From 83dcb6f4856198b92431a0281922f26f0734ff56 Mon Sep 17 00:00:00 2001
From: "C. McEnroe" <june@causal.agency>
Date: Mon, 23 Nov 2020 16:49:22 -0500
Subject: Unlink existing UNIX socket if it can't be connected to

I think this emulates SO_REUSEADDR, which for some reason doesn't work
on PF_UNIX. If the socket exists, check if connect(2) works, rather than
clobbering the socket being used by a still-running instance.
---
 local.c | 36 ++++++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/local.c b/local.c
index 21e7cb1..ff879c8 100644
--- a/local.c
+++ b/local.c
@@ -158,21 +158,37 @@ static void unixUnlink(void) {
 	if (error) warn("unlinkat");
 }
 
-size_t localUnix(int fds[], size_t cap, const char *path) {
-	if (!cap) return 0;
-
-	int sock = socket(PF_UNIX, SOCK_STREAM, 0);
-	if (sock < 0) err(EX_OSERR, "socket");
-
+static int unixBind(int sock, const char *path) {
 	struct sockaddr_un addr = { .sun_family = AF_UNIX };
-	int len = snprintf(
-		addr.sun_path, sizeof(addr.sun_path), "%s", path
-	);
+	int len = snprintf(addr.sun_path, sizeof(addr.sun_path), "%s", path);
 	if ((size_t)len >= sizeof(addr.sun_path)) {
 		errx(EX_CONFIG, "path too long: %s", path);
 	}
 
 	int error = bind(sock, (struct sockaddr *)&addr, SUN_LEN(&addr));
+	if (!error || errno != EADDRINUSE) return error;
+
+	int check = socket(PF_UNIX, SOCK_STREAM, 0);
+	if (check < 0) err(EX_OSERR, "socket");
+
+	error = connect(check, (struct sockaddr *)&addr, SUN_LEN(&addr));
+	close(check);
+	if (!error) {
+		errno = EADDRINUSE;
+		return -1;
+	}
+
+	unlink(path);
+	return bind(sock, (struct sockaddr *)&addr, SUN_LEN(&addr));
+}
+
+size_t localUnix(int fds[], size_t cap, const char *path) {
+	if (!cap) return 0;
+
+	int sock = socket(PF_UNIX, SOCK_STREAM, 0);
+	if (sock < 0) err(EX_OSERR, "socket");
+
+	int error = unixBind(sock, path);
 	if (error) err(EX_UNAVAILABLE, "%s", path);
 
 	char dir[PATH_MAX] = ".";
@@ -227,8 +243,8 @@ int localAccept(struct tls **client, int bind) {
 
 	if (unix) {
 		int sent = recvfd(fd);
-		if (sent < 0) err(EX_IOERR, "recvfd");
 		close(fd);
+		if (sent < 0) return sent;
 		fd = sent;
 	}
 
-- 
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