fix(rpc): handle more cases when identifying loopback #36100

Problem:

On MacOS it is a relatively common pattern to set XDG_RUNTIME_DIR under
`/tmp` or `/var`. Both of these are symlinks to `/private/tmp` and
`/private/var`. When checking for loopback the input address is
normalized using `fix_fname`, however this is not applied to the
addresses of the sockets. As a result of one address being normalized
and the other not the comparison would fail.

Solution:

Normalize both sides of the comparison using `fix_fname`.

src/nvim/channel.c

@@ -41,7 +41,6 @@
 #include "nvim/os/fs.h"
 #include "nvim/os/os_defs.h"
 #include "nvim/os/shell.h"
-#include "nvim/path.h"
 #include "nvim/terminal.h"
 #include "nvim/types_defs.h"
 
@@ -461,10 +460,7 @@ uint64_t channel_connect(bool tcp, const char *address, bool rpc, CallbackReader
   Channel *channel;
 
   if (!tcp && rpc) {
-    char *path = fix_fname(address);
-    bool loopback = server_owns_pipe_address(path);
-    xfree(path);
-    if (loopback) {
+    if (server_owns_pipe_address(address)) {
       // Create a loopback channel. This avoids deadlock if nvim connects to
       // its own named pipe.
       channel = channel_alloc(kChannelStreamInternal);

src/nvim/msgpack_rpc/server.c

@@ -19,6 +19,7 @@
 #include "nvim/os/os.h"
 #include "nvim/os/os_defs.h"
 #include "nvim/os/stdpaths_defs.h"
+#include "nvim/path.h"
 #include "nvim/types_defs.h"
 
 #define MAX_CONNECTIONS 32
@@ -135,15 +136,20 @@ char *server_address_new(const char *name)
 }
 
 /// Check if this instance owns a pipe address.
-/// The argument must already be resolved to an absolute path!
-bool server_owns_pipe_address(const char *path)
+bool server_owns_pipe_address(const char *address)
 {
+  bool result = false;
+  char *path = fix_fname(address);
   for (int i = 0; i < watchers.ga_len; i++) {
-    if (!strcmp(path, ((SocketWatcher **)watchers.ga_data)[i]->addr)) {
-      return true;
+    char *addr = fix_fname(((SocketWatcher **)watchers.ga_data)[i]->addr);
+    result = strequal(path, addr);
+    xfree(addr);
+    if (result) {
+      break;
     }
   }
-  return false;
+  xfree(path);
+  return result;
 }
 
 /// Starts listening for RPC calls.