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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 19:33:14 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 19:33:14 +0000 |
commit | 36d22d82aa202bb199967e9512281e9a53db42c9 (patch) | |
tree | 105e8c98ddea1c1e4784a60a5a6410fa416be2de /docs/nspr/layeredpoll.rst | |
parent | Initial commit. (diff) | |
download | firefox-esr-36d22d82aa202bb199967e9512281e9a53db42c9.tar.xz firefox-esr-36d22d82aa202bb199967e9512281e9a53db42c9.zip |
Adding upstream version 115.7.0esr.upstream/115.7.0esr
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/nspr/layeredpoll.rst')
-rw-r--r-- | docs/nspr/layeredpoll.rst | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/docs/nspr/layeredpoll.rst b/docs/nspr/layeredpoll.rst new file mode 100644 index 0000000000..6f5cb80efe --- /dev/null +++ b/docs/nspr/layeredpoll.rst @@ -0,0 +1,118 @@ +PR_Poll() and the layered I/O +============================= + +*[last edited by AOF 8 August 1998]* +This memo discusses some of the nuances of using PR_Poll() in +conjunction with *layered I/O*. This is a relatively new feature in NSPR +2.0, not that it hasn't been in the source tree for a while, but in that +it has had no clients. + +Implementation +-------------- + +NSPR provides a public API function, PR_Poll() that is modeled after +UNIX' ``poll()`` system call. + +The implementation of :ref:`PR_Poll` is somewhat complicated. Not only +does it map the :ref:`PRPollDesc` array into structures needed by the +underlying OS, it also must deal with layered I/O. This is done despite +the fact that :ref:`PR_Poll` itself is *not* layered. For every element +of the :ref:`PRPollDesc` array that has a non-NULL :ref:`PRFileDesc` and whose +``in_flags`` are not zero, it calls the file descriptor's +``poll() method``. +The ``poll()`` method is one of the vector contained in the +:ref:`PRIOMethods` table. In the case of layered I/O, the elements (the +methods) of the methods table may be overridden by the implementor of +that layer. The layers are then *stacked.* I/O using that *stack* will +call through the method at the top layer, and each layer may make +altering decisions regarding how the I/O operation should proceed. + +The purpose of the ``poll()`` method is to allow a layer to modify the +flags that will ultimately be used in the call to the underlying OS' +``poll()`` (or equivalent) function. Such modification might be useful +if one was implementing an augmented stream protocol (*e.g.,* **SSL**). +SSL stands for **Secure Socket Layer**, hence the obvious applicability +as an example. But it is way to complicated to describe in this memo, so +this memo will use a much simpler layered protocol. +The example protocol is one that, in order to send *n* bytes, it must +first ask the connection's peer if the peer is willing to receive that +many bytes. The form of the request is 4 bytes (binary) stating the +number of bytes the sender wishes to transmit. The peer will send back +the number of bytes it is willing to receive (in the test code there are +no error conditions, so don't even ask). + +The implication of the protocol is obvious. In order to do a +:ref:`PR_Send` operation, the layer must first do a *different* send and +then *receive* a response. Doing this and keeping the *stack's* client +unaware is the goal. **It is not a goal of NSPR 2.0 to hide the nuances +of synchronous verses non-blocking I/O**. + +The layered methods +------------------- + +Each layer must implement a suitable function for *every* element of the +methods table. One can get a copy of default methods by calling +:ref:`PR_GetDefaultIOMethods` These methods simply pass all calls +through the layer on to the next lower layer of the stack. + +A layer implementor might copy the elements of the ``PRIOMethods`` +acquired from this function into a methods table of its own, then +override just those methods of interest. *Usually* (with only a single +exception) a layered method will perform its design duties and then call +the next lower layer's equivalent function. + +Layered ``poll()`` +------------------ + +One of the more interesting methods is the ``poll()``. It is called by +the runtime whenever the client calls :ref:`PR_Poll`. It may be called at +the *top* layer for *every* file descriptor in the poll descriptor. It +may be called zero or more times. The purpose of the ``poll()`` method +is to provide the layer an opportunity to adjust the polling bits as +needed. For instance, if a client (*i.e.*, top layer) is calling +:ref:`PR_Poll` for a particular file descriptor with a *read* poll +request, a lower layer might decide that it must perform a *write* +first. +In that case, the layer's ``poll()`` method would be called with +**``in_flags``** including a ``PR_POLL_READ`` flag. However, the +``poll()`` method would call the next lower layer's ``poll()`` method +with a ``PR_POLL_WRITE`` bit set. This process of re-assigning the poll +flags can happen as many times as there are layers in the stack. It is +the final value, the one returned to the caller of the top layer's +``poll()`` method (:ref:`PR_Poll`) that will be used by the runtime when +calling the OS' ``poll()`` (or equivalent) system call. + +It is expected that the modification of the polling bits propagate from +the top of the stack down, allowing the layer closest to the bottom of +the stack to provide the final setting. The implication is that there +should be no modifications of the **``in_flags``** during the *return* +phase of the layered function. + +For example: + +It is not advised to modify the ``final_in_flags`` between the call to +the lower layer's ``poll()`` method and the ``return`` statement. +The third argument of the ``poll()`` method is a pointer to a 16-bit +word. If the layer sets a value in memory through that pointer *and* +returns with a value that has *corresponding* bits, the runtime assumes +that the file descriptor is ready immediately. + +There are two important deviations from the normal. First, this is the +one (known) exception to having a layered routine call the stack's next +lower layer method. If bits are set in the ``out_flags`` the method +should return *directly*. Second, the runtime will observe that the +layer claims this file descriptor is ready and suppress the call to the +OS' ``poll()`` system call. + +At this time the only known use for this feature is to allow a layer to +indicate it has buffered *input*. Note that it is not appropriate for +buffered *output* since in order to write/send output the runtime must +still confirm with the OS that such an operation is permitted. + +Since the ``poll()`` method may be called zero or more times it must +therefore be *idempotent* or at least *functional*. It will need to look +at the layer's state, but must not make modifications to that state that +would cause subsequent calls within the same :ref:`PR_Poll` call to +return a different answer. Since the ``poll()`` method may not be called +at all, so there is not guarantee that any modifications that would have +been performed by the routine will every happen. |