Commit 57a25a01 authored by Stephen Morris's avatar Stephen Morris
Browse files

[2980] Changes as a result for the first part of the review

parent d63e340a
......@@ -52,12 +52,12 @@ CalloutHandle::~CalloutHandle() {
context_collection_.clear();
// Normal destruction of the remaining variables will include the
// destruction of lm_collection_, wn action that will decrement the
// reference count on the library manager collection (which holds the
// libraries that could have allocated memory in the argument and context
// members). When that goes to zero, the libraries will be unloaded:
// at that point nothing in the hooks framework will be pointing to memory
// in the libraries' address space.
// destruction of lm_collection_, an action that decrements the reference
// count on the library manager collection (which holds the libraries that
// could have allocated memory in the argument and context members.) When
// that goes to zero, the libraries will be unloaded: at that point nothing
// in the hooks framework will be pointing to memory in the libraries'
// address space.
//
// It is possible that some other data structure in the server (the program
// using the hooks library) still references the address space and attempts
......
......@@ -80,11 +80,11 @@ class LibraryManagerCollection;
/// "context_destroy" callout. The information is accessed through the
/// {get,set}Context() methods.
///
/// - Per-library handle. Allows the callout to dynamically register and
/// deregister callouts. (In the latter case, only functions registered by
/// functions in the same library as the callout doing the deregistration
/// can be removed: callouts registered by other libraries cannot be
/// modified.)
/// - Per-library handle (LibraryHandle). The library handle allows the
/// callout to dynamically register and deregister callouts. In the latter
/// case, only functions registered by functions in the same library as the
/// callout doing the deregistration can be removed: callouts registered by
/// other libraries cannot be modified.
class CalloutHandle {
public:
......
......@@ -17,60 +17,65 @@
@section hooksComponentIntroduction Introduction
The hooks framework is a BIND 10 library that simplifies the way that
The hooks framework is a BIND 10 system that simplifies the way that
users can write code to modify the behavior of BIND 10. Instead of
altering the BIND 10 source code, they write functions that are compiled
and linked into a shared library. The library is specified in the BIND 10
configuration database and run time, BIND 10 dynamically loads the library
into its address space. At various points in the processing, the server
into its address space. At various points in the processing, the component
"calls out" to functions in the library, passing to them the data is it
currently working on. They can examine and modify the data as required.
The document @ref hooksDevelopersGuide describes how to write a library
that interfaces into a BIND 10 component. This guide describes how to
write or modify a BIND 10 component so that it can load a shared library
and call out to functions in it.
This guide is aimed at BIND 10 developers who want to write or modify a
BIND 10 component to use hooks. It shows how the component should be written
to load a shared library at run-time and how to call functions in it.
For information about writing a hooks library containing functions called by BIND 10
during its execution, see the document @ref hooksDevelopersGuide.
@subsection hooksComponentTerminology Terminology
In the remainder of this guide, the following terminology is used:
- Component - a BIND 10 process, e.g. the authoritative DNS server or the
DHCPv4 server.
- Hook/Hook Point - used interchageably, this is a point in the code at
which a call to user-written functions is made. Each hook has a name and
each hook can have any number (including 0) of user-written functions
attached to it.
- Callout - a user-written function called by the server at a hook
point. This is so-named because the server "calls out" to the library
- Callout - a user-written function called by the component at a hook
point. This is so-named because the component "calls out" to the library
to execute a user-written function.
- Component - a BIND 10 process, e.g. the authoritative server or the
DHCPv4 server.
- User code/user library - non-BIND 10 code that is compiled into a
shared library and loaded by BIND 10 into its address space. Multiple
user libraries can be loaded at the same time, each containing callouts for
the same hooks. The hooks framework calls these libraries one after the
other. (See the document @hooksDevelopersGuide for more details.)
other. (See the document @ref hooksDevelopersGuide for more details.)
@subsection hooksComponentLanguages Languages
The core of BIND 10 is written in C++ with some parts in Python. While it is
the intention to provide the hooks framework for all languages, the initial
versions are for C++. All examples in this guide are in that language.
version is for C++. All examples in this guide are in that language.
@section hooksComponentBasicIdeas Basic Ideas
From the point of view of the component author, the basic ideas of the hooks
framework are quite simple:
- The hook points need to be defined.
- The location of hook points in the code need to be determined.
- At each hook point, the component needs to:
- copy data into the object used to pass information to the callout.
