event-httpd: add event httpd support

Add event httpd, which is a small and basic http server designed to
provide server sent events to clients. The server accepts updates
through a simple "UPDATE" method which are then broadcast to clients
listening using the "GET" method.

Signed-off-by: Russell King <rmk@armlinux.org.uk>

9 files changed, 849 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..0b729c9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,5 @@
+*.o
+*.service
+*.swp
+event-httpd
+event-httpd.service
diff --git a/COPYING b/COPYING
new file mode 100644
index 0000000..d159169
--- /dev/null
+++ b/COPYING
@@ -0,0 +1,339 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 2, June 1991
+
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diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..b062b10
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,19 @@
+CFLAGS		:=-O2 -Wall -g
+CPPFLAGS	:=$(shell pkg-config --cflags gio-2.0)
+LDLIBS		:=$(shell pkg-config --libs gio-2.0)
+INSTALL		:=install
+
+prefix		:=/usr/local
+sbindir		:=$(prefix)/sbin
+systemdsystemunitdir :=/etc/systemd/system
+
+all:		event-httpd event-httpd.service
+
+install:
+		$(INSTALL) -s -m 755 event-httpd $(sbindir)
+		$(INSTALL) -m 644 event-httpd.service $(systemdsystemunitdir)
+
+event-httpd:	event-httpd.o resource.o
+
+event-httpd.service: event-httpd.service.in
+		sed "s|@sbindir@|$(sbindir)|" $< > $@
diff --git a/README b/README
new file mode 100644
index 0000000..57d9ac6
--- /dev/null
+++ b/README
@@ -0,0 +1,21 @@
+This is a very basic HTTP server, designed for real-time server sent
+event streams. BEWARE: the server offers little to no security
+features.
+
+The server supports:
+- the standard HTTP "GET" method for retrieval of events.
+- a custom "UPDATE" method to send events.
+- HTTP 1.0 and 1.1 connections. HTTP 1.1 uses chunked mode.
+
+The server is hard-coded to listen on TCP *:1180.
+
+Design
+------
+The design concept is:
+
+public ------ apache reverse ----- event ------ data gathering
+internet          proxy         httpd server      application
+
+The reverse proxy is responsible for controlling public access to the
+event streams served by the mini-httpd event server; the event server
+itself should not be publically accessible.
diff --git a/event-httpd.c b/event-httpd.c
new file mode 100644
index 0000000..3ec2790
--- /dev/null
+++ b/event-httpd.c
@@ -0,0 +1,302 @@
+// mini-httpd: this is a single threaded, event driven very basic http
+// server designed for event streams.
+//
+// Copyright (C) 2021 Russell King.
+// Licensed under GPL version 2. See COPYING.
+//
+// This is designed *not* to be a publically accessible HTTP server,
+// but is designed to be used behind e.g. an Apache reverse proxy that
+// sends X-Forwarded-* headers. The presence of these headers prevents
+// the served data being updated maliciously - updates must be done
+// directly to this server.
