packer: prevent accidental overwrites on boot partition

related to gokrazy/gokrazy#264

Currently, both the gokrazy/firmware and the gokrazy/kernel.rpi repository
contain an overlays directory. The desired state is that only gokrazy/kernel.rpi
contains an overlays directory, as that is the repository from which the
overlays are built (whereas the are only copied into the firmware repository).

To prevent confusion between different versions, the packer now returns an error
instead of silently overwriting files on the boot partition when there are
multiple sources for a file.

Author: stapelberg

go.mod

@@ -5,7 +5,7 @@ go 1.21
 require (
 	github.com/breml/rootcerts v0.2.10
 	github.com/donovanhide/eventsource v0.0.0-20210830082556-c59027999da0
-	github.com/gokrazy/internal v0.0.0-20240607160728-3c7bf66a62e9
+	github.com/gokrazy/internal v0.0.0-20240623083220-71d9c6c36ba4
 	github.com/gokrazy/updater v0.0.0-20230215172637-813ccc7f21e2
 	github.com/google/go-cmp v0.5.9
 	github.com/google/renameio/v2 v2.0.0

go.sum

@@ -3,8 +3,8 @@ github.com/breml/rootcerts v0.2.10/go.mod h1:24FDtzYMpqIeYC7QzaE8VPRQaFZU5TIUDly
 github.com/cpuguy83/go-md2man/v2 v2.0.2/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
 github.com/donovanhide/eventsource v0.0.0-20210830082556-c59027999da0 h1:C7t6eeMaEQVy6e8CarIhscYQlNmw5e3G36y7l7Y21Ao=
 github.com/donovanhide/eventsource v0.0.0-20210830082556-c59027999da0/go.mod h1:56wL82FO0bfMU5RvfXoIwSOP2ggqqxT+tAfNEIyxuHw=
-github.com/gokrazy/internal v0.0.0-20240607160728-3c7bf66a62e9 h1:SLjEgU2ufDL13xVW4+BWTcwhGFTOwf2LXAiJQiolASU=
-github.com/gokrazy/internal v0.0.0-20240607160728-3c7bf66a62e9/go.mod h1:t3ZirVhcs9bH+fPAJuGh51rzT7sVCZ9yfXvszf0ZjF0=
+github.com/gokrazy/internal v0.0.0-20240623083220-71d9c6c36ba4 h1:kkesqri5ZNKIF/QuFjivbr04R+SRx5JpZ0TaD120K/c=
+github.com/gokrazy/internal v0.0.0-20240623083220-71d9c6c36ba4/go.mod h1:t3ZirVhcs9bH+fPAJuGh51rzT7sVCZ9yfXvszf0ZjF0=
 github.com/gokrazy/updater v0.0.0-20230215172637-813ccc7f21e2 h1:kBY5R1tSf+EYZ+QaSrofLaVJtBqYsVNVBWkdMq3Smcg=
 github.com/gokrazy/updater v0.0.0-20230215172637-813ccc7f21e2/go.mod h1:PYOvzGOL4nlBmuxu7IyKQTFLaxr61+WPRNRzVtuYOHw=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=

internal/packer/write.go

@@ -27,11 +27,18 @@ import (
 	"github.com/gokrazy/tools/third_party/systemd-250.5-1"
 )
 
-func copyFile(fw *fat.Writer, dest string, src fs.File) error {
+func copyFile(fw *fat.Writer, dest string, src fs.File, srcName string) error {
 	st, err := src.Stat()
 	if err != nil {
 		return err
 	}
+	exists, err := fw.Exists(dest)
+	if err != nil {
+		return err
+	}
+	if exists {
+		return fmt.Errorf("copyFile(%s, %s): file already exists", dest, srcName)
+	}
 	w, err := fw.File(dest, st.ModTime())
 	if err != nil {
 		return err
@@ -181,7 +188,7 @@ func (p *Pack) copyGlobsToBoot(fw *fat.Writer, srcDir string, globs []string) er
 			if err != nil {
 				return err
 			}
-			if err := copyFile(fw, "/"+relPath, src); err != nil {
+			if err := copyFile(fw, "/"+relPath, src, m); err != nil {
 				return err
 			}
 		}
@@ -330,15 +337,15 @@ func (p *Pack) writeBoot(f io.Writer, mbrfilename string) error {
 		if err != nil {
 			return err
 		}
-		if err := copyFile(fw, "/EFI/BOOT/BOOTX64.EFI", srcX86); err != nil {
+		if err := copyFile(fw, "/EFI/BOOT/BOOTX64.EFI", srcX86, "<embedded>"); err != nil {
 			return err
 		}
 
 		srcAA86, err := systemd.SystemdBootAA64.Open("systemd-bootaa64.efi")
 		if err != nil {
 			return err
 		}
-		if err := copyFile(fw, "/EFI/BOOT/BOOTAA64.EFI", srcAA86); err != nil {
+		if err := copyFile(fw, "/EFI/BOOT/BOOTAA64.EFI", srcAA86, "<embedded>"); err != nil {
 			return err
 		}
 	}