[musl] graceful nscd fallback on kernels without AF_UNIX support

[Joakim Sindholt <opensource@zhasha.com>]

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I was reminded of this coincidentally on IRC and apparently it was never
applied, presumably because I never actually sent a proper patch.

5 years ago someone came in asking about musl on a system without
AF_UNIX support and this patch is what we came up with. It does require
access to /dev/null which might itself be a problem now that musl is
shedding filesystem requirements.

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From 9984ac4ad320844a91bbcdbc9f5fa07d3154621e Mon Sep 17 00:00:00 2001
From: Joakim Sindholt <opensource@zhasha.com>
Date: Sat, 3 Apr 2021 12:50:18 +0200
Subject: [PATCH] nscd: fall back gracefully on kernels without AF_UNIX support

---
 src/passwd/nscd_query.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/passwd/nscd_query.c b/src/passwd/nscd_query.c
index d38e371b..8641e4f0 100644
--- a/src/passwd/nscd_query.c
+++ b/src/passwd/nscd_query.c
@@ -40,7 +40,13 @@ retry:
 	buf[0] = NSCDVERSION;
 
 	fd = socket(PF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
-	if (fd < 0) return NULL;
+	if (fd < 0) {
+		if (errno == EACCES || errno == EAFNOSUPPORT || errno == EPROTONOSUPPORT) {
+			errno = errno_save;
+			return fopen("/dev/null", "re");
+		}
+		return 0;
+	}
 
 	if(!(f = fdopen(fd, "r"))) {
 		close(fd);
-- 
2.26.2