Allow for user defined functions

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

README.md

@@ -614,13 +614,10 @@ target "db" {
 Complete list of valid target fields:
 	args, cache-from, cache-to, context, dockerfile, inherits, labels, no-cache, output, platform, pull, secrets, ssh, tags, target
 
-#### HCL variable interpolation
+#### HCL variables and functions
+
+Similar to how Terraform provides a way to [define variables](https://www.terraform.io/docs/configuration/variables.html#declaring-an-input-variable), the HCL file format also supports variable block definitions. These can be used to define variables with values provided by the current environment or a default value when unset.
 
-Similar to how Terraform provides a way to
-[define variables](https://www.terraform.io/docs/configuration/variables.html#declaring-an-input-variable),
-the HCL file format also supports variable block definitions. These can be used
-to define variables with values provided by the current environment or a
-default value when unset.
 
 
 Example of using interpolation to tag an image with the git sha:
@@ -667,6 +664,78 @@ $ TAG=$(git rev-parse --short HEAD) docker buildx bake --print webapp
 }
 ```
 
+
+A [set of generally useful functions](https://github.com/docker/buildx/blob/master/bake/hcl.go#L19-L65) provided by [go-cty](https://github.com/zclconf/go-cty/tree/master/cty/function/stdlib) are avaialble for use in HCL files. In addition, [user defined functions](https://github.com/hashicorp/hcl/tree/hcl2/ext/userfunc) are also supported.
+
+
+
+Example of using the `add` function:
+
+```
+$ cat <<'EOF' > docker-bake.hcl
+variable "TAG" {
+	default = "latest"
+}
+
+group "default" {
+	targets = ["webapp"]
+}
+
+target "webapp" {
+	args = {
+		buildno = "${add(123, 1)}"
+	}
+}
+EOF
+
+$ docker buildx bake --print webapp
+{
+   "target": {
+      "webapp": {
+         "context": ".",
+         "dockerfile": "Dockerfile",
+         "args": {
+            "buildno": "124"
+         }
+      }
+   }
+}
+```
+
+Example of defining an `increment` function:
+
+```
+$ cat <<'EOF' > docker-bake.hcl
+function "increment" {
+	params = [number]
+	result = number + 1
+}
+
+group "default" {
+	targets = ["webapp"]
+}
+
+target "webapp" {
+	args = {
+		buildno = "${increment(123)}"
+	}
+}
+EOF
+
+$ docker buildx bake --print webapp
+{
+   "target": {
+      "webapp": {
+         "context": ".",
+         "dockerfile": "Dockerfile",
+         "args": {
+            "buildno": "124"
+         }
+      }
+   }
+}
+```
+
 ### `buildx imagetools create [OPTIONS] [SOURCE] [SOURCE...]`
 
 Imagetools contains commands for working with manifest lists in the registry. These commands are useful for inspecting multi-platform build results.

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" {

vendor/github.com/hashicorp/hcl/v2/ext/userfunc/README.md

@@ -0,0 +1,28 @@
+# HCL User Functions Extension
+
+This HCL extension allows a calling application to support user-defined
+functions.
+
+Functions are defined via a specific block type, like this:
+
+```hcl
+function "add" {
+  params = [a, b]
+  result = a + b
+}
+
+function "list" {
+  params         = []
+  variadic_param = items
+  result         = items
+}
+```
+
+The extension is implemented as a pre-processor for `cty.Body` objects. Given
+a body that may contain functions, the `DecodeUserFunctions` function searches
+for blocks that define functions and returns a functions map suitable for
+inclusion in a `hcl.EvalContext`. It also returns a new `cty.Body` that
+contains the remainder of the content from the given body, allowing for
+further processing of remaining content.
+
+For more information, see [the godoc reference](http://godoc.org/github.com/hashicorp/hcl/v2/ext/userfunc).

