On 8/30/2022 3:35 AM, Anton Khirnov wrote:
Quoting James Almer (2022-08-29 17:00:54)
On 8/29/2022 11:07 AM, Anton Khirnov wrote:
---
libavutil/fifo.h | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/libavutil/fifo.h b/libavutil/fifo.h
index 6c6bd78842..89872d0972 100644
--- a/libavutil/fifo.h
+++ b/libavutil/fifo.h
@@ -97,7 +97,13 @@ void av_fifo_auto_grow_limit(AVFifo *f, size_t max_elems);
size_t av_fifo_can_read(const AVFifo *f);
/**
- * @return number of elements that can be written into the given FIFO.
+ * @return Number of elements that can be written into the given FIFO without
+ * growing it.
+ *
+ * In other words, this number of elements or less is guaranteed to fit
+ * into the FIFO. More data may be written when the
+ * AV_FIFO_FLAG_AUTO_GROW flag was specified at FIFO creation, but this
+ * may involve memory allocation, which can fail.
This patch is an API break, because before it i was told
av_fifo_can_write() would tell me the amount of elements i could write
into the FIFO, regardless of how it was created, but now it legitimates
the one scenario where it was not reliable. An scenario i stumbled upon
in my code by following the documentation, which is in at least one
release, the LTS one.
Instead of changing the documentation to fit the behavior, the behavior
should match the documentation. This means that if a call to
av_fifo_write() can succeed, then av_fifo_can_write() should reflect that.
That said, it would be great if making av_fifo_can_write() tell the real
amount of elements one can write into the FIFO was possible without
breaking anything, but the doxy for av_fifo_grow2() says "On success,
the FIFO will be large enough to hold exactly inc + av_fifo_can_read() +
av_fifo_can_write()", a line that was obviously aware of the fact
av_fifo_can_write() ignored the autogrow feature, and would no longer be
true if said function is fixed.
I disagree that this is a break.
The issue in my view is that 'can be written' is ambiguous here, so we
are interpreting it differently. Your interpretation is apparently
'maximum number of elements for which a write can possibly succeeed',
whereas my intended interpretation was 'maximum number of elements for
which a write is always guaranteed to succeed'.
IMO it's not really ambiguous. If you don't state that's the intention,
which you're doing in this patch, then "can be written" has one literal
meaning.
One of these interpretations is correct, because it matches the actual
behaviour. So the right solution IMO is to clarify the documentation so
it is no longer ambiguous, but I do not consider this an API break.
av_fifo_write() says "In case nb_elems > av_fifo_can_write(f), nothing
is written and an error is returned.", which is definitely not
ambiguous, and you're changing it in patch 3/3 to include the case where
having enabled autogrow could result in the function succeeding when
nb_elems > av_fifo_can_write(f).
The behavior of the function remains intact, but a library user reading
the documentation in ffmpeg 5.1 and the documentation in what will be
5.2 after this patch could rightly assume the function was changed and
will behave differently between versions (Which is not the case, but to
find out you'll have to read the implementation, or the git history, or
test code with both versions). So this is technically an API break.
More generally:
- a FIFO conceptually has a well-defined size at any given moment
- that size is can_read() + can_write()
But this could (should?) have been av_fifo_size2(). That way can_write()
could effectively become a generic "can write", instead of begin stuck
as "can write without the chance of failure".
- you can change the size by growing the FIFO
- AV_FIFO_FLAG_AUTO_GROW does not fundamentally change the above
concepts, it merely calls av_fifo_grow2() when a write would
otherwise fail
Now we can introduce a function for 'maximum number that can possibly
succeed' if you think it's useful - something like av_fifo_max_write().
Maybe, but only if we find a usecase for it.
Hendrik had an opinion about what av_fifo_can_write() should report, but
it was on IRC. But otherwise, knowing that anything we do will be
breaking, if the current behavior was your intention all along as the
author of the API, then i guess this set can go in unless someone else
chimes.
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