Re: [Qemu-devel] [PATCH 04/11] snapshot: new function bdrv_snapshot_find_by_id_and_name()

[Fam Zheng]

On Sat, 06/08 15:58, Wenchao Xia wrote:
> 于 2013-6-8 15:31, Fam Zheng 写道:
> >On Sat, 06/08 14:58, Wenchao Xia wrote:
> >>To make it clear about id and name in searching, add this API
> >>to distinguish them. Caller can choose to search by id or name,
> >>*errp will be set only for exception.
> >>
> >>Some code are modified based on Pavel's patch.
> >>
> >>Signed-off-by: Wenchao Xia <address@hidden>
> >>Signed-off-by: Pavel Hrdina <address@hidden>
> >>---
> >>  block/snapshot.c         |   74 
> >> ++++++++++++++++++++++++++++++++++++++++++++++
> >>  include/block/snapshot.h |    6 ++++
> >>  2 files changed, 80 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/block/snapshot.c b/block/snapshot.c
> >>index 6c6d9de..0a9af4e 100644
> >>--- a/block/snapshot.c
> >>+++ b/block/snapshot.c
> >>@@ -48,6 +48,80 @@ int bdrv_snapshot_find(BlockDriverState *bs, 
> >>QEMUSnapshotInfo *sn_info,
> >>      return ret;
> >>  }
> >>
> >>+/**
> >>+ * Look up an internal snapshot by @id and @name.
> >>+ * @bs: block device to search
> >>+ * @id: unique snapshot ID, or NULL
> >>+ * @name: snapshot name, or NULL
> >>+ * @sn_info: location to store information on the snapshot found
> >>+ * @errp: location to store error, will be set only for exception
> >>+ *
> >>+ * This function will traverse snapshot list in @bs to search the matching
> >>+ * one, @id and @name are the matching condition:
> >>+ * If both @id and @name are specified, find the first one with id @id and
> >>+ * name @name.
> >>+ * If only @id is specified, find the first one with id @id.
> >>+ * If only @name is specified, find the first one with name @name.
> >>+ * if none is specified, abort().
> >>+ *
> >>+ * Returns: true when a snapshot is found and @sn_info will be filled, 
> >>false
> >>+ * when error or not found. If all operation succeed but no matching one is
> >>+ * found, @errp will NOT be set.
> >>+ */
> >>+bool bdrv_snapshot_find_by_id_and_name(BlockDriverState *bs,
> >>+                                       const char *id,
> >>+                                       const char *name,
> >>+                                       QEMUSnapshotInfo *sn_info,
> >>+                                       Error **errp)
> >>+{
> >>+    QEMUSnapshotInfo *sn_tab, *sn;
> >>+    int nb_sns, i;
> >>+    bool ret = false;
> >>+
> >>+    nb_sns = bdrv_snapshot_list(bs, &sn_tab);
> >>+    if (nb_sns < 0) {
> >>+        error_setg_errno(errp, -nb_sns, "Failed to get a snapshot list");
> >>+        return false;
> >>+    } else if (nb_sns == 0) {
> >>+        return false;
> >>+    }
> >>+
> >>+    if (id && name) {
> >>+        for (i = 0; i < nb_sns; i++) {
> >>+            sn = &sn_tab[i];
> >>+            if (!strcmp(sn->id_str, id) && !strcmp(sn->name, name)) {
> >>+                *sn_info = *sn;
> >>+                ret = true;
> >>+                break;
> >>+            }
> >>+        }
> >>+    } else if (id) {
> >>+        for (i = 0; i < nb_sns; i++) {
> >>+            sn = &sn_tab[i];
> >>+            if (!strcmp(sn->id_str, id)) {
> >>+                *sn_info = *sn;
> >>+                ret = true;
> >>+                break;
> >>+            }
> >>+        }
> >>+    } else if (name) {
> >>+        for (i = 0; i < nb_sns; i++) {
> >>+            sn = &sn_tab[i];
> >>+            if (!strcmp(sn->name, name)) {
> >>+                *sn_info = *sn;
> >>+                ret = true;
> >>+                break;
> >>+            }
> >>+        }
> >>+    } else {
> >>+        /* program error */
> >>+        abort();
> >>+    }
> >
> >Looks duplicated. How about:
> >
> >     if (id || name) {
> >         for (i = 0; i < nb_sns; i++) {
> >             sn = &sn_tab[i];
> >             if ((!id || !strcmp(sn->id_str, id)) &&
> >                 (!name || !strcmp(sn->name, name))) {
> >                 *sn_info = *sn;
> >                 ret = true;
> >                 break;
> >             }
> >         }
> >     } else {
> >         abort();
> >     }
> >
>   Less code, but slightly slower since more "if" inside "for". I think
> three "for" also show more clear about judgement logic.

No I don't think if-in-for or for-in-if *here* makes any meaningful
difference in performance, if we really need it fast, we'd better sort the
list it first and binary search. And I don't see it clearer to duplicate
the same logic for three times, If I want to understand it, I need to
compare if#1 and if#2 to get they are the same, and then compare #2 and
#3 again, just to know that the three are no different.

> 
> >And why do we have to abort here? It is not completely nonsense to me to
> >return first snapshot with id == NULL and name == NULL.
> >
>   Just to tip program error. An snapshot with id == NULL and name ==
> NULL is not possible, isn't it?.

OK.

-- 
Fam