+
+#include <gio/gio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "event-httpd.h"
+#include "resource.h"
+
+static GHashTable *resource_hash;
+
+void close_client(struct client *c)
+{
+	if (c->request)
+		g_free(c->request);
+	g_object_unref(c->data);
+	g_object_unref(c->conn);
+	g_free(c);
+}
+
+void respond_header(struct client *c, int error_code, const char *reason,
+		    const char *headers)
+{
+	g_output_stream_printf(c->out, NULL, NULL, NULL,
+			       "HTTP/1.1 %d %s\r\n%s\r\n",
+			       error_code, reason, headers);
+}
+
+void respond_chunk(struct client *c, GString *s)
+{
+	if (c->can_chunk)
+		g_output_stream_printf(c->out, NULL, NULL, NULL,
+				       "%x\r\n%s\r\n", s->len, s->str);
+	else
+		g_output_stream_write(c->out, s->str, s->len, NULL, NULL);
+}
+
+static void respond_error(struct client *c, int error_code, const char *reason)
+{
+	GString *headers, *body = NULL;
+
+	headers = g_string_new("Cache-Control: no-cache\r\n"
+			       "Connection: close\r\n");
+
+	if (error_code != 204) {
+		g_string_append(headers,
+			       	"Content-type: text/html; charset=UTF-8\r\n");
+		if (c->can_chunk)
+			g_string_append(headers,
+				"Transfer-Encoding: chunked\r\n");
+
+		body = g_string_sized_new(1024);
+		g_string_printf(body,
+				"<html><head><title>%d %s</title></head>"
+				"<body>%s</body></html>",
+				error_code, reason, reason);
+	}
+
+	respond_header(c, error_code, reason, headers->str);
+	if (body)
+		respond_chunk(c, body);
+	g_string_free(body, TRUE);
+	g_string_free(headers, TRUE);
+
+	close_client(c);
+}
+
+static void finish(GObject *source, GAsyncResult *res, gpointer user_data)
+{
+	struct client *c = user_data;
+	GError *error = NULL;
+	gsize len;
+
+	g_data_input_stream_read_line_finish(c->data, res, &len, &error);
+
+	if (c->resource->close) {
+		c->resource->close(c, c->resource);
+		c->resource = NULL;
+	}
+
+	close_client(c);
+}
+
+static void update(GObject *source, GAsyncResult *res, gpointer user_data)
+{
+	struct client *c = user_data;
+	GError *error = NULL;
+	char *line;
+	gsize len;
+
+	line = g_data_input_stream_read_line_finish(c->data, res, &len, &error);
+	if (error || !line) {
+		if (error)
+			g_free(error);
+		close_client(c);
+		return;
+	}
+
+	c->resource->update(c, c->resource, line);
+	g_data_input_stream_read_line_async(c->data, 0, NULL, update, c);
+}
+
+enum method {
+	GET,
+	UPDATE,
+};
+
+static void receive(GObject *source, GAsyncResult *res, gpointer user_data)
+{
+	struct client *c = user_data;
+	struct resource *resource;
+	enum method method;
+	GError *error = NULL;
+	gsize len;
+	char *line, *uri, *query, *unescaped, *version;
+
+	line = g_data_input_stream_read_line_finish(c->data, res, &len, &error);
+	if (error || !line) {
+		if (error)
+			g_free(error);
+		close_client(c);
+		return;
+	}
+
+	// In the interest of robustness, servers SHOULD ignore any empty
+	// line(s) received where a Request-Line is expected.
+	if (!c->request) {
+		if (line[0])
+			c->request = g_strdup(line);
+
+		g_data_input_stream_read_line_async(c->data, 0, NULL,
+						    receive, c);
+		return;
+	}
+
+	// Continue reading the request headers (we discard them)
+	if (line[0]) {
+		// Detect any X-Forwarded-* header
+		// FIXME: should this be case-insensitive?
+		if (g_str_has_prefix(line, "X-Forwarded-"))
+			c->forwarded = TRUE;
+
+		g_data_input_stream_read_line_async(c->data, 0, NULL,
+						    receive, c);
+		return;
+	}
+
+	// End of request. Parse it.
+	// Find the URI
+	uri = strchr(c->request, ' ');
+	if (!uri) {
+		respond_error(c, 400, "Invalid request");
+		return;
+	}
+
+	*uri++ = '\0';
+
+	// Find the version
+	version = strchr(uri, ' ');
+	if (!version) {
+		respond_error(c, 400, "Invalid request");
+		return;
+	}
+
+	*version++ = '\0';
+
+	// Check that the version is HTTP/1.x. We probably ought to
+	// parse the major version better, as leading zeros should be
+	// accepted.
+	if (!g_str_has_prefix(version, "HTTP/1.")) {
+		respond_error(c, 505, "HTTP Version Not Supported");
+		return;
+	}
+
+	// Split the query string
+	query = strchr(uri, '?');
+	if (query)
+		*query++ = '\0';
+
+	c->uri = uri;
+	c->query = query;
+	c->version = version;
+
+	// HTTP/1.1 and later can use chunked mode. Note that http 1.1
+	// allows leading zeros.
+	c->can_chunk = atoi(version + 7) != '0';
+
+	// Check the method
+	if (!strcmp(c->request, "GET")) {
+		method = GET;
+	} else if (!strcmp(c->request, "UPDATE") && !c->forwarded) {
+		// Update is only permitted if not forwarded through a
+		// proxy. NOTE: this is the only way we control access.