vendor/github.com/hashicorp/hcl/v2/ext/userfunc/decode.go

@@ -0,0 +1,156 @@
+package userfunc
+
+import (
+	"github.com/hashicorp/hcl/v2"
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+var funcBodySchema = &hcl.BodySchema{
+	Attributes: []hcl.AttributeSchema{
+		{
+			Name:     "params",
+			Required: true,
+		},
+		{
+			Name:     "variadic_param",
+			Required: false,
+		},
+		{
+			Name:     "result",
+			Required: true,
+		},
+	},
+}
+
+func decodeUserFunctions(body hcl.Body, blockType string, contextFunc ContextFunc) (funcs map[string]function.Function, remain hcl.Body, diags hcl.Diagnostics) {
+	schema := &hcl.BodySchema{
+		Blocks: []hcl.BlockHeaderSchema{
+			{
+				Type:       blockType,
+				LabelNames: []string{"name"},
+			},
+		},
+	}
+
+	content, remain, diags := body.PartialContent(schema)
+	if diags.HasErrors() {
+		return nil, remain, diags
+	}
+
+	// first call to getBaseCtx will populate context, and then the same
+	// context will be used for all subsequent calls. It's assumed that
+	// all functions in a given body should see an identical context.
+	var baseCtx *hcl.EvalContext
+	getBaseCtx := func() *hcl.EvalContext {
+		if baseCtx == nil {
+			if contextFunc != nil {
+				baseCtx = contextFunc()
+			}
+		}
+		// baseCtx might still be nil here, and that's okay
+		return baseCtx
+	}
+
+	funcs = make(map[string]function.Function)
+Blocks:
+	for _, block := range content.Blocks {
+		name := block.Labels[0]
+		funcContent, funcDiags := block.Body.Content(funcBodySchema)
+		diags = append(diags, funcDiags...)
+		if funcDiags.HasErrors() {
+			continue
+		}
+
+		paramsExpr := funcContent.Attributes["params"].Expr
+		resultExpr := funcContent.Attributes["result"].Expr
+		var varParamExpr hcl.Expression
+		if funcContent.Attributes["variadic_param"] != nil {
+			varParamExpr = funcContent.Attributes["variadic_param"].Expr
+		}
+
+		var params []string
+		var varParam string
+
+		paramExprs, paramsDiags := hcl.ExprList(paramsExpr)
+		diags = append(diags, paramsDiags...)
+		if paramsDiags.HasErrors() {
+			continue
+		}
+		for _, paramExpr := range paramExprs {
+			param := hcl.ExprAsKeyword(paramExpr)
+			if param == "" {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid param element",
+					Detail:   "Each parameter name must be an identifier.",
+					Subject:  paramExpr.Range().Ptr(),
+				})
+				continue Blocks
+			}
+			params = append(params, param)
+		}
+
+		if varParamExpr != nil {
+			varParam = hcl.ExprAsKeyword(varParamExpr)
+			if varParam == "" {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid variadic_param",
+					Detail:   "The variadic parameter name must be an identifier.",
+					Subject:  varParamExpr.Range().Ptr(),
+				})
+				continue
+			}
+		}
+
+		spec := &function.Spec{}
+		for _, paramName := range params {
+			spec.Params = append(spec.Params, function.Parameter{
+				Name: paramName,
+				Type: cty.DynamicPseudoType,
+			})
+		}
+		if varParamExpr != nil {
+			spec.VarParam = &function.Parameter{
+				Name: varParam,
+				Type: cty.DynamicPseudoType,
+			}
+		}
+		impl := func(args []cty.Value) (cty.Value, error) {
+			ctx := getBaseCtx()
+			ctx = ctx.NewChild()
+			ctx.Variables = make(map[string]cty.Value)
+
+			// The cty function machinery guarantees that we have at least
+			// enough args to fill all of our params.
+			for i, paramName := range params {
+				ctx.Variables[paramName] = args[i]
+			}
+			if spec.VarParam != nil {
+				varArgs := args[len(params):]
+				ctx.Variables[varParam] = cty.TupleVal(varArgs)
+			}
+
+			result, diags := resultExpr.Value(ctx)
+			if diags.HasErrors() {
+				// Smuggle the diagnostics out via the error channel, since
+				// a diagnostics sequence implements error. Caller can
+				// type-assert this to recover the individual diagnostics
+				// if desired.
