feat(daemon): Add support for PEBBLE_COPY_ONCE.

README.md

@@ -181,6 +181,9 @@ pebble run
 ...
 ```
 
+To initialise the `$PEBBLE` directory with the contents of another, in a one time copy, set the `PEBBLE_COPY_ONCE` environment
+variable to the source directory. This will only copy the contents if the target directory, `$PEBBLE`, is empty.
+
 ### Viewing, starting, and stopping services
 
 You can view the status of one or more services by using `pebble services`:

internals/cli/cli.go

@@ -375,6 +375,10 @@ func getEnvPaths() (pebbleDir string, socketPath string) {
 	return pebbleDir, socketPath
 }
 
+func getCopySource() string {
+	return os.Getenv("PEBBLE_COPY_ONCE")
+}
+
 type cliState struct {
 	NoticesLastListed time.Time `json:"notices-last-listed"`
 	NoticesLastOkayed time.Time `json:"notices-last-okayed"`

internals/cli/cmd_run.go

@@ -17,8 +17,11 @@ package cli
 import (
 	"errors"
 	"fmt"
+	"io"
+	"io/fs"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"strconv"
 	"syscall"
 	"time"
@@ -157,6 +160,11 @@ func runDaemon(rcmd *cmdRun, ch chan os.Signal, ready chan<- func()) error {
 			return err
 		}
 	}
+	err := maybeCopyPebbleDir(getCopySource(), pebbleDir)
+	if err != nil {
+		return err
+	}
+
 	dopts := daemon.Options{
 		Dir:        pebbleDir,
 		SocketPath: socketPath,
@@ -275,3 +283,109 @@ func convertArgs(args [][]string) (map[string][]string, error) {
 
 	return mappedArgs, nil
 }
+
+func maybeCopyPebbleDir(srcDir, destDir string) error {
+	if srcDir == "" {
+		return nil
+	}
+	dirEnts, err := os.ReadDir(destDir)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return err
+	} else if len(dirEnts) != 0 {
+		// Skip non-empty dir.
+		return nil
+	}
+	fsys := os.DirFS(srcDir)
+	// TODO: replace with os.CopyFS
+	err = _go_os_CopyFS(destDir, fsys)
+	if err != nil {
+		return fmt.Errorf("cannot copy %q to %q: %w", srcDir, destDir, err)
+	}
+	return nil
+}
+
+// Implementation from https://go-review.googlesource.com/c/go/+/558995 until accepted into go.
+// Copyright (c) 2009 The Go Authors. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//   - Redistributions of source code must retain the above copyright
+//
+// notice, this list of conditions and the following disclaimer.
+//   - Redistributions in binary form must reproduce the above
+//
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//   - Neither the name of Google Inc. nor the names of its
+//
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// _go_os_CopyFS copies the file system fsys into the directory dir,
+// creating dir if necessary.
+//
+// Newly created directories and files have their default modes
+// where any bits from the file in fsys that are not part of the
+// standard read, write, and execute permissions will be zeroed
+// out, and standard read and write permissions are set for owner,
+// group, and others while retaining any existing execute bits from
+// the file in fsys.
+//
+// Symbolic links in fsys are not supported, a *PathError with Err set
+// to ErrInvalid is returned on symlink.
+//
+// Copying stops at and returns the first error encountered.
+func _go_os_CopyFS(dir string, fsys fs.FS) error {
+	return fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		newPath := filepath.Join(dir, path)
+		if d.IsDir() {
+			return os.MkdirAll(newPath, 0777)
+		}
+
+		// TODO(panjf2000): handle symlinks with the help of fs.ReadLinkFS
+		// 		once https://go.dev/issue/49580 is done.
+		//		we also need safefilepath.IsLocal from https://go.dev/cl/564295.
+		if !d.Type().IsRegular() {
+			return &fs.PathError{Op: "CopyFS", Path: path, Err: fs.ErrInvalid}
+		}
+
+		r, err := fsys.Open(path)
+		if err != nil {
+			return err
+		}
+		defer r.Close()
+		info, err := r.Stat()
+		if err != nil {
+			return err
+		}
+		w, err := os.OpenFile(newPath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0666|info.Mode()&0777)
+		if err != nil {
+			return err
+		}
+
+		if _, err := io.Copy(w, r); err != nil {
+			w.Close()
+			return &fs.PathError{Op: "Copy", Path: newPath, Err: err}
+		}
+		return w.Close()
+	})
+}