+		method = UPDATE;
+	} else {
+		respond_error(c, 501, "Not Implemented");
+		return;
+	}
+
+	// Unescape the URI
+	unescaped = g_uri_unescape_string(uri, NULL);
+
+	// Lookup the resource handler
+	resource = g_hash_table_lookup(resource_hash, unescaped);
+	if (!resource) {
+		respond_error(c, 404, "Not Found");
+		return;
+	}
+
+	c->resource = resource;
+
+	// We have a valid resource, start the response
+	switch (method) {
+	case GET:
+		if (!resource->get) {
+			respond_error(c, 204, "No Content");
+			close_client(c);
+			return;
+		}
+
+		if (resource->get(c, resource)) {
+			g_data_input_stream_read_line_async(c->data, 0, NULL,
+							    finish, c);
+		} else {
+			close_client(c);
+		}
+		break;
+
+	case UPDATE:
+		if (!resource->update) {
+			respond_error(c, 204, "No Content");
+			close_client(c);
+			return;
+		}
+
+		g_data_input_stream_read_line_async(c->data, 0, NULL,
+						    update, c);
+		break;
+	}
+}
+
+static gboolean incoming(GSocketService *service, GSocketConnection *connection,
+			 GObject *source_object, gpointer user_data)
+{
+	struct client *c;
+
+	c = g_new0(struct client, 1);
+	c->conn = g_object_ref(connection);
+	c->out = g_io_stream_get_output_stream(G_IO_STREAM(connection));
+	c->in = g_io_stream_get_input_stream(G_IO_STREAM(connection));
+	c->data = g_data_input_stream_new(c->in);
+
+	g_tcp_connection_set_graceful_disconnect(G_TCP_CONNECTION(connection),
+						 TRUE);
+
+	/* Be tolerant of input */
+	g_data_input_stream_set_newline_type(c->data,
+					     G_DATA_STREAM_NEWLINE_TYPE_ANY);
+
+	g_data_input_stream_read_line_async(c->data, 0, NULL, receive, c);
+
+	return TRUE;
+}
+
+int mini_httpd_init(int port, const char *progname)
+{
+	GSocketService *service;
+	GError *error = NULL;
+
+	service = g_socket_service_new();
+	if (!g_socket_listener_add_inet_port(G_SOCKET_LISTENER(service), port,
+					     NULL, &error)) {
+		g_printerr("%s: %s\n", progname, error->message);
+		return 0;
+	}
+
+	g_signal_connect(service, "incoming", G_CALLBACK(incoming), NULL);
+
+	return 1;
+}
+
+int main(int argc, char *argv[])
+{
+	resource_hash = g_hash_table_new(g_str_hash, g_str_equal);
+	resource_init(resource_hash);
+
+	if (!mini_httpd_init(1180, argv[0]))
+		return 1;
+
+	g_main_loop_run(g_main_loop_new(NULL, FALSE));
+	g_assert_not_reached();
+}
diff --git a/event-httpd.h b/event-httpd.h
new file mode 100644
index 0000000..7818bd5
--- /dev/null
+++ b/event-httpd.h
@@ -0,0 +1,28 @@
+// Copyright (C) 2021 Russell King.
+// Licensed under GPL version 2. See COPYING.
+#ifndef MINI_HTTPD_H
+#define MINI_HTTPD_H
+
+struct client {
+	GSocketConnection *conn;
+	GDataInputStream *data;
+	GOutputStream *out;
+	GInputStream *in;
+
+	gboolean forwarded;
+	gboolean can_chunk;
+	char *request;
+	char *uri;
+	char *query;
+	char *version;
+
+	struct resource *resource;
+	void *resource_data;
+};
+
+void close_client(struct client *c);
+void respond_header(struct client *c, int error_code, const char *reason,
+		    const char *headers);
+void respond_chunk(struct client *c, GString *s);
+
+#endif
diff --git a/event-httpd.service.in b/event-httpd.service.in
new file mode 100644
index 0000000..85983b9
--- /dev/null
+++ b/event-httpd.service.in
@@ -0,0 +1,13 @@
+[Unit]
+Description=event stream service
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=@sbindir@/event-httpd
+Restart=on-failure
+NoNewPrivileges=yes
+DynamicUser=yes
+
+[Install]
+WantedBy=multi-user.target
diff --git a/resource.c b/resource.c
new file mode 100644
index 0000000..c1473e7
--- /dev/null
+++ b/resource.c
@@ -0,0 +1,105 @@
+// Copyright (C) 2021 Russell King.
