Add server module for accepting API connections

The `NEOVIM_LISTEN_ADDRESS` environment variable can be set to customize the
address where Neovim will listen for connections. If it's not set, a random
socket/pipe will be created, and the `NEOVIM_LISTEN_ADDRESS` will be updated
accordingly.

4 files changed, 284 insertions, 0 deletions

diff --git a/src/os/event.c b/src/os/event.c
index a4b9d1ac5a..2c25852778 100644
--- a/src/os/event.c
+++ b/src/os/event.c
@@ -8,6 +8,7 @@
 #include "os/event.h"
 #include "os/input.h"
 #include "os/channel.h"
+#include "os/server.h"
 #include "os/signal.h"
 #include "os/rstream.h"
 #include "os/job.h"
@@ -39,6 +40,8 @@ void event_init()
   job_init();
   // Channels
   channel_init();
+  // Servers
+  server_init();
   uv_timer_init(uv_default_loop(), &timer);
   // This prepare handle that actually starts the timer
   uv_prepare_init(uv_default_loop(), &timer_prepare);
@@ -48,6 +51,7 @@ void event_teardown()
 {
   channel_teardown();
   job_teardown();
+  server_teardown();
 }
 
 // Wait for some event
diff --git a/src/os/server.c b/src/os/server.c
new file mode 100644
index 0000000000..bda9602936
--- /dev/null
+++ b/src/os/server.c
@@ -0,0 +1,243 @@
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+#include <uv.h>
+
+#include "os/channel_defs.h"
+#include "os/channel.h"
+#include "os/server.h"
+#include "os/os.h"
+#include "vim.h"
+#include "memory.h"
+#include "message.h"
+#include "fileio.h"
+#include "map.h"
+
+#define MAX_CONNECTIONS 32
+#define ADDRESS_MAX_SIZE 256
+#define NEOVIM_DEFAULT_TCP_PORT 7450
+
+typedef enum {
+  kServerTypeTcp,
+  kServerTypePipe
+} ServerType;
+
+typedef struct {
+  // Protocol for channels established through this server
+  ChannelProtocol protocol;
+  // Type of the union below
+  ServerType type;
+
+  // This is either a tcp server or unix socket(named pipe on windows)
+  union {
+    struct {
+      uv_tcp_t handle;
+      struct sockaddr_in addr;
+    } tcp;
+    struct {
+      uv_pipe_t handle;
+      char addr[ADDRESS_MAX_SIZE];
+    } pipe;
+  } socket;
+} Server;
+
+static Map *servers = NULL;
+
+static void close_server(Map *map, const char *endpoint, void *server);
+static void connection_cb(uv_stream_t *server, int status);
+static void free_client(uv_handle_t *handle);
+static void free_server(uv_handle_t *handle);
+
+void server_init()
+{
+  servers = map_new();
+
+  if (!os_getenv("NEOVIM_LISTEN_ADDRESS")) {
+    char *listen_address = (char *)vim_tempname('s');
+    os_setenv("NEOVIM_LISTEN_ADDRESS", listen_address, 1);
+    free(listen_address);
+  }
+
+  server_start((char *)os_getenv("NEOVIM_LISTEN_ADDRESS"),
+               kChannelProtocolMsgpack);
+}
+
+void server_teardown()
+{
+  if (!servers) {
+    return;
+  }
+
+  map_foreach(servers, close_server);
+}
+
+void server_start(char *endpoint, ChannelProtocol prot)
+{
+  char addr[ADDRESS_MAX_SIZE];
+
+  // Trim to `ADDRESS_MAX_SIZE`
+  strncpy(addr, endpoint, sizeof(addr));
+
+  // Check if the server already exists
+  if (map_has(servers, addr)) {
+    EMSG2("Already listening on %s", addr);
+    return;
+  }
+
+  ServerType server_type = kServerTypeTcp;
+  Server *server = xmalloc(sizeof(Server));
+  char ip[16], *ip_end = strrchr(addr, ':');
+
+  server->protocol = prot;
+
+  if (!ip_end) {
+    ip_end = strchr(addr, NUL);
+  }
+
+  uint32_t addr_len = ip_end - addr;
+
+  if (addr_len > sizeof(ip) - 1) {
+    // Maximum length of a ip address buffer is 15(eg: 255.255.255.255)
+    addr_len = sizeof(ip);
+  }
+
+  // Extract the address part
+  strncpy(ip, addr, addr_len);
+
+  int port = NEOVIM_DEFAULT_TCP_PORT;
+
+  if (*ip_end == ':') {
+    char *port_end;
+    // Extract the port
+    port = strtol(ip_end + 1, &port_end, 10);
+  
+    errno = 0;
+    if (errno != 0 || port == 0 || port > 0xffff) {
+      // Invalid port, treat as named pipe or unix socket
+      server_type = kServerTypePipe;
+    }
+  }
+
+  if (server_type == kServerTypeTcp) {
+    // Try to parse ip address
+    if (uv_ip4_addr(ip, port, &server->socket.tcp.addr)) {
+      // Invalid address, treat as named pipe or unix socket
+      server_type = kServerTypePipe;
+    }
+  }
+
+  int result;
+
+  if (server_type == kServerTypeTcp) {
+    // Listen on tcp address/port
+    uv_tcp_init(uv_default_loop(), &server->socket.tcp.handle);
