Now it is possible for a reader to subscribe and wait for new events
sent to a ring buffer. When new events are written to a ring buffer,
the applets that are subscribed are woken up to display new events.
For now we only support this with the CLI applet called by "show events"
since the I/O handler is indeed a CLI I/O handler. But it's not
complicated to add other mechanisms to consume events and forward them
to external log servers for example. The wait mode is enabled by adding
"-w" after "show events <sink>". An extra "-n" was added to directly
seek to new events only.
There are two problems with the way we count watchers on a ring:
- the test for >=255 was accidently kept to 1 used during QA
- if the producer deletes all data up to the reader's position
and the reader is called, cannot write, and is called again,
it will have a zero offset, causing it to initialize itself
multiple times and each time adding a new refcount.
Let's change this and instead use ~0 as the initial offset as it's
not possible to have it as a valid one. We now store it into the
CLI context's size_t o0 instead of casting it to a void*.
No backport needed.
The three functions (attach, IO handler, and release) are meant to be
called by any CLI command which requires to dump the contents of a ring
buffer. We do not implement anything generic to dump any ring buffer on
the CLI since it's meant to be used by other functionalities above.
However these functions deal with locking and everything so it's trivial
to embed them in other code.
This function tries to write to the ring buffer, possibly removing enough
old messages to make room for the new one. It takes two arrays of fragments
on input to ease the insertion of prefixes by the caller. It atomically
writes the message, possibly truncating it if desired, and returns the
operation's status.
Our circular buffers are well suited for being used as ring buffers for
not-so-structured data. The machanism here consists in making room in a
buffer before inserting a new record which is prefixed by its size, and
looking up next record based on the previous one's offset and size. We
can have up to 255 consumers watching for data (dump in progress, tail)
which guarantee that entrees are not recycled while they're being dumped.
The complete representation is described in the header file. For now only
ring_new(), ring_resize() and ring_free() are created.