+// Licensed under GPL version 2. See COPYING.
+#include <gio/gio.h>
+#include "event-httpd.h"
+#include "resource.h"
+
+struct resource_object {
+	GList *client_list;
+	char *str;
+};
+
+static struct resource_object position_object;
+static struct resource_object signal_object;
+
+static void object_v1_send(struct client *c, struct resource_object *obj)
+{
+	const char *str = obj->str;
+	GString *s;
+
+	if (!str)
+		return;
+
+	// Format the text/event-stream response
+	// The double newline terminates this event
+	// See https://www.html5rocks.com/en/tutorials/eventsource/basics/
+	s = g_string_sized_new(1024);
+	g_string_printf(s, "data:%s\n", str);
+	g_string_append_c(s, '\n');
+
+	respond_chunk(c, s);
+	g_string_free(s, TRUE);
+}
+
+static int object_v1_get(struct client *c, struct resource *r)
+{
+	struct resource_object *obj = r->data;
+
+	respond_header(c, 200, "OK",
+		       "Cache-Control: no-cache\r\n"
+		       "Connection: keep-alive\r\n"
+		       "Content-Type: text/event-stream; charset=UTF-8\r\n"
+		       "Transfer-Encoding: chunked\r\n");
+
+	object_v1_send(c, obj);
+
+	// Add this client to the object list so it receives updates
+	obj->client_list = g_list_append(obj->client_list, c);
+
+	return 1;
+}
+
+// Update this client with the new resource object data
+static void object_v1_client_update(gpointer data, gpointer user_data)
+{
+	struct resource_object *obj = user_data;
+	struct client *c = data;
+
+	object_v1_send(c, obj);
+}
+
+// Update all attached clients with the new resource object data
+static int object_v1_update(struct client *c, struct resource *r, const char *m)
+{
+	struct resource_object *obj = r->data;
+	char *n, *o;
+
+	n = g_strdup(m);
+	g_strchomp(n);
+	o = obj->str;
+	obj->str = n;
+	if (o)
+		g_free(o);
+
+	g_list_foreach(obj->client_list, object_v1_client_update, obj);
+
+	return 0;
+}
+
+static void object_v1_close(struct client *c, struct resource *r)
+{
+	struct resource_object *obj = r->data;
+
+	// Remove this client from the object list
+	obj->client_list = g_list_remove(obj->client_list, c);
+}
+
+static struct resource resource_position1 = {
+	.get = object_v1_get,
+	.update = object_v1_update,
+	.close = object_v1_close,
+	.data = &position_object,
+};
+
+static struct resource resource_signal1 = {
+	.get = object_v1_get,
+	.update = object_v1_update,
+	.close = object_v1_close,
+	.data = &signal_object,
+};
+
+void resource_init(GHashTable *hash)
+{
+	g_hash_table_insert(hash, "/api/1/position", &resource_position1);
+	g_hash_table_insert(hash, "/api/1/signal", &resource_signal1);
+}
diff --git a/resource.h b/resource.h
new file mode 100644
index 0000000..44d0d74
--- /dev/null
+++ b/resource.h
@@ -0,0 +1,17 @@
+// Copyright (C) 2021 Russell King.
+// Licensed under GPL version 2. See COPYING.
+#ifndef RESOURCE_H
+#define RESOURCE_h
+
+struct client;
+
+struct resource {
+	int (*get)(struct client *c, struct resource *r);
+	int (*update)(struct client *c, struct resource *r, const char *m);
+	void (*close)(struct client *c, struct resource *r);
+	void *data;
+};
+
+void resource_init(GHashTable *hash);
+
+#endif