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https://issues.apache.org/jira/browse/ZOOKEEPER-311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12697996#action_12697996
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Chris Darroch commented on ZOOKEEPER-311:
-----------------------------------------
Mostly because the existing logic is a little counter-intuitive. I spent quite
a bit of time looking at it while trying to determine it's behaviour in various
cases --- e.g., what happens if my buffer is 1 byte long? Because I was
writing a wrapper library (Net::ZooKeeper) and not a client application, I
didn't know what "max path len" the user might supply and I wanted to ensure
consistent behaviour.
Typical C functions that take a buffer and fill it (e.g., snprintf()) rarely do
nothing if the buffer is 1 byte long; instead, they fill it with a null
terminator. So that's surprising and counterintuitive.
What I ended up doing in Net::ZooKeeper was preventing users from supplying
"max path len" values less than 2.
My other concern is really just about future-proofing. Suppose in a future
release ZooKeeper returns null paths from create() calls for some reason ---
because of some new feature ("virtual nodes"?) or whatever. Anyway, in this
case, the user might reasonably expect a 1-byte buffer to be (marginally)
useful. Or perhaps the code gets copied and re-used in another context where
1-byte buffers make more sense.
The code rewrite, although simple, I think clarifies what's going on in a way
that will help prevent buffer problems in the future --- always a sore point
when coding in C. First we calculate the len of the buffer that would be
required for the full path, including space for the null terminator. If that's
longer than what we've been given, we truncate to what we have. If there's
anything to do at this point (i.e., the supplied buffer had at least one byte),
then we do it.
> handle small path lengths in zoo_create()
> -----------------------------------------
>
> Key: ZOOKEEPER-311
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-311
> Project: Zookeeper
> Issue Type: Improvement
> Components: c client
> Affects Versions: 3.0.0, 3.0.1, 3.1.0
> Reporter: Chris Darroch
> Assignee: Chris Darroch
> Priority: Minor
> Fix For: 4.0.0
>
> Attachments: ZOOKEEPER-311.patch, ZOOKEEPER-311.patch
>
>
> The synchronous completion for zoo_create() contains the following code:\\
> {noformat}
> if (sc->u.str.str_len > strlen(res.path)) {
> len = strlen(res.path);
> } else {
> len = sc->u.str.str_len-1;
> }
> if (len > 0) {
> memcpy(sc->u.str.str, res.path, len);
> sc->u.str.str[len] = '\0';
> }
> {noformat}
> In the case where the max_realpath_len argument to zoo_create() is 0, none of
> this code executes, which is OK. In the case where max_realpath_len is 1, a
> user might expect their buffer to be filled with a null terminator, but
> again, nothing will happen (even if strlen(res.path) is 0, which is unlikely
> since new node's will have paths longer than "/").
> The name of the argument to zoo_create() is also a little misleading, as is
> its description ("the maximum length of real path you would want") in
> zookeeper.h, and the example usage in the Programmer's Guide:
> {noformat}
> int rc = zoo_create(zh,"/xyz","value", 5, &CREATE_ONLY, ZOO_EPHEMERAL,
> buffer, sizeof(buffer)-1);
> {noformat}
> In fact this value should be the actual length of the buffer, including space
> for the null terminator. If the user supplies a max_realpath_len of 10 and a
> buffer of 11 bytes, and strlen(res.path) is 10, the code will truncate the
> returned value to 9 bytes and put the null terminator in the second-last
> byte, leaving the final byte of the buffer unused.
> It would be better, I think, to rename the realpath and max_realpath_len
> arguments to something like path_buffer and path_buffer_len, akin to
> zoo_set(). The path_buffer_len would be treated as the full length of the
> buffer (as the code does now, in fact, but the docs suggest otherwise).
> The code in the synchronous completion could then be changed as per the
> attached patch.
> Since this would change, slightly, the behaviour or "contract" of the API, I
> would be inclined to suggest waiting until 4.0.0 to implement this change.
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