misc: Also check for .zst-compressed variants of files (MR 49)

Initially I thought of breaking off the Stat + error check call into its
own function as to reduce repetition, but given that it's only useful in
this situation where it only happens twice anyway, I'm not sure it
actually would reduce complexity.

Additionally, this means that .zst-compressed variants of files will be
searched for in all contexts where this function is used. I'm not sure
this is desirable.

Tested and works with arrow-db820c. I didn't test it on actual hardware,
but I verified that the firmware ended up in the initramfs via
$ pmbootstrap initfs ls. I choose this device because it uses firmware
from linux-firmware and also needs said firmware present in the
initramfs.

Closes https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/issues/39

internal/misc/getfiles.go

@@ -41,10 +41,22 @@ func getFile(file string, required bool) (files []string, err error) {
 
 	fileInfo, err := os.Stat(file)
 	if err != nil {
-		if required {
-			return files, fmt.Errorf("getFile: failed to stat file %q: %w", file, err)
+		// Check if there is a Zstd-compressed version of the file
+		fileZstd := file + ".zst" // .zst is the extension used by linux-firmware
+		fileInfoZstd, errZstd := os.Stat(fileZstd)
+
+		if errZstd == nil {
+			file = fileZstd
+			fileInfo = fileInfoZstd
+			// Unset nil so we don't retain the error from the os.Stat call for the uncompressed version.
+			err = nil
+		} else {
+			if required {
+				return files, fmt.Errorf("getFile: failed to stat file %q: %w (also tried %q: %w)", file, err, fileZstd, errZstd)
+			}
+
+			return files, nil
 		}
-		return files, nil
 	}
 
 	if fileInfo.IsDir() {