fix: double go download buildpacks

pkg/buildpacks/builder.go

@@ -19,6 +19,7 @@ import (
 	"github.com/buildpacks/pack/pkg/project/types"
 	"github.com/docker/docker/client"
 	"github.com/heroku/color"
+	"golang.org/x/mod/modfile"
 
 	"knative.dev/func/pkg/builders"
 	"knative.dev/func/pkg/docker"
@@ -186,6 +187,17 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 		opts.Env["BPE_DEFAULT_LISTEN_ADDRESS"] = "[::]:8080"
 	}
 
+	// use "explicit default" go version in function's go.mod file via variable
+	// if not explicitly set via buildEnvs (in func.yaml). This ensures single
+	// go version in buildpacks. See https://github.com/knative/func/issues/3178
+	if f.Runtime == "go" {
+		if _, ok := opts.Env["BP_GO_VERSION"]; !ok {
+			if goVersion := parseGoModVersion(f.Root); goVersion != "" {
+				opts.Env["BP_GO_VERSION"] = goVersion
+			}
+		}
+	}
+
 	var bindings = make([]string, 0, len(f.Build.Mounts))
 	for _, m := range f.Build.Mounts {
 		bindings = append(bindings, fmt.Sprintf("%s:%s", m.Source, m.Destination))
@@ -297,6 +309,24 @@ func BuilderImage(f fn.Function, builderName string) (string, error) {
 	return builders.Image(f, builderName, DefaultBuilderImages)
 }
 
+// parseGoModVersion reads the go.mod file and returns the Go version directive.
+// Returns empty string if go.mod doesn't exist or has no go directive.
+func parseGoModVersion(root string) string {
+	goModPath := filepath.Join(root, "go.mod")
+	data, err := os.ReadFile(goModPath)
+	if err != nil {
+		return ""
+	}
+	f, err := modfile.Parse(goModPath, data, nil)
+	if err != nil {
+		return ""
+	}
+	if f.Go == nil {
+		return ""
+	}
+	return f.Go.Version
+}
+
 // Errors
 
 type ErrRuntimeRequired struct{}

pkg/buildpacks/builder_test.go

@@ -232,6 +232,35 @@ func TestBuild_Errors(t *testing.T) {
 	}
 }
 
+// TestParseGoModVersion tests parsing of Go version from go.mod
+func TestParseGoModVersion(t *testing.T) {
+	tests := []struct {
+		name      string
+		content   string
+		writeFile bool
+		expected  string
+	}{
+		{"valid go version", "module test\n\ngo 1.25", true, "1.25"},
+		{"go version with toolchain", "module test\n\ngo 1.25\ntoolchain go1.25.4", true, "1.25"},
+		{"no go directive", "module test", true, ""},
+		{"no go.mod file", "", false, ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tempDir := t.TempDir()
+			if tt.writeFile {
+				if err := os.WriteFile(filepath.Join(tempDir, "go.mod"), []byte(tt.content), 0644); err != nil {
+					t.Fatal(err)
+				}
+			}
+			if got := parseGoModVersion(tempDir); got != tt.expected {
+				t.Errorf("parseGoModVersion() = %q, want %q", got, tt.expected)
+			}
+		})
+	}
+}
+
 type mockImpl struct {
 	BuildFn func(context.Context, pack.BuildOptions) error
 }