- call the callout.
- copy data back from the object used to exchange information.
- take action based on information returned.
- Name the hook points and register them.
- At each hook point, the component needs to complete the following steps to
execute callouts registered by the user-library:
-# copy data into the object used to pass information to the callout.
-# call the callout.
-# copy data back from the object used to exchange information.
-# take action based on information returned.
Of course, to set up the system the libraries need to be loaded in the first
place. The component also needs to:
......@@ -82,59 +87,64 @@ component.
The following sections will describe these tasks in more detail.
@section hooksComponentDefinition Defining the Hook Points
Before any other action takes place, the hook points in the code need to be
defined. Each hook point has a name that must be unique amongst all hook
points for the server, and the first step is to register those names. The
naming is done using the static method isc::hooks::HooksManager::registerHook():
@section hooksComponentDefinition Determing the Hook Points
Before any other action takes place, the location of the hook points
in the code need to be determined. This of course depends on the
component but as a general guideline, hook locations should be chosen
where a callout is able to obtain useful information from BIND 10 and/or
affect processing. Typically this means at the start or end of a major
step in the processing of a request, at a point where either useful
information can be passed to a callout and/or the callout can affect
the processing of the component. The latter is achieved in either or both
of the following eays:
- Setting the "skip" flag. This is a boolean flag that the callout can set
and is a quick way of passing information back to the component. It is used
to indicate that the component should skip the processing step associated with
the hook. The exact action is up to the component, but is likely to be one
of skipping the processing step (probably because the callout has
done its own processing for the action) or dropping the current packet
and starting on a new request.
- Modifying data passed to it. The component should be prepared to continue
processing with the data returned by the callout. It is up to the component
author whether the data is validated before being used, but doing so will
have performance implications.
@section hooksComponentRegistration Naming and Registering the Hooks Points
Once the location of the hook point has been determined, it should be
given a name. This name should be unique amongst all hook points and is
subject to certain restrictions (see below).
Before the callouts at any hook point are called and any user libraries
loaded - so typically during component initialization - the component must
register the names of all the hooks. The registration is done using
the static method isc::hooks::HooksManager::registerHook():
@code
#include <hooks/hooks_manager.h>
:
int example_index = HooksManager::registerHook("manager");
int example_index = HooksManager::registerHook("lease_allocate");
@endcode
The name of the hooks is passed as the sole argument to the
HooksManager::registerHook() method. The value returned is the index of that
hook point and should be retained - it is needed to call the hook.
All hooks used by the component must be registered before the component
starts operations.
@subsection hooksComponentHookNames Hook Names
The name of the hook is passed as the sole argument to the registerHook()
method. The value returned is the index of that hook point and should
be retained - it is needed to call the callouts attached to that hook.
Hook names are strings and in principle, any string can be used as the
name of a hook, even one containing spaces and non-printable characters.
However, the following guidelines should be observed:
- The names <b>context_create</b> and <b>context_destroy</b> are reserved to
the hooks system and are automatically registered: an attempt to register
one of these will lead to a isc::hooks::DuplicateHook exception being thrown.
- The hook name should be a valid function name in C. If a user gives a
callout the same name as one of the hooks, the hooks framework will
automatically load that callout and attach it to the hook: the user does not
have to explicitly register it. <b>TBD: do we still want this given
the possibility of confusion with functions in system libraries?</b>
- The hook name should not conflict with the name of a function in any of
the system libraries (e.g. naming a hook "sqrt" could lead to the
square-root function in the system's maths library being attached to the hook
as a callout).
- Although hook names can be in any case (including mixed case), the BIND 10
convention is that they are lower-case.
Note that a hook only needs to be registered once. There is no mechanism for
unregistering a hook and there is no need to do so.
@subsection hooksComponentAutomaticRegistration Automatic Registration of Hooks
In some components, it may be convenient to set up a separate
initialization function that registers all hooks. For others, it may
be more convenient for each module within the component to perform its
own initialization. Since the HooksManager object is a singleton and
is created when first requested, a useful trick is to automatically
register the hooks when the module is loaded.
In some components, it may be convenient to set up a single initialization
function that registers all hooks. For others, it may be more convenient
for each module within the component to perform its own initialization.
Since the isc::hooks::HooksManager object is a singleton and is created when first
accessed, a useful trick is to automatically register the hooks when
the module is loaded.
