From f7583f538ee6e33c1f1a0d8b386f202cc540a020 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Thu, 13 Oct 2022 13:34:29 +0100
Subject: [PATCH] Read selector from client

---
 main.c | 49 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/main.c b/main.c
index 077cc26..74ce543 100644
--- a/main.c
+++ b/main.c
@@ -24,6 +24,8 @@
 #include <sys/socket.h>
 #include <unistd.h>
 
+#define SBUF_SIZE 1024
+
 static bool exit_requested = false;
 
 void handle_exit_signal(int signum)
@@ -82,13 +84,19 @@ int main(int argc, char *argv[])
 	struct sockaddr_in6 paddr;
 	socklen_t paddr_size = sizeof(paddr);
 	int cfd;
+	char sbuf[SBUF_SIZE];
+	ssize_t n, slen;
 	while (!exit_requested) {
 		/*
 		 * Accept incoming connection.
 		 *
-		 * If accept() returns with an error and errno is EINTR, this
-		 * should not exit the program as it just indicates that a
-		 * signal arrived while waiting for a connection.
+		 * If this is interrupted we go to the next loop iteration
+		 * rather than retrying directly in case the received signal
+		 * was requesting that the server exits.
+		 *
+		 * System calls after this point should be retried before
+		 * checking if we need to exit, as we don't want to leave the
+		 * client hanging.
 		 */
 		cfd = accept(sfd, (struct sockaddr *)&paddr, &paddr_size);
 		if (cfd == -1) {
@@ -98,7 +106,40 @@ int main(int argc, char *argv[])
 			return 1;
 		}
 
-		printf("Accepted connection\n");
+		/*
+		 * Read selector from client socket.
+		 *
+		 * For now, assuming that the selector is less than RDBUF
+		 * bytes long.
+		 */
+		do {
+			errno = 0;
+			n = read(cfd, sbuf, SBUF_SIZE);
+		} while (errno == EINTR);
+		if (n == -1) {
+			fprintf(stderr, "Error reading selector from client\n");
+			return 1;
+		}
+
+		/*
+		 * Locate the end of the selector.
+		 *
+		 * The end of the selector string is indicated with CRLF. Most
+		 * of the time this will be at the end of the received data,
+		 * so might as well start from there.
+		 */
+		slen = n;
+		for (unsigned i = n - 2; i < n; --i) {
+			if (sbuf[i] == 0x0d && sbuf[i + 1] == 0x0a)
+				slen = i;
+		}
+		if (slen == n) {
+			fprintf(stderr, "Received invalid selector (no CRLF)\n");
+			return 1;
+		}
+
+		sbuf[slen] = '\0';
+		printf("Received selector: \"%s\"\n", sbuf);
 
 		close(cfd);
 	}