+				return cty.DynamicVal, diags
+			}
+			return result, nil
+		}
+		spec.Type = func(args []cty.Value) (cty.Type, error) {
+			val, err := impl(args)
+			return val.Type(), err
+		}
+		spec.Impl = func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+			return impl(args)
+		}
+		funcs[name] = function.New(spec)
+	}
+
+	return funcs, remain, diags
+}

vendor/github.com/hashicorp/hcl/v2/ext/userfunc/doc.go

@@ -0,0 +1,22 @@
+// Package userfunc implements a HCL extension that allows user-defined
+// functions in HCL configuration.
+//
+// Using this extension requires some integration effort on the part of the
+// calling application, to pass any declared functions into a HCL evaluation
+// context after processing.
+//
+// The function declaration syntax looks like this:
+//
+//     function "foo" {
+//       params = ["name"]
+//       result = "Hello, ${name}!"
+//     }
+//
+// When a user-defined function is called, the expression given for the "result"
+// attribute is evaluated in an isolated evaluation context that defines variables
+// named after the given parameter names.
+//
+// The block name "function" may be overridden by the calling application, if
+// that default name conflicts with an existing block or attribute name in
+// the application.
+package userfunc

vendor/github.com/hashicorp/hcl/v2/ext/userfunc/public.go

@@ -0,0 +1,42 @@
+package userfunc
+
+import (
+	"github.com/hashicorp/hcl/v2"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+// A ContextFunc is a callback used to produce the base EvalContext for
+// running a particular set of functions.
+//
+// This is a function rather than an EvalContext directly to allow functions
+// to be decoded before their context is complete. This will be true, for
+// example, for applications that wish to allow functions to refer to themselves.
+//
+// The simplest use of a ContextFunc is to give user functions access to the
+// same global variables and functions available elsewhere in an application's
+// configuration language, but more complex applications may use different
+// contexts to support lexical scoping depending on where in a configuration
+// structure a function declaration is found, etc.
+type ContextFunc func() *hcl.EvalContext
+
+// DecodeUserFunctions looks for blocks of the given type in the given body
+// and, for each one found, interprets it as a custom function definition.
+//
+// On success, the result is a mapping of function names to implementations,
+// along with a new body that represents the remaining content of the given
+// body which can be used for further processing.
+//
+// The result expression of each function is parsed during decoding but not
+// evaluated until the function is called.
+//
+// If the given ContextFunc is non-nil, it will be called to obtain the
+// context in which the function result expressions will be evaluated. If nil,
+// or if it returns nil, the result expression will have access only to
+// variables named after the declared parameters. A non-nil context turns
+// the returned functions into closures, bound to the given context.
+//
+// If the returned diagnostics set has errors then the function map and
+// remain body may be nil or incomplete.
+func DecodeUserFunctions(body hcl.Body, blockType string, context ContextFunc) (funcs map[string]function.Function, remain hcl.Body, diags hcl.Diagnostics) {
+	return decodeUserFunctions(body, blockType, context)
+}

vendor/modules.txt

@@ -212,6 +212,7 @@ github.com/hashicorp/golang-lru/simplelru
 # github.com/hashicorp/hcl/v2 v2.4.0
 github.com/hashicorp/hcl/v2
 github.com/hashicorp/hcl/v2/ext/customdecode
+github.com/hashicorp/hcl/v2/ext/userfunc
 github.com/hashicorp/hcl/v2/gohcl
 github.com/hashicorp/hcl/v2/hclsimple
 github.com/hashicorp/hcl/v2/hclsyntax