Allow for user defined functions

Signed-off-by: Patrick Van Stee <patrick@vanstee.me>

bake/bake.go

@@ -11,6 +11,7 @@ import (
 	"github.com/docker/buildx/build"
 	"github.com/docker/buildx/util/platformutil"
 	"github.com/docker/docker/pkg/urlutil"
+	hcl "github.com/hashicorp/hcl/v2"
 	"github.com/moby/buildkit/session/auth/authprovider"
 	"github.com/pkg/errors"
 )
@@ -73,6 +74,7 @@ type Config struct {
 	Variables []*Variable `json:"-" hcl:"variable,block"`
 	Groups    []*Group    `json:"groups" hcl:"group,block"`
 	Targets   []*Target   `json:"targets" hcl:"target,block"`
+	Remain    hcl.Body    `json:"-" hcl:",remain"`
 }
 
 func mergeConfig(c1, c2 Config) Config {

bake/hcl.go

@@ -5,7 +5,9 @@ import (
 	"strings"
 
 	hcl "github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/ext/userfunc"
 	"github.com/hashicorp/hcl/v2/hclsimple"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/function"
 	"github.com/zclconf/go-cty/cty/function/stdlib"
@@ -14,7 +16,7 @@ import (
 // Collection of generally useful functions in cty-using applications, which
 // HCL supports. These functions are available for use in HCL files.
 var (
-	functions = map[string]function.Function{
+	stdlibFunctions = map[string]function.Function{
 		"absolute":               stdlib.AbsoluteFunc,
 		"add":                    stdlib.AddFunc,
 		"and":                    stdlib.AndFunc,
@@ -71,6 +73,21 @@ type staticConfig struct {
 }
 
 func ParseHCL(dt []byte, fn string) (*Config, error) {
+	// Decode user defined functions.
+	file, diags := hclsyntax.ParseConfig(dt, fn, hcl.Pos{Line: 1, Column: 1})
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	userFunctions, _, diags := userfunc.DecodeUserFunctions(file.Body, "function", func() *hcl.EvalContext {
+		return &hcl.EvalContext{
+			Functions: stdlibFunctions,
+		}
+	})
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
 	var sc staticConfig
 
 	// Decode only variable blocks without interpolation.
@@ -78,9 +95,8 @@ func ParseHCL(dt []byte, fn string) (*Config, error) {
 		return nil, err
 	}
 
-	variables := make(map[string]cty.Value)
-
 	// Set all variables to their default value if defined.
+	variables := make(map[string]cty.Value)
 	for _, variable := range sc.Variables {
 		variables[variable.Name] = cty.StringVal(variable.Default)
 	}
@@ -94,6 +110,14 @@ func ParseHCL(dt []byte, fn string) (*Config, error) {
 		}
 	}
 
+	functions := make(map[string]function.Function)
+	for k, v := range stdlibFunctions {
+		functions[k] = v
+	}
+	for k, v := range userFunctions {
+		functions[k] = v
+	}
+
 	ctx := &hcl.EvalContext{
 		Variables: variables,
 		Functions: functions,

bake/hcl_test.go

@@ -93,6 +93,36 @@ func TestParseHCL(t *testing.T) {
 		require.Equal(t, "124", c.Targets[0].Args["buildno"])
 	})
 
+	t.Run("WithUserDefinedFunctions", func(t *testing.T) {
+		dt := []byte(`
+		function "increment" {
+			params = [number]
+			result = number + 1
+		}
+
+		group "default" {
+			targets = ["webapp"]
+		}
+
+		target "webapp" {
+			args = {
+				buildno = "${increment(123)}"
+			}
+		}
+		`)
+
+		c, err := ParseHCL(dt, "docker-bake.hcl")
+		require.NoError(t, err)
+
+		require.Equal(t, 1, len(c.Groups))
+		require.Equal(t, "default", c.Groups[0].Name)
+		require.Equal(t, []string{"webapp"}, c.Groups[0].Targets)
+
+		require.Equal(t, 1, len(c.Targets))
+		require.Equal(t, c.Targets[0].Name, "webapp")
+		require.Equal(t, "124", c.Targets[0].Args["buildno"])
+	})
+
 	t.Run("WithVariables", func(t *testing.T) {
 		dt := []byte(`
 		variable "BUILD_NUMBER" {