httpd: keep the login sessions in bank 3

The web server's bank had no room left for the four sessions once the
original firmware's more robust header handling came in. Picking a slot,
checking a cookie and starting a session move to sessions.c in bank 3;
httpd.c only calls session_init(), session_check() and session_new().
The random id is written straight into the reply, which drops
gen_random_hex_chars() from bank 1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sessions.c

@@ -0,0 +1,109 @@
+/*
+ * Login sessions of the web interface.
+ *
+ * Up to SESSIONS logins at once, each with its own cookie and its own expiry.
+ * With a single one, logging in from a second browser (or a phone) ended the
+ * first browser's session at its next request.
+ *
+ * This sits in bank 3: the web server's own bank is full, so httpd.c only
+ * calls in here. It shares three things with httpd.c: the cookie value it
+ * parsed (session), the time read for the request (now) and the result
+ * (authenticated); the reply is written to outbuf at slen.
+ */
+
+#pragma codeseg BANK3
+#pragma constseg BANK3
+
+#include <stdint.h>
+#include "rtl837x_common.h"
+#include "sessions.h"
+
+extern volatile __xdata uint8_t sfr_data[4];
+extern __xdata uint16_t session_timeout;
+extern __xdata uint32_t now;
+extern __xdata uint8_t authenticated;
+extern __xdata uint8_t * __xdata session;
+extern __xdata uint8_t outbuf[];
+extern __xdata uint16_t slen;
+
+__xdata char session_id[SESSIONS][SESSION_ID_LENGTH + 1];
+__xdata uint32_t last_session_use[SESSIONS];
+
+static __xdata uint8_t ss_slot, ss_i, ss_oldest, ss_c;
+
+static __code const char ss_hex[] = "0123456789abcdef";
+/* What may follow a cookie value: ';' separates cookies in a Cookie header */
+static __code const char ss_end[] = " \t:?=\n\r;";
+
+/* Leaf function, so the compiler keeps its 32-bit scratch in the overlay
+ * rather than in internal RAM. */
+static uint8_t ss_stale(void)
+{
+	if (now - last_session_use[ss_slot] > session_timeout)
+		return 1;
+	return 0;
+}
+
+/* Whether the cookie value is the id of slot ss_slot */
+static uint8_t ss_match(void)
+{
+	for (ss_i = 0; ss_i < SESSION_ID_LENGTH; ss_i++) {
+		if (session[ss_i] != session_id[ss_slot][ss_i])
+			return 0;
+	}
+	ss_c = session[SESSION_ID_LENGTH];
+	if (!ss_c)
+		return 1;
+	for (ss_i = 0; ss_end[ss_i]; ss_i++) {
+		if (ss_c == ss_end[ss_i])
+			return 1;
+	}
+	return 0;
+}
+
+void session_init(void) __banked
+{
+	for (ss_slot = 0; ss_slot < SESSIONS; ss_slot++)
+		session_id[ss_slot][0] = NUL;
+}
+
+void session_check(void) __banked
+{
+	for (ss_slot = 0; ss_slot < SESSIONS; ss_slot++) {
+		if (!session_id[ss_slot][0])
+			continue;
+		if (ss_stale()) {
+			session_id[ss_slot][0] = NUL;
+		} else if (ss_match()) {
+			authenticated = 1;
+			last_session_use[ss_slot] = now;
+			return;
+		}
+	}
+}
+
+void session_new(void) __banked
+{
+	/* A free or expired slot, else the least recently used */
+	ss_oldest = 0;
+	for (ss_slot = 0; ss_slot < SESSIONS; ss_slot++) {
+		if (!session_id[ss_slot][0] || ss_stale())
+			break;
+		if (last_session_use[ss_slot] < last_session_use[ss_oldest])
+			ss_oldest = ss_slot;
+	}
+	if (ss_slot == SESSIONS)
+		ss_slot = ss_oldest;
+
+	last_session_use[ss_slot] = now;
+	for (ss_i = 0; ss_i < SESSION_ID_LENGTH; ss_i++) {
+		if (!(ss_i & 7))
+			get_random_32();
+		ss_c = sfr_data[(ss_i >> 1) & 3];
+		if (ss_i & 1)
+			ss_c >>= 4;
+		session_id[ss_slot][ss_i] = ss_hex[ss_c & 0xf];
+		outbuf[slen++] = ss_hex[ss_c & 0xf];
+	}
+	session_id[ss_slot][SESSION_ID_LENGTH] = NUL;
+}