+    server->socket.tcp.handle.data = server;
+    uv_tcp_bind(&server->socket.tcp.handle,
+                         (const struct sockaddr *)&server->socket.tcp.addr,
+                         0);
+    result = uv_listen((uv_stream_t *)&server->socket.tcp.handle,
+               MAX_CONNECTIONS,
+               connection_cb);
+    if (result) {
+      uv_close((uv_handle_t *)&server->socket.tcp.handle, free_server);
+    }
+  } else {
+    // Listen on named pipe or unix socket
+    strcpy(server->socket.pipe.addr, addr);
+    uv_pipe_init(uv_default_loop(), &server->socket.pipe.handle, 0);
+    server->socket.pipe.handle.data = server;
+    uv_pipe_bind(&server->socket.pipe.handle, server->socket.pipe.addr);
+    result = uv_listen((uv_stream_t *)&server->socket.pipe.handle,
+               MAX_CONNECTIONS,
+               connection_cb);
+
+    if (result) {
+      uv_close((uv_handle_t *)&server->socket.pipe.handle, free_server);
+    }
+  }
+
+  if (result) {
+    EMSG2("Failed to start server: %s", uv_strerror(result));
+    return;
+  }
+
+
+  server->type = server_type;
+
+  // Add the server to the hash table
+  map_put(servers, addr, server);
+}
+
+void server_stop(char *endpoint)
+{
+  Server *server;
+  char addr[ADDRESS_MAX_SIZE];
+
+  // Trim to `ADDRESS_MAX_SIZE`
+  strncpy(addr, endpoint, sizeof(addr));
+
+  if ((server = map_get(servers, addr)) == NULL) {
+    EMSG2("Not listening on %s", addr);
+    return;
+  }
+
+  if (server->type == kServerTypeTcp) {
+    uv_close((uv_handle_t *)&server->socket.tcp.handle, free_server);
+  } else {
+    uv_close((uv_handle_t *)&server->socket.pipe.handle, free_server);
+  }
+
+  map_del(servers, addr);
+}
+
+static void connection_cb(uv_stream_t *server, int status)
+{
+  int result;
+  uv_stream_t *client;
+  Server *srv = server->data;
+
+  if (status < 0) {
+    abort();
+  }
+
+  if (srv->type == kServerTypeTcp) {
+    client = xmalloc(sizeof(uv_tcp_t));
+    uv_tcp_init(uv_default_loop(), (uv_tcp_t *)client);
+  } else {
+    client = xmalloc(sizeof(uv_pipe_t));
+    uv_pipe_init(uv_default_loop(), (uv_pipe_t *)client, 0);
+  }
+
+  result = uv_accept(server, client);
+
+  if (result) {
+    EMSG2("Failed to accept connection: %s", uv_strerror(result));
+    uv_close((uv_handle_t *)client, free_client);
+    return;
+  }
+
+  channel_from_stream(client, srv->protocol);
+}
+
+static void close_server(Map *map, const char *endpoint, void *srv)
+{
+  Server *server = srv;
+
+  if (server->type == kServerTypeTcp) {
+    uv_close((uv_handle_t *)&server->socket.tcp.handle, free_server);
+  } else {
+    uv_close((uv_handle_t *)&server->socket.pipe.handle, free_server);
+  }
+}
+
+static void free_client(uv_handle_t *handle)
+{
+  free(handle);
+}
+
+static void free_server(uv_handle_t *handle)
+{
+  free(handle->data);
+}
diff --git a/src/os/server.h b/src/os/server.h
new file mode 100644
index 0000000000..b9459a81af
--- /dev/null
+++ b/src/os/server.h
@@ -0,0 +1,30 @@
+#ifndef NEOVIM_OS_SERVER_H
+#define NEOVIM_OS_SERVER_H
+
+#include "os/channel_defs.h"
+
+/// Initializes the module
+void server_init();
+
+/// Teardown the server module
+void server_teardown();
+
+/// Starts listening on arbitrary tcp/unix addresses specified by
+/// `endpoint` for API calls. The type of socket used(tcp or unix/pipe) will 
+/// be determined by parsing `endpoint`: If it's a valid tcp address in the
+/// 'ip:port' format, then it will be tcp socket, else it will be a unix
+/// socket or named pipe.
+///
+/// @param endpoint Address of the server. Either a 'ip:port' string or an
+///        arbitrary identifier(trimmed to 256 bytes) for the unix socket or
+///        named pipe.
+/// @param prot The rpc protocol to be used
+void server_start(char *endpoint, ChannelProtocol prot);
+
+/// Stops listening on the address specified by `endpoint`.
+///
+/// @param endpoint Address of the server.
+void server_stop(char *endpoint);
+
+#endif  // NEOVIM_OS_SERVER_H
+
diff --git a/src/os/server_defs.h b/src/os/server_defs.h
new file mode 100644
index 0000000000..cbaefe3949
--- /dev/null
+++ b/src/os/server_defs.h
@@ -0,0 +1,7 @@
+#ifndef NEOVIM_OS_SERVER_DEFS_H
+#define NEOVIM_OS_SERVER_DEFS_H
+
+typedef struct server Server;
+
+#endif  // NEOVIM_OS_SERVER_DEFS_H
+