github.com/vugu/vugu@v0.3.5/devutil/compiler.go

package devutil

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
)

// NOTE: https://webassembly.org/ says "Wasm" not "WASM" or "WAsm", so that's what I went with on the name.

// NewWasmCompiler returns a WasmCompiler instance.
func NewWasmCompiler() *WasmCompiler {
	return &WasmCompiler{
		logWriter: os.Stderr,
	}
}

// WasmCompiler provides a convenient way to call `go generate` and `go build` and produce Wasm executables for your system.
type WasmCompiler struct {
	beforeFunc      func() error
	generateCmdFunc func() *exec.Cmd
	buildCmdFunc    func(outpath string) *exec.Cmd
	afterFunc       func(outpath string, err error) error
	logWriter       io.Writer
}

// SetLogWriter sets the writer to use for logging output.  Setting it to nil disables logging.
// The default from NewWasmCompiler is os.Stderr
func (c *WasmCompiler) SetLogWriter(w io.Writer) *WasmCompiler {
	if w == nil {
		w = ioutil.Discard
	}
	c.logWriter = w
	return c
}

// SetDir sets both the build and generate directories.
func (c *WasmCompiler) SetDir(dir string) *WasmCompiler {
	return c.SetBuildDir(dir).SetGenerateDir(dir)
}

// SetBuildDir sets the directory of the main package, where `go build` will be run.
// Relative paths are okay and will be resolved with filepath.Abs.
func (c *WasmCompiler) SetBuildDir(dir string) *WasmCompiler {
	return c.SetBuildCmdFunc(func(outpath string) *exec.Cmd {
		cmd := exec.Command("go", "build", "-o", outpath)
		cmd.Dir = dir
		cmd.Env = os.Environ()
		cmd.Env = append(cmd.Env, "GOOS=js", "GOARCH=wasm")
		return cmd
	})
}

// SetBuildCmdFunc provides a function to create the exec.Cmd used when running `go build`.
// It overrides any other build-related setting.
func (c *WasmCompiler) SetBuildCmdFunc(cmdf func(outpath string) *exec.Cmd) *WasmCompiler {
	c.buildCmdFunc = cmdf
	return c
}

// SetGenerateDir sets the directory of where `go generate` will be run.
// Relative paths are okay and will be resolved with filepath.Abs.
func (c *WasmCompiler) SetGenerateDir(dir string) *WasmCompiler {
	return c.SetGenerateCmdFunc(func() *exec.Cmd {
		cmd := exec.Command("go", "generate")
		cmd.Dir = dir
		return cmd
	})
}

// SetGenerateCmdFunc provides a function to create the exec.Cmd used when running `go generate`.
// It overrides any other generate-related setting.
func (c *WasmCompiler) SetGenerateCmdFunc(cmdf func() *exec.Cmd) *WasmCompiler {
	c.generateCmdFunc = cmdf
	return c
}

// SetBeforeFunc specifies a function to be executed before anything else during Execute().
func (c *WasmCompiler) SetBeforeFunc(f func() error) *WasmCompiler {
	c.beforeFunc = f
	return c
}

// SetAfterFunc specifies a function to be executed after everthing else during Execute().
func (c *WasmCompiler) SetAfterFunc(f func(outpath string, err error) error) *WasmCompiler {
	c.afterFunc = f
	return c
}

// Execute runs the generate command (if any) and then invokes the Go compiler
// and produces a wasm executable (or an error).
// The value of outpath is the absolute path to the output file on disk.
// It will be created with a temporary name and if no error is returned
// it is the caller's responsibility to delete the file when it is no longer needed.
// If an error occurs during any of the steps it will be returned with (possibly multi-line)
// descriptive output in it's error message, as produced by the underlying tool.
func (c *WasmCompiler) Execute() (outpath string, err error) {

	logerr := func(e error) error {
		if e == nil {
			return nil
		}
		fmt.Fprintln(c.logWriter, e)
		return e
	}

	if c.buildCmdFunc == nil {
		return "", logerr(errors.New("WasmCompiler: no build command set, cannot continue (did you forget to call SetBulidDir?)"))
	}

	if c.beforeFunc != nil {
		err := c.beforeFunc()
		if err != nil {
			return "", logerr(err)
		}
	}

	if c.generateCmdFunc != nil {
		cmd := c.generateCmdFunc()
		b, err := cmd.CombinedOutput()
		if err != nil {
			return "", logerr(fmt.Errorf("WasmCompiler: generate error: %w; full output:\n%s", err, b))
		}
		fmt.Fprintln(c.logWriter, "WasmCompiler: Successful generate")
	}

	tmpf, err := ioutil.TempFile("", "WasmCompiler")
	if err != nil {
		return "", logerr(fmt.Errorf("WasmCompiler: error creating temporary file: %w", err))
	}

	outpath = tmpf.Name()

	err = tmpf.Close()
	if err != nil {
		return outpath, logerr(fmt.Errorf("WasmCompiler: error closing temporary file: %w", err))
	}

	cmd := c.buildCmdFunc(outpath)
	b, err := cmd.CombinedOutput()
	if err != nil {
		return "", logerr(fmt.Errorf("WasmCompiler: build error: %w; full output:\n%s", err, b))
	}
	fmt.Fprintln(c.logWriter, "WasmCompiler: Successful build")

	if c.afterFunc != nil {
		err = c.afterFunc(outpath, err)
	}

	return outpath, logerr(err)

}

// WasmExecJS returns the contents of the wasm_exec.js file bundled with the Go compiler.
func (c *WasmCompiler) WasmExecJS() (r io.Reader, err error) {

	b1, err := exec.Command("go", "env", "GOROOT").CombinedOutput()
	if err != nil {
		return nil, err
	}

	b2, err := ioutil.ReadFile(filepath.Join(strings.TrimSpace(string(b1)), "misc/wasm/wasm_exec.js"))
	return bytes.NewReader(b2), err

}