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Signed-off-by: Andrea Bolognani <abologna@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
299 lines
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299 lines
12 KiB
ReStructuredText
=================================
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Implementing a new API in Libvirt
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=================================
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.. contents::
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This document walks you through the process of implementing a new API in
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libvirt. Remember that new API consists of any new public functions, as
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well as the addition of flags or extensions of XML used by existing
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functions.
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Before you begin coding, it is critical that you propose your changes on
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the libvirt mailing list and get feedback on your ideas to make sure
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what you're proposing fits with the general direction of the project.
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Even before doing a proof of concept implementation, send an email
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giving an overview of the functionality you think should be added to
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libvirt. Someone may already be working on the feature you want. Also,
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recognize that everything you write is likely to undergo significant
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rework as you discuss it with the other developers, so don't wait too
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long before getting feedback.
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Adding a new API to libvirt is not difficult, but there are quite a few
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steps. This document assumes that you are familiar with C programming
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and have checked out the libvirt code from the source code repository
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and successfully built the existing tree. Instructions on how to check
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out and build the code can be found at:
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https://libvirt.org/downloads.html
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Once you have a working development environment, the steps to create a
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new API are:
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#. define the public API
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#. define the internal driver API
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#. implement the public API
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#. implement the remote protocol:
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#. define the wire protocol format
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#. implement the RPC client
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#. implement the server side dispatcher
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#. use new API where appropriate in drivers
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#. add virsh support
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#. add common handling for new API
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#. for each driver that can support the new API:
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#. add prerequisite support
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#. fully implement new API
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It is, of course, possible to implement the pieces in any order, but if
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the development tasks are completed in the order listed, the code will
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compile after each step. Given the number of changes required,
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verification after each step is highly recommended.
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Submit new code in the form of one patch per step. That's not to say
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submit patches before you have working functionality--get the whole
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thing working and make sure you're happy with it. Then use git to break
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the changes into pieces so you don't drop a big blob of code on the
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mailing list in one go. Also, you should follow the upstream tree, and
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rebase your series to adapt your patches to work with any other changes
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that were accepted upstream during your development.
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Don't mix anything else into the patches you submit. The patches should
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be the minimal changes required to implement the functionality you're
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adding. If you notice a bug in unrelated code (i.e., code you don't have
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to touch to implement your API change) during development, create a
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patch that just addresses that bug and submit it separately.
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Defining the public API
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-----------------------
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The first task is to define the public API. If the new API involves an
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XML extension, you have to enhance the RelaxNG schema and document the
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new elements or attributes:
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* ``src/conf/schemas/domaincommon.rng``
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* ``docs/formatdomain.rst``
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If the API extension involves a new function, you have to add a
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declaration in the public header, and arrange to export the function
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name (symbol) so other programs can link against the libvirt library and
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call the new function:
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* ``include/libvirt/libvirt-$MODULE.h.in``
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* ``src/libvirt_public.syms``
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Please consult our `coding
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style <coding-style.html#xml-element-and-attribute-naming>`__ guide on
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elements and attribute names.
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This task is in many ways the most important to get right, since once
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the API has been committed to the repository, it's libvirt's policy
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never to change it. Mistakes in the implementation are bugs that you can
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fix. Make a mistake in the API definition and you're stuck with it, so
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think carefully about the interface and don't be afraid to rework it as
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you go through the process of implementing it.
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Defining the internal API
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-------------------------
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Each public API call is associated with a driver, such as a host
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virtualization driver, a network virtualization driver, a storage
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virtualization driver, a state driver, or a device monitor. Adding the
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internal API is ordinarily a matter of adding a new member to the struct
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representing one of these drivers.
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Of course, it's possible that the new API will involve the creation of
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an entirely new driver type, in which case the changes will include the
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creation of a new struct type to represent the new driver type.
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The driver structs are defined in:
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* ``src/driver-$MODULE.h``
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To define the internal API, first typedef the driver function prototype
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and then add a new field for it to the relevant driver struct. Then,
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update all existing instances of the driver to provide a ``NULL`` stub
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for the new function.
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Implementing the public API
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---------------------------
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Implementing the public API is largely a formality in which we wire up
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public API to the internal driver API. The public API implementation
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takes care of some basic validity checks before passing control to the
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driver implementation. In RFC 2119 vocabulary, this function:
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#. SHOULD log a message with VIR_DEBUG() indicating that it is being
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called and its parameters;
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#. MUST call virResetLastError();
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#. SHOULD confirm that the connection is valid with
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virCheckConnectReturn() or virCheckConnectGoto();
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#. **SECURITY: If the API requires a connection with write privileges,
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MUST confirm that the connection flags do not indicate that the
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connection is read-only with virCheckReadOnlyGoto();**
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#. SHOULD do basic validation of the parameters that are being passed
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in, using helpers like virCheckNonNullArgGoto();
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#. MUST confirm that the driver for this connection exists and that it
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implements this function;
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#. MUST call the internal API;
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#. SHOULD log a message with VIR_DEBUG() indicating that it is
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returning, its return value, and status.
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#. MUST return status to the caller.
