Change 'grub-mkrescue' to honor the 'GRUB_FAT_SERIAL_NUMBER'
environment variable.  That way, the caller can specify a fixed
serial number (instead of the randomly chosen one) to create EFI
images (the 'efi.img' file) that are reproducible bit-for-bit.

Patch by Ludovic Courtès <ludo@gnu.org>.
Mangled (for GRUB 2.04) by Tobias Geerinckx-Rice <me@tobias.gr>.

--- grub-2.04/util/grub-mkrescue.c	2019-05-20 13:01:11.000000000 +0200
+++ grub-2.04/util/grub-mkrescue.c	2019-07-08 23:57:36.912104652 +0200
@@ -809,8 +809,15 @@
       free (efidir_efi_boot);
 
       efiimgfat = grub_util_path_concat (2, iso9660_dir, "efi.img");
-      rv = grub_util_exec ((const char * []) { "mformat", "-C", "-f", "2880", "-L", "16", "-i",
-	    efiimgfat, "::", NULL });
+
+      const char *fat_serial_number = getenv ("GRUB_FAT_SERIAL_NUMBER");
+      const char *mformat_args[] =
+       { "mformat", "-C", "-f", "2880", "-L", "16",
+         fat_serial_number != NULL ? "-N" : "-C",
+         fat_serial_number != NULL ? fat_serial_number : "-C",
+         "-i", efiimgfat, "::", NULL };
+
+      rv = grub_util_exec (mformat_args);
       if (rv != 0)
 	grub_util_error ("`%s` invocation failed\n", "mformat");
       rv = grub_util_exec ((const char * []) { "mcopy", "-s", "-i", efiimgfat, efidir_efi, "::/", NULL });
ect name='qt'>
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</td></tr></table>
<div class='path'>path: <a href='/guix/log/'>root</a>/<a href='/guix/log/tests'>tests</a>/<a href='/guix/log/tests/file-systems.scm'>file-systems.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/file-systems.scm?showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-20 08:30:35 -0400'>2020-05-20</span></td><td><a href='/guix/commit/tests/file-systems.scm?id=b460ba7992a0b4af2ddb5927dcf062784539ef7b'>bootloader: grub: Allow booting from a Btrfs subvolume.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/bootloader/grub.scm (strip-mount-point): Remove procedure.
(normalize-file): Add procedure.
(grub-configuration-file): New BTRFS-SUBVOLUME-FILE-NAME parameter.  When
defined, prepend its value to the kernel and initrd file names, using the
NORMALIZE-FILE procedure.  Adjust the call to EYE-CANDY to pass the
BTRFS-SUBVOLUME-FILE-NAME argument.  Normalize the KEYMAP file as well.
(eye-candy): Add a BTRFS-SUBVOLUME-FILE-NAME parameter, and use it, along with
the NORMALIZE-FILE procedure, to normalize the FONT-FILE and IMAGE nested
variables.  Adjust doc.
* gnu/bootloader/depthcharge.scm (depthcharge-configuration-file): Adapt.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Likewise.
* gnu/system/file-systems.scm (btrfs-subvolume?)
(btrfs-store-subvolume-file-name): New procedures.
* gnu/system.scm (operating-system-bootcfg): Specify the Btrfs
subvolume file name the store resides on to the
`operating-system-bootcfg' procedure, using the new
BTRFS-SUBVOLUME-FILE-NAME argument.
* doc/guix.texi (File Systems): Add a Btrfs subsection to document the use of
subvolumes.
* gnu/tests/install.scm (%btrfs-root-on-subvolume-os)
(%btrfs-root-on-subvolume-os-source)
(%btrfs-root-on-subvolume-installation-script)
(%test-btrfs-root-on-subvolume-os): New variables.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-05-20 08:30:34 -0400'>2020-05-20</span></td><td><a href='/guix/commit/tests/file-systems.scm?id=fa35fb58c84d1c1741e4e63c0b37074e35ed2a61'>file-systems: Add helpers for parsing the options string into an alist.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/file-systems.scm (file-system-options-&gt;alist)
(alist-&gt;file-system-options): New procedures.
* tests/file-systems.scm: New tests.
* doc/guix.texi (File Systems): Add note about the newly added procedures.
</span></span></td><td>Maxim Cournoyer</td></tr>