9pfs: Don't use file descriptors in core code

v9fs_getattr() currently peeks into V9fsFidOpenState to know if a fid
has a valid file descriptor or directory stream. Even though the fields
are accessible, this is an implementation detail of the local backend
that should not be manipulated directly by the server code.

Abstract that with a new has_valid_file_handle() backend operation.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20250312152933.383967-3-groug@kaod.org>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

fsdev/file-op-9p.h

@@ -164,6 +164,7 @@ struct FileOperations {
     int (*renameat)(FsContext *ctx, V9fsPath *olddir, const char *old_name,
                     V9fsPath *newdir, const char *new_name);
     int (*unlinkat)(FsContext *ctx, V9fsPath *dir, const char *name, int flags);
+    bool (*has_valid_file_handle)(int fid_type, V9fsFidOpenState *fs);
 };
 
 #endif

hw/9pfs/9p-local.c

@@ -1572,6 +1572,13 @@ static int local_parse_opts(QemuOpts *opts, FsDriverEntry *fse, Error **errp)
     return 0;
 }
 
+static bool local_has_valid_file_handle(int fid_type, V9fsFidOpenState *fs)
+{
+    return
+        (fid_type == P9_FID_FILE && fs->fd != -1) ||
+        (fid_type == P9_FID_DIR && fs->dir.stream != NULL);
+}
+
 FileOperations local_ops = {
     .parse_opts = local_parse_opts,
     .init  = local_init,
@@ -1609,4 +1616,5 @@ FileOperations local_ops = {
     .name_to_path = local_name_to_path,
     .renameat  = local_renameat,
     .unlinkat = local_unlinkat,
+    .has_valid_file_handle = local_has_valid_file_handle,
 };

hw/9pfs/9p-synth.c

@@ -615,6 +615,11 @@ static int synth_init(FsContext *ctx, Error **errp)
     return 0;
 }
 
+static bool synth_has_valid_file_handle(int fid_type, V9fsFidOpenState *fs)
+{
+    return false;
+}
+
 FileOperations synth_ops = {
     .init         = synth_init,
     .lstat        = synth_lstat,
@@ -650,4 +655,5 @@ FileOperations synth_ops = {
     .name_to_path = synth_name_to_path,
     .renameat     = synth_renameat,
     .unlinkat     = synth_unlinkat,
+    .has_valid_file_handle = synth_has_valid_file_handle,
 };

hw/9pfs/9p.c

@@ -1593,6 +1593,11 @@ out_nofid:
     pdu_complete(pdu, err);
 }
 
+static bool fid_has_valid_file_handle(V9fsState *s, V9fsFidState *fidp)
+{
+    return s->ops->has_valid_file_handle(fidp->fid_type, &fidp->fs);
+}
+
 static void coroutine_fn v9fs_getattr(void *opaque)
 {
     int32_t fid;
@@ -1615,9 +1620,7 @@ static void coroutine_fn v9fs_getattr(void *opaque)
         retval = -ENOENT;
         goto out_nofid;
     }
-    if ((fidp->fid_type == P9_FID_FILE && fidp->fs.fd != -1) ||
-        (fidp->fid_type == P9_FID_DIR && fidp->fs.dir.stream))
-    {
+    if (fid_has_valid_file_handle(pdu->s, fidp)) {
         retval = v9fs_co_fstat(pdu, fidp, &stbuf);
     } else {
         retval = v9fs_co_lstat(pdu, &fidp->path, &stbuf);