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The public API calls are implemented in:
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* ``src/libvirt-$MODULE.c``
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Implementing the remote protocol
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--------------------------------
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Implementing the remote protocol is essentially a straightforward
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exercise which is probably most easily understood by referring to the
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existing code.
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Defining the wire protocol format
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Defining the wire protocol involves making additions to:
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* ``src/remote/remote_protocol.x``
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First, create two new structs for each new function that you're adding
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to the API. One struct describes the parameters to be passed to the
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remote function, and a second struct describes the value returned by the
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remote function. The one exception to this rule is that functions that
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return only 0 or -1 for status do not require a struct for returned
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data.
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Second, add values to the remote_procedure enum for each new function
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added to the API.
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Once these changes are in place, it's necessary to run 'make rpcgen' in
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the src directory to create the .c and .h files required by the remote
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protocol code. This must be done on a Linux host using the GLibC rpcgen
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program. Other rpcgen versions may generate code which results in bogus
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compile time warnings. This regenerates the following files:
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* ``src/remote/remote_daemon_dispatch_stubs.h``
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* ``src/remote/remote_daemon_dispatch.h``
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* ``src/remote/remote_daemon_dispatch.c``
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* ``src/remote/remote_protocol.c``
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* ``src/remote/remote_protocol.h``
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Implement the RPC client
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~~~~~~~~~~~~~~~~~~~~~~~~
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Implementing the RPC client uses the rpcgen generated .h files. The
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remote method calls go in:
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* ``src/remote/remote_driver.c``
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Each remote method invocation does the following:
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#. locks the remote driver;
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#. sets up the method arguments;
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#. invokes the remote function;
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#. checks the return value, if necessary;
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#. extracts any returned data;
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#. frees any returned data;
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#. unlocks the remote driver.
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Implement the server side dispatcher
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Implementing the server side of the remote function call is simply a
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matter of deserializing the parameters passed in from the remote caller
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and passing them to the corresponding internal API function. The server
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side dispatchers are implemented in:
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* ``src/remote/remote_daemon_dispatch.c``
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Again, this step uses the .h files generated by make rpcgen.
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After all three pieces of the remote protocol are complete, and the
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generated files have been updated, it will be necessary to update the
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file:
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* ``src/remote_protocol-structs``
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This file should only have new lines added; modifications to existing
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lines probably imply a backwards-incompatible API change.
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Use the new API internally
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--------------------------
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Sometimes, a new API serves as a superset of existing API, by adding
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more granularity in what can be managed. When this is the case, it makes
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sense to share a common implementation by making the older API become a
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trivial wrapper around the new API, rather than duplicating the common
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code. This step should not introduce any semantic differences for the
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old API, and is not necessary if the new API has no relation to existing
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API.
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Expose the new API in virsh
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---------------------------
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All new API should be manageable from the virsh command line shell. This
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proves that the API is sufficient for the intended purpose, and helps to
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identify whether the proposed API needs slight changes for easier usage.
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However, remember that virsh is used to connect to hosts running older
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versions of libvirtd, so new commands should have fallbacks to an older
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API if possible; implementing the virsh hooks at this point makes it
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very easy to test these fallbacks. Also remember to document virsh
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additions.
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A virsh command is composed of a few pieces of code. You need to define
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an array of vshCmdInfo structs for each new command that contain the
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help text and the command description text. You also need an array of
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vshCmdOptDef structs to describe the command options. Once you have
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those pieces in place you can write the function implementing the virsh
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command. Finally, you need to add the new command to the commands[]
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array. The following files need changes:
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* ``tools/virsh-$MODULE.c``
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* ``tools/virsh.pod``
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Implement the driver methods
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----------------------------
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So, after all that, we get to the fun part. All functionality in libvirt
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is implemented inside a driver. Thus, here is where you implement
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whatever functionality you're adding to libvirt. You'll either need to
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add additional files to the src directory or extend files that are
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already there, depending on what functionality you're adding.
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Implement common handling
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~~~~~~~~~~~~~~~~~~~~~~~~~
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If the new API is applicable to more than one driver, it may make sense
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to provide some utility routines, or to factor some of the work into the
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dispatcher, to avoid reimplementing the same code in every driver. In
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the example code, this involved adding a member to the virDomainDef
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struct for mapping between the XML API addition and the in-memory
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representation of a domain, along with updating all clients to use the
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new member. Up to this point, there have been no changes to existing
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semantics, and the new APIs will fail unless they are used in the same
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way as the older API wrappers.
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Implement driver handling
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The remaining patches should only touch one driver at a time. It is
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possible to implement all changes for a driver in one patch, but for
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review purposes it may still make sense to break things into simpler
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steps. Here is where the new APIs finally start working.
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It is always a good idea to patch the test driver in addition to the
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target driver, to prove that the API can be used for more than one
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driver.
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Any cleanups resulting from the changes should be added as separate
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patches at the end of the series.
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Once you have working functionality, run ninja test on each patch of the
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series before submitting patches. It may also be worth writing tests for
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the libvirt-TCK testsuite to exercise your new API, although those
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patches are not kept in the libvirt repository.
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