This technique involves declaring an object outside of any execution
unit in the module. When the module is loaded, the object's constructor
......@@ -151,8 +161,8 @@ namespace {
// Declare structure to perform initialization and store the hook indexes.
//
struct MyHooks {
int pkt_rcvd; // Packet received
int pkt_sent; // Packet sent
int pkt_rcvd; // Index of "packet received" hook
int pkt_sent; // Index of "packet sent" hook
// Constructor
MyHooks() {
......@@ -161,23 +171,47 @@ struct MyHooks {
}
};
// Instantiate a "Hooks" object. The constructor is run when the module is
// loaded and so the hook indexes will be defined before any method in this
// Declare a "MyHooks" object. As this is outside any function or method, it
// will be instantiated (and the constructor run) when the module is loaded.
// As a result, the hook indexes will be defined before any method in this
// module is called.
Hooks hooks;
MyHooks my_hooks;
} // Anonymous namespace
void Someclass::someFunction() {
:
// Check if any callouts are defined on the pkt_rcvd hook.
if (HooksManager::calloutPresent(hooks.pkt_rcvd)) {
if (HooksManager::calloutPresent(my_hooks.pkt_rcvd)) {
:
}
:
}
@endcode
@subsection hooksComponentHookNames Hook Names
Hook names are strings and in principle, any string can be used as the
name of a hook, even one containing spaces and non-printable characters.
However, the following guidelines should be observed:
- The names <b>context_create</b> and <b>context_destroy</b> are reserved to
the hooks system and are automatically registered: an attempt to register
one of these will lead to a isc::hooks::DuplicateHook exception being thrown.
- The hook name should be a valid "C" function name. If a user gives a
callout the same name as one of the hooks, the hooks framework will
automatically load that callout and attach it to the hook: the user does not
have to explicitly register it.
- The hook name should not conflict with the name of a function in any of
the system libraries (e.g. naming a hook "sqrt" could lead to the
square-root function in the system's maths library being attached to the hook
as a callout).
- Although hook names can be in any case (including mixed case), the BIND 10
convention is that they are lower-case.
@section hooksComponentCallingCallouts Calling Callouts on a Hook
@subsection hooksComponentArgument The Callout Handle
......@@ -187,12 +221,16 @@ how to pass data to it.
Each user callout has the signature:
@code
int callout_name(CalloutHandle& handle);
int callout_name(isc::hooks::CalloutHandle& handle);
@endcode
The isc::hooks::CalloutHandle object is the object used to pass data to
and from the callout. This holds the data as a set of name/value pairs,
each pair being considered an argument to the callout.
each pair being considered an argument to the callout. If there are
multiple callouts attached to a hook, the CalloutHandle is passed to
each in turn. Should a callout modify an argument, the updated data is
passed subsequent callouts (each of which could also modify it) before
being returned to the component.
Two methods are provided to get and set the arguments passed to
the callout called (naturally enough) getArgument and SetArgument.
......@@ -202,23 +240,25 @@ Their usage is illustrated by the following code snippets.
int count = 10;
boost::shared_ptr<Pkt4> pktptr = ... // Set to appropriate value
// Assume that "handle" has been created
handle.setArgument("data_count", count);
handle.setArgument("inpacket", pktptr);
// Assume that "handle_ptr" has been created and is a pointer to a
// CalloutHandle.
handle_ptr->setArgument("data_count", count);
handle_ptr->setArgument("inpacket", pktptr);
// Call the hook code...
...
// Call the hook code. lease_assigned_index is the value returned from
// HooksManager::registerHook() when the hook was registered.
HooksManager::callCallouts(lease_assigned_index, *handle_ptr);
// Retrieve the modified values
handle.getArgument("data_count", count);
handle.getArgument("inpacket", pktptr);
handle_ptr->getArgument("data_count", count);
handle_ptr->getArgument("inpacket", pktptr);
@endcode
As can be seen "getArgument" is used to retrieve data from the
isc::hooks::CalloutHandle, and setArgument used to put data into it.
If a callout wishes to alter data and pass it back to the server,
it should retrieve the data with getArgument, modify it, and call
setArgument to send it back.
CalloutHandle, and "setArgument" used to put data into it. If a callout
wishes to alter data and pass it back to the component, it should retrieve
the data with getArgument, modify it, and call setArgument to send
it back.
