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docs: Convert 'errors' page to rST

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Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
This commit is contained in:
Peter Krempa 2022-03-07 14:46:22 +01:00
parent ac5c17a2fb
commit 3c489dbbe3
3 changed files with 110 additions and 85 deletions
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84
docs/errors.html.in
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@ -1,84 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<body>
<h1 >Handling of errors</h1>
<p>The main goals of libvirt when it comes to error handling are:</p>
<ul>
<li>provide as much detail as possible</li>
<li>provide the information as soon as possible</li>
<li>dont force the library user into one style of error handling</li>
</ul>
<p>As result the library provide both synchronous, callback based and
asynchronous error reporting. When an error happens in the library code the
error is logged, allowing to retrieve it later and if the user registered an
error callback it will be called synchronously. Once the call to libvirt ends
the error can be detected by the return value and the full information for
the last logged error can be retrieved.</p>
<p>To avoid as much as possible troubles with a global variable in a
multithreaded environment, libvirt will associate when possible the errors to
the current connection they are related to, that way the error is stored in a
dynamic structure which can be made thread specific. Error callback can be
set specifically to a connection with</p>
<p>So error handling in the code is the following:</p>
<ol>
<li>if the error can be associated to a connection for example when failing
to look up a domain
<ol><li>if there is a callback associated to the connection set with <a href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
call it with the error information</li><li>otherwise if there is a global callback set with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
call it with the error information</li><li>otherwise call <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
which is the default error function of the library issuing the error
on stderr</li><li>save the error in the connection for later retrieval with <a href="html/libvirt-virterror.html#virConnGetLastError">virConnGetLastError</a></li></ol></li>
<li>otherwise like when failing to create a hypervisor connection:
<ol><li>if there is a global callback set with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
call it with the error information</li><li>otherwise call <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
which is the default error function of the library issuing the error
on stderr</li><li>save the error in the connection for later retrieval with <a href="html/libvirt-virterror.html#virGetLastError">virGetLastError</a></li></ol></li>
</ol>
<p>In all cases the error information is provided as a <a href="html/libvirt-virterror.html#virErrorPtr">virErrorPtr</a> pointer to
read-only structure <a href="html/libvirt-virterror.html#virError">virError</a> containing the
following fields:</p>
<ul>
<li>code: an error number from the <a href="html/libvirt-virterror.html#virErrorNumber">virErrorNumber</a>
enum</li>
<li>domain: an enum indicating which part of libvirt raised the error see
<a href="html/libvirt-virterror.html#virErrorDomain">virErrorDomain</a></li>
<li>level: the error level, usually VIR_ERR_ERROR, though there is room for
warnings like VIR_ERR_WARNING</li>
<li>message: the full human-readable formatted string of the error</li>
<li>conn: if available a pointer to the <a href="html/libvirt-libvirt-host.html#virConnectPtr">virConnectPtr</a>
connection to the hypervisor where this happened</li>
<li>dom: if available a pointer to the <a href="html/libvirt-libvirt-domain.html#virDomainPtr">virDomainPtr</a> domain
targeted in the operation</li>
</ul>
<p>and then extra raw information about the error which may be initialized
to 0 or NULL if unused</p>
<ul>
<li>str1, str2, str3: string information, usually str1 is the error
message format</li>
<li>int1, int2: integer information</li>
</ul>
<p>So usually, setting up specific error handling with libvirt consist of
registering a handler with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a> or
with <a href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
check the value of the code value, take appropriate action, if needed let
libvirt print the error on stderr by calling <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>.
For asynchronous error handing, set such a function doing nothing to avoid
the error being reported on stderr, and call virConnGetLastError or
virGetLastError when an API call returned an error value. It can be a good
idea to use <a href="html/libvirt-virterror.html#virResetLastError">virResetError</a> or <a href="html/libvirt-virterror.html#virConnResetLastError">virConnResetLastError</a>
once an error has been processed fully.</p>
<p>At the python level, there only a global reporting callback function at
this point, see the error.py example about it:</p>
<pre>def handler(ctxt, