9pfs: fix 'total_open_fd' decrementation

According to 'man 2 close' errors returned by close() should only be used
for either diagnostic purposes or for catching data loss due to a previous
write error, as an error result of close() usually indicates a deferred
error of a previous write operation.

Therefore not decrementing 'total_open_fd' on a close() error is wrong
and would yield in a higher open file descriptor count than actually the
case, leading to 9p server reclaiming open file descriptors too soon.

Based-on: <20250312152933.383967-7-groug@kaod.org>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <E1tvEyJ-004dMa-So@kylie.crudebyte.com>

hw/9pfs/9p.c

@@ -510,7 +510,15 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
             err = (f->fid_type == P9_FID_DIR) ?
                 s->ops->closedir(&s->ctx, &f->fs_reclaim) :
                 s->ops->close(&s->ctx, &f->fs_reclaim);
-            if (!err) {
+
+            /* 'man 2 close' suggests to ignore close() errors except of EBADF */
+            if (unlikely(err && errno == EBADF)) {
+                /*
+                 * unexpected case as FIDs were picked above by having a valid
+                 * file descriptor
+                 */
+                error_report("9pfs: v9fs_reclaim_fd() WARNING: close() failed with EBADF");
+            } else {
                 /* total_open_fd must only be mutated on main thread */
                 nclosed++;
             }

hw/9pfs/codir.c

@@ -20,6 +20,7 @@
 #include "fsdev/qemu-fsdev.h"
 #include "qemu/thread.h"
 #include "qemu/main-loop.h"
+#include "qemu/error-report.h"
 #include "coth.h"
 #include "9p-xattr.h"
 #include "9p-util.h"
@@ -353,7 +354,11 @@ int coroutine_fn v9fs_co_closedir(V9fsPDU *pdu, V9fsFidOpenState *fs)
                 err = -errno;
             }
         });
-    if (!err) {
+    /* 'man 2 close' suggests to ignore close() errors except of EBADF */
+    if (unlikely(err && errno == EBADF)) {
+        /* unexpected case as we should have checked for a valid file handle */
+        error_report("9pfs: WARNING: v9fs_co_closedir() failed with EBADF");
+    } else {
         total_open_fd--;
     }
     return err;

hw/9pfs/cofile.c

@@ -20,6 +20,7 @@
 #include "fsdev/qemu-fsdev.h"
 #include "qemu/thread.h"
 #include "qemu/main-loop.h"
+#include "qemu/error-report.h"
 #include "coth.h"
 
 int coroutine_fn v9fs_co_st_gen(V9fsPDU *pdu, V9fsPath *path, mode_t st_mode,
@@ -197,7 +198,11 @@ int coroutine_fn v9fs_co_close(V9fsPDU *pdu, V9fsFidOpenState *fs)
                 err = -errno;
             }
         });
-    if (!err) {
+    /* 'man 2 close' suggests to ignore close() errors except of EBADF */
+    if (unlikely(err && errno == EBADF)) {
+        /* unexpected case as we should have checked for a valid file handle */
+        error_report("9pfs: WARNING: v9fs_co_close() failed with EBADF");
+    } else {
         total_open_fd--;
     }
     return err;