There are a couple points to be aware of:
......@@ -234,7 +274,7 @@ data pointed to by pointers, e.g. if an argument is defined as a "char*",
an exception will be thrown if an attempt is made to retrieve it into
a variable of type "const char*". (However, if an argument is set as a
"const int", it can be retrieved into an "int".) The documentation of
each hook point should detail the exact data type of each argument.
a hook point should detail the exact data type of each argument.
- If a pointer to an object is passed to a callout (either a "raw"
pointer, or a boost smart pointer (as in the example above), and the
......@@ -242,6 +282,47 @@ underlying object is altered through that pointer, the change will be
reflected in the component even if the callout makes no call to setArgument.
This can be avoided by passing a pointer to a "const" object.
@subsection hooksComponentSkipFlag The Skip Flag
Although information is passed back to the component from callouts through
CalloutHandle arguments, a common action for callouts is to inform the component
that its flow of control should be altered. For example:
- In the DHCP servers, there is a hook at the point at which a lease is
about to be assigned. Callouts attached to this hooks may handle the
lease assignment in special cases, in which case they set the skip flag
to indicate that the server should not perform lease assignment in this
case.
- A server may define a hook just after a packet is received. A callout
attached to the hook might inspect the source address and compare it
against a blacklist. If the address is on the list, the callout could set
the skip flag to indicate to the server that the packet should be dropped.
For ease of processing, the CalloutHandle contains
two methods, isc::hooks::CalloutHandle::getSkip() and
isc::hooks::CalloutHandle::setSkip(). It is only meaningful for the
component to use the "get" method. The skip flag is cleared by the hooks
framework when the component requests that callouts be executed, so any
value set by the component is lost. Callouts can both inspect the flag (it
might have been set by callouts earlier in the callout list for the hook)
and set it. Note that the setting of the flag by a callout does not
prevent callouts later in the list from being called: the skip flag is
just a boolean flag - the only significance comes from its interpretation
by the component.
An example of use could be:
@code
// Set up arguments for DHCP lease assignment.
handle->setArgument("query", query);
handle->setArgument("response", response);
HooksManager::callCallouts(lease_hook_index, *handle_ptr);
if (! handle_ptr->getSkip()) {
// Skip flag not set, do the address allocation
:
}
@endcode
@subsection hooksComponentGettingHandle Getting the Callout Handle
The CalloutHandle object is linked to the loaded libraries
......@@ -249,15 +330,17 @@ for lifetime reasons (described below). Components
should retrieve a isc::hooks::CalloutHandle using
isc::hooks::HooksManager::createCalloutHandle():
@code
CalloutHandlePtr handle = HooksManager::createCalloutHandle();
CalloutHandlePtr handle_ptr = HooksManager::createCalloutHandle();
@endcode
(isc::hooks::CalloutHandlePtr is a typedef for a boost shared pointer to a
(isc::hooks::CalloutHandlePtr is a typedef for a Boost shared pointer to a
CalloutHandle.) The CalloutHandle so retrieved may be used for as
long as the libraries are loaded.
The handle is deleted by resetting the pointer:
@code
handle.reset();
handle_ptr.reset();
@endcode
... or by letting the handle object go out of scope. The actual deletion
... or by letting the handle pointer go out of scope. The actual deletion
occurs when the CallHandle's reference count goes to zero. (The
current version of the hooks framework does not maintain any other
pointer to the returned CalloutHandle, so it gets destroyed when the
......@@ -271,52 +354,25 @@ isc::hooks::HooksManager::callCallouts() method for the hook index in
question. For example, with the hook index pkt_sent defined as above,
the hook can be executed by:
@code
HooksManager::callCallouts(pkt_rcvd, *handle);
HooksManager::callCallouts(pkt_sent, *handle_ptr);
@endcode
... where "*handle" is a reference (note: not a pointer) to the
... where "*handle_ptr" is a reference (note: not a pointer) to the
isc::hooks::CalloutHandle object holding the arguments. No status code
is returned. If a component needs to get data returned, it should define
an argument through which the callout can do so.
Actually, the statement "no status code is returned" is not strictly true. At
many hooks, the following logic applies:
@code
call hook_code
if (hook_code has not performed an action) {
perform the action
}
@endcode
For example, in a DHCP server an address should be allocated for a client.
The DHCP server does that by default, but the hook code may want to override
it in some situations.
As this logic is so common, the CalloutHandle includes a "skip" flag. This
is a boolean flag that can be set by the callout to pass a basic yes/no
response to the component. Its use is illustrated by the following code
snippet:
@code
// Set up arguments for lease assignment
handle->setArgument("query", query);
handle->setArgument("response", response);
HooksManager::callCallouts(lease_hook_index, *handle);
if (! handle->getSkip()) {
// Skip flag not set, do the address allocation
:
}
@endcode
<b>SHOULD WE GET RID OF THE SKIP FLAG AND WHERE APPROPRIATE, SIGNAL SUCH
PROCESSING THROUGH AN ARGUMENT?</b>
is returned. If a component needs to get data returned (other than that
provided by the "skip" flag), it should define an argument through which
the callout can do so.
@subsubsection hooksComponentConditionalCallout Conditionally Calling Hook Callouts
Most hooks in a server will not have callouts attached to them. To avoid the
overhead of setting up arguments in the CalloutHandle, a component can
check for callouts before doing that processing. The
isc::hooks::HooksManager::calloutsPresent() method performs this check.
Taking the index of a hook as its sole argument, it returns true if there
are any callouts attached to the hook and false otherwise.
Most hooks in a component will not have callouts attached to them. To
avoid the overhead of setting up arguments in the CalloutHandle, a
component can check for callouts before doing that processing using
isc::hooks::HooksManager::calloutsPresent(). Taking the index of a
hook as its sole argument, the function returns true if there are any
callouts attached to the hook and false otherwise.
With this check, the above example can be modified to:
With this check, the code in the component for calling a hook would look
something like:
@code
if (HooksManager::calloutsPresent(lease_hook_index)) {
// Set up arguments for lease assignment
......@@ -332,7 +388,7 @@ if (HooksManager::calloutsPresent(lease_hook_index)) {
@section hooksComponentLoadLibraries Loading the User Libraries
Once hooks are defined, all the hooks code describe above will
Once hooks are defined, all the hooks code described above will
work, even if no libraries are loaded (and even if the library
loading method is not called). The CalloutHandle returned by
isc::hooks::HooksManager::createCalloutHandle() will be valid,
......@@ -365,8 +421,8 @@ loadLibraries() with an empty vector as an argument.
@subsection hooksComponentUnloadIssues Unload and Reload Issues
Unloading a shared library works by unmapping the part of the process's
virtual address space in which the library lies. This may lead to problems
consequences if there are still references to that address space elsewhere
virtual address space in which the library lies. This may lead to
problems if there are still references to that address space elsewhere
in the process.
In many operating systems, heap storage allowed by a shared library will
......@@ -376,21 +432,21 @@ in the hooks framework because:
- Argument information stored in a CalloutHandle by a callout in a library
may lie in the library's address space.
- Data modified in objects passed as arguments may lie in the address
space. For example, it is common for a DHCP callout to add "options" to
a packet: the memory allocated for those options will like in library address
space.
space. For example, it is common for a DHCP callout to add "options"
to a packet: the memory allocated for those options will most likely
lie in library address space.
The problem really arises because of the extensive use by BIND 10 of boost
smart pointers. When the pointer is destroyed, the pointed-to memory is
deallocated. If the pointer points to address space that is unmapped because
a library has been unloaded, the deletion causes a segmentation fault.
The hooks framework addresses the issue for CalloutHandles by keeping
in that object a shared pointer to the object controlling library
unloading. Although a library can be unloaded at any time, it is only when
all CalloutHandles that could possibly reference address space in the
library have been deleted that the library will be unloaded and the address
space unmapped.
The hooks framework addresses the issue for CalloutHandles by keeping in
that object a shared pointer to the object controlling library unloading.
Although a library can be unloaded at any time, it is only when all
CalloutHandles that could possibly reference address space in the library
have been deleted that the library will actually be unloaded and the
address space unmapped.
The hooks framework cannot solve the second issue as the objects in
question are under control of the component. It is up to the component
......@@ -421,7 +477,7 @@ the LibraryHandle to register and deregister callouts is described in
Finally, it should be noted that callouts registered in this way only
remain registered until the next call to isc::hooks::loadLibraries().
It is up to the server to re-register the callouts after this
It is up to the component to re-register the callouts after this
method has been called.
*/
......@@ -151,7 +151,7 @@ private:
///
/// Constructor is declared private to enforce the singleton nature of
/// the object. A reference to the singleton is obtainable through the
/// ggetServerHooks() static method.
/// getServerHooks() static method.
///
/// @throws isc::Unexpected if the registration of the pre-defined hooks
/// fails in some way.
......
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