source:  source,
+		EnvVars: envVars,
 	}
 }
 
-func (g *Go) goContainer() *dagger.Container {
-	return dag.Container().
+func (g *Go) goContainer() (*dagger.Container, error) {
+	ctr := dag.Container().
 		From("golang:1.26-bookworm").
 		WithMountedCache("/go/pkg/mod", dag.CacheVolume("go-mod")).
 		WithMountedCache("/root/.cache/go-build", dag.CacheVolume("go-build")).
 		WithWorkdir("/src").
 		WithDirectory("/src", g.Source)
+
+	// Apply caller-provided environment variables.
+	for _, env := range g.EnvVars {
+		parts := strings.SplitN(env, "=", 2)
+		if len(parts) != 2 || parts[0] == "" {
+			return nil, fmt.Errorf("invalid env var %q: must be in KEY=VALUE format", env)
+		}
+		ctr = ctr.WithEnvVariable(parts[0], parts[1])
+	}
+
+	return ctr, nil
 }
 
 // CheckGoModTidy runs "go mod tidy" and fails if it produces any changes to
 // go.mod or go.sum, indicating that the caller forgot to tidy before committing.
 //
 // +check
 func (g *Go) CheckGoModTidy(ctx context.Context) (string, error) {
-	out, err := g.goContainer().
+	ctr, err := g.goContainer()
+	if err != nil {
+		return "", fmt.Errorf("could not create go container: %w", err)
+	}
+
+	out, err := ctr.
 		WithExec([]string{"cp", "go.mod", "go.mod.HEAD"}).
 		WithExec([]string{"cp", "go.sum", "go.sum.HEAD"}).
 		WithExec([]string{"go", "mod", "tidy"}).
@@ -62,7 +92,12 @@ func (g *Go) CheckGoModTidy(ctx context.Context) (string, error) {
 //
 // +check
 func (g *Go) CheckGoVet(ctx context.Context) (string, error) {
-	return g.goContainer().
+	ctr, err := g.goContainer()
+	if err != nil {
+		return "", fmt.Errorf("could not create go container: %w", err)
+	}
+
+	return ctr.
 		WithExec([]string{"go", "vet", "./..."}).
 		Stdout(ctx)
 }