Have 'salloc' use $SHELL or /bin/sh as the fallback shell.

This is useful in situations where glibc’s Name Service Switch (NSS) is
dysfunctional (with ‘getpwuid’ returning NULL), which can happen when
/etc/nsswitch.conf specifies plugins that fail to be loaded in user code, for
instance because user code is linked against an incompatible libc.

Similar patch submitted upstream: https://bugs.schedmd.com/show_bug.cgi?id=19896

diff --git a/src/salloc/opt.c b/src/salloc/opt.c
index ffff7c8..74563ad 100644
--- a/src/salloc/opt.c
+++ b/src/salloc/opt.c
@@ -329,6 +329,7 @@ static void _opt_args(int argc, char **argv, int het_job_offset)
  * NOTE: This function is NOT reentrant (see getpwuid_r if needed) */
 static char *_get_shell(void)
 {
+	char *shell;
 	struct passwd *pw_ent_ptr;
 
 	if (opt.uid == SLURM_AUTH_NOBODY)
@@ -336,11 +337,13 @@ static char *_get_shell(void)
 	else
 		pw_ent_ptr = getpwuid(opt.uid);
 
-	if (!pw_ent_ptr) {
-		pw_ent_ptr = getpwnam("nobody");
-		warning("no user information for user %u", opt.uid);
+	if (pw_ent_ptr) {
+		shell = pw_ent_ptr->pw_shell;
+	} else {
+		shell = getenv("SHELL") ?: "/bin/sh";
+		warning("no user information for user %u, using '%s' as the shell", opt.uid, shell);
 	}
-	return pw_ent_ptr->pw_shell;
+	return shell;
 }
 
 static void _salloc_default_command(int *argcp, char **argvp[])
ref='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>commit</a><a href='/guix/diff/gnu/system/examples/bare-hurd.tmpl?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/system/examples/bare-hurd.tmpl'>
<input type='hidden' name='id' value='4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>root</a>/<a href='/guix/log/gnu?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>gnu</a>/<a href='/guix/log/gnu/system?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>system</a>/<a href='/guix/log/gnu/system/examples?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>examples</a>/<a href='/guix/log/gnu/system/examples/bare-hurd.tmpl?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3'>bare-hurd.tmpl</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/gnu/system/examples/bare-hurd.tmpl?id=4f3e5548fcc2c0bbd9a5ecaf76a4fc63d2439aa3&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-08-29 01:05:26 -0400'>2021-08-29</span></td><td><a href='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=da4e40948326387d5bcda58783914d5f9ce8b095'>Migrate to the new 'targets' field of bootloader-configuration.</a><span class='msg-avail'>...<span class='msg-tooltip'>The old 'target' field is deprecated; adjust the sources to use the new
'targets' one instead.

* doc/guix-cookbook.texi&lt;target&gt;: Replace by 'targets'.
* gnu/bootloader/grub.scm: Likewise.
* gnu/installer/parted.scm: Likewise.
* gnu/machine/digital-ocean.scm: Likewise.
* gnu/system/examples/asus-c201.tmpl: Likewise
* gnu/system/examples/bare-bones.tmpl: Likewise
* gnu/system/examples/bare-hurd.tmpl: Likewise
* gnu/system/examples/beaglebone-black.tmpl: Likewise
* gnu/system/examples/desktop.tmpl: Likewise
* gnu/system/examples/docker-image.tmpl: Likewise
* gnu/system/examples/lightweight-desktop.tmpl: Likewise
* gnu/system/examples/vm-image.tmpl: Likewise
* gnu/system/examples/yggdrasil.tmpl: Likewise
* gnu/system/hurd.scm: Likewise
* gnu/system/images/hurd.scm: Likewise
* gnu/system/images/novena.scm: Likewise
* gnu/system/images/pine64.scm: Likewise
* gnu/system/images/pinebook-pro.scm: Likewise
* gnu/system/images/rock64.scm: Likewise
* gnu/system/install.scm: Likewise
* gnu/system/vm.scm: Likewise
* gnu/tests.scm: Likewise
* gnu/tests/ganeti.scm: Likewise
* gnu/tests/install.scm: Likewise
* gnu/tests/nfs.scm: Likewise
* gnu/tests/telephony.scm: Likewise
* tests/boot-parameters.scm: Likewise
* tests/system.scm: Likewise
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-01-19 17:16:56 +0100'>2021-01-19</span></td><td><a href='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=e74baa124592428f05b17562f180469e405037f3'>system: Rename 'disk-image' command 'image'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/system.scm (system-derivation-for-action): Rename 'disk-image'
command 'image'. Warn when using the now deprecated 'disk-image' command.
(show-help): Adapt accordingly.
(guix-system): Ditto.
* tests/guix-system.sh: Ditto.
* gnu/system/examples/bare-hurd.tmpl: Ditto.
* doc/guix.texi (Building the Installation Image,
Building the Installation Image for ARM Boards,
Invoking guix pack,
Invoking guix system): Adapt documentation.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-10-09 23:25:33 +0200'>2020-10-09</span></td><td><a href='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=16f8ea064cde3ee4ef36212cdbdef90e98ba6914'>system: hurd: Add PAM support with shadow and sudo.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system.scm (hurd-default-essential-services): Add
setuid-program-service-type.
* gnu/system/hurd.scm (%base-packages/hurd): Add shadow, sudo.
(%setuid-programs/hurd): New variable.
(%hurd-default-operating-system)[setuid-program]: Use it.
[pam-services, sudoers-file]: Remove overrides; enabling regular defaults.
* gnu/system/examples/bare-hurd.tmpl (%hurd-os)[users]: New field.
[services]: Do not disable PAM in SSH.
</span></span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>
<tr><td><span title='2020-06-13 09:11:47 +0200'>2020-06-13</span></td><td><a href='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=edbc067138fa417d03cbb204511d8b08b61a3d4d'>system: bare-hurd.tmpl: Add openssh-sans-x client and service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/examples/bare-hurd.tmpl (%hurd-os)[packages]: Add openssh-sans-x.
[services]: Add openssh-service.  Add example usage.
</span></span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>
<tr><td><span title='2020-06-08 13:51:19 +0200'>2020-06-08</span></td><td><a href='/guix/commit/gnu/system/examples/bare-hurd.tmpl?id=59bcffa314171294278444aad4b7012abe50257a'>system: examples: Add bare-hurd.tmpl.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/hurd.scm (%hurd-def%hurd-default-operating-system-kernel,
%hurd-default-operating-system): New exported variables.
* gnu/system/examples/bare-hurd.tmpl: New file.
* Makefile.am (EXAMPLES): Add it.
* tests/guix-system.sh: Add --target=i586-pc-gnu when testing it.
</span></span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>