package loader

// This file constructs a new temporary GOROOT directory by merging both the
// standard Go GOROOT and the GOROOT from Moxie using symlinks.
//
// The goal is to replace specific packages from Go with a Moxie version. It's
// never a partial replacement, either a package is fully replaced or it is not.
// This is important because if we did allow to merge packages (e.g. by adding
// files to a package), it would lead to a dependency on implementation details
// with all the maintenance burden that results in. Only allowing to replace
// packages as a whole avoids this as packages are already designed to have a
// public (backwards-compatible) API.

import (
	"crypto/sha512"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"io/fs"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"runtime"
	"sort"
	"sync"

	"moxie/compileopts"
	"moxie/goenv"
)

var gorootCreateMutex sync.Mutex

// GetCachedGoroot creates a new GOROOT by merging both the standard GOROOT and
// the GOROOT from Moxie using lots of symbolic links.
func GetCachedGoroot(config *compileopts.Config) (string, error) {
	goroot := goenv.Get("GOROOT")
	if goroot == "" {
		return "", errors.New("could not determine GOROOT")
	}
	moxieroot := goenv.Get("MOXIEROOT")
	if moxieroot == "" {
		return "", errors.New("could not determine MOXIEROOT")
	}

	// Find the overrides needed for the goroot.
	overrides := pathsToOverride(config.GoMinorVersion, needsSyscallPackage(config.BuildTags()))

	// Resolve the merge links within the goroot.
	merge, err := listGorootMergeLinks(goroot, moxieroot, overrides)
	if err != nil {
		return "", err
	}

	// Hash the merge links to create a cache key.
	data, err := json.Marshal(merge)
	if err != nil {
		return "", err
	}
	hash := sha512.Sum512_256(data)

	// Do not try to create the cached GOROOT in parallel, that's only a waste
	// of I/O bandwidth and thus speed. Instead, use a mutex to make sure only
	// one goroutine does it at a time.
	// This is not a way to ensure atomicity (a different Moxie invocation
	// could be creating the same directory), but instead a way to avoid
	// creating it many times in parallel when running tests in parallel.
	gorootCreateMutex.Lock()
	defer gorootCreateMutex.Unlock()

	// Check if the goroot already exists.
	cachedGorootName := "goroot-" + hex.EncodeToString(hash[:])
	cachedgoroot := filepath.Join(goenv.Get("GOCACHE"), cachedGorootName)
	if _, err := os.Stat(cachedgoroot); err == nil {
		return cachedgoroot, nil
	}

	// Create the cache directory if it does not already exist.
	err = os.MkdirAll(goenv.Get("GOCACHE"), 0777)
	if err != nil {
		return "", err
	}

	// Create a temporary directory to construct the goroot within.
	tmpgoroot, err := os.MkdirTemp(goenv.Get("GOCACHE"), cachedGorootName+".tmp")
	if err != nil {
		return "", err
	}

	// Remove the temporary directory if it wasn't moved to the right place
	// (for example, when there was an error).
	defer os.RemoveAll(tmpgoroot)

	// Create the directory structure.
	// The directories are created in sorted order so that nested directories are created without extra work.
	{
		var dirs []string
		for dir, merge := range overrides {
			if merge {
				dirs = append(dirs, filepath.Join(tmpgoroot, "src", dir))
			}
		}
		sort.Strings(dirs)

		for _, dir := range dirs {
			err := os.Mkdir(dir, 0777)
			if err != nil {
				return "", err
			}
		}
	}

	// Create all symlinks.
	for dst, src := range merge {
		err := symlink(src, filepath.Join(tmpgoroot, dst))
		if err != nil {
			return "", err
		}
	}

	// Rename the new merged gorooot into place.
	err = os.Rename(tmpgoroot, cachedgoroot)
	if err != nil {
		if errors.Is(err, fs.ErrExist) {
			// Another invocation of Moxie also seems to have created a GOROOT.
			// Use that one instead. Our new GOROOT will be automatically
			// deleted by the defer above.
			return cachedgoroot, nil
		}
		if runtime.GOOS == "windows" && errors.Is(err, fs.ErrPermission) {
			// On Windows, a rename with a destination directory that already
			// exists does not result in an IsExist error, but rather in an
			// access denied error. To be sure, check for this case by checking
			// whether the target directory exists.
			if _, err := os.Stat(cachedgoroot); err == nil {
				return cachedgoroot, nil
			}
		}
		return "", err
	}
	return cachedgoroot, nil
}

// listGorootMergeLinks searches goroot and moxieroot for all symlinks that must be created within the merged goroot.
func listGorootMergeLinks(goroot, moxieroot string, overrides map[string]bool) (map[string]string, error) {
	goSrc := filepath.Join(goroot, "src")
	moxieSrc := filepath.Join(moxieroot, "src")
	merges := make(map[string]string)
	for dir, merge := range overrides {
		if !merge {
			// Use the Moxie version.
			merges[filepath.Join("src", dir)] = filepath.Join(moxieSrc, dir)
			continue
		}

		// Add files from Moxie.
		moxieDir := filepath.Join(moxieSrc, dir)
		moxieEntries, err := os.ReadDir(moxieDir)
		if err != nil {
			return nil, err
		}
		var hasMoxieFiles bool
		for _, e := range moxieEntries {
			if e.IsDir() {
				continue
			}

			// Link this file.
			name := e.Name()
			merges[filepath.Join("src", dir, name)] = filepath.Join(moxieDir, name)

			hasMoxieFiles = true
		}

		// Add all directories from $GOROOT that are not part of the Moxie
		// overrides.
		goDir := filepath.Join(goSrc, dir)
		goEntries, err := os.ReadDir(goDir)
		if err != nil {
			return nil, err
		}
		for _, e := range goEntries {
			isDir := e.IsDir()
			if hasMoxieFiles && !isDir {
				// Only merge files from Go if Moxie does not have any files.
				// Otherwise we'd end up with a weird mix from both Go
				// implementations.
				continue
			}

			name := e.Name()
			if _, ok := overrides[path.Join(dir, name)+"/"]; ok {
				// This entry is overridden by Moxie.
				// It has/will be merged elsewhere.
				continue
			}

			// Add a link to this entry
			merges[filepath.Join("src", dir, name)] = filepath.Join(goDir, name)
		}
	}

	// Merge the special directories from goroot.
	for _, dir := range []string{"bin", "lib", "pkg"} {
		merges[dir] = filepath.Join(goroot, dir)
	}

	// Required starting in Go 1.21 due to https://github.com/golang/go/issues/61928
	if _, err := os.Stat(filepath.Join(goroot, "go.env")); err == nil {
		merges["go.env"] = filepath.Join(goroot, "go.env")
	}

	return merges, nil
}

// needsSyscallPackage returns whether the syscall package should be overridden
// with the Moxie version. This is the case on some targets.
func needsSyscallPackage(buildTags []string) bool {
	for _, tag := range buildTags {
		if tag == "baremetal" || tag == "nintendoswitch" || tag == "moxie.wasm" {
			return true
		}
	}
	return false
}

// The boolean indicates whether to merge the subdirs. True means merge, false
// means use the moxie version entirely.
//
// Moxie's src/ tree is self-contained — all stdlib source is in-tree.
// The root entry (false) means use moxie's src/ for everything.
// syscall/ is the one exception: moxie has compiler-intrinsic stubs that
// conflict with Go's full implementation. The merge system provides Go's
// syscall files for hosted targets while moxie's stubs serve baremetal.
func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
	return map[string]bool{
		"": false,
	}
}

// symlink creates a symlink or something similar. On Unix-like systems, it
// always creates a symlink. On Windows, it tries to create a symlink and if
// that fails, creates a hardlink or directory junction instead.
//
// Note that while Windows 10 does support symlinks and allows them to be
// created using os.Symlink, it requires developer mode to be enabled.
// Therefore provide a fallback for when symlinking is not possible.
// Unfortunately this fallback only works when Moxie is installed on the same
// filesystem as the Moxie cache and the Go installation (which is usually the
// C drive).
func symlink(oldname, newname string) error {
	symlinkErr := os.Symlink(oldname, newname)
	if runtime.GOOS == "windows" && symlinkErr != nil {
		// Fallback for when developer mode is disabled.
		// Note that we return the symlink error even if something else fails
		// later on. This is because symlinks are the easiest to support
		// (they're also used on Linux and MacOS) and enabling them is easy:
		// just enable developer mode.
		st, err := os.Stat(oldname)
		if err != nil {
			return symlinkErr
		}
		if st.IsDir() {
			// Make a directory junction. There may be a way to do this
			// programmatically, but it involves a lot of magic. Use the mklink
			// command built into cmd instead (mklink is a builtin, not an
			// external command).
			err := exec.Command("cmd", "/k", "mklink", "/J", newname, oldname).Run()
			if err != nil {
				return symlinkErr
			}
		} else {
			// Try making a hard link.
			err := os.Link(oldname, newname)
			if err != nil {
				// Making a hardlink failed. Try copying the file as a last
				// fallback.
				inf, err := os.Open(oldname)
				if err != nil {
					return err
				}
				defer inf.Close()
				outf, err := os.Create(newname)
				if err != nil {
					return err
				}
				defer outf.Close()
				_, err = io.Copy(outf, inf)
				if err != nil {
					os.Remove(newname)
					return err
				}
				// File was copied.
			}
		}
		return nil // success
	}
	return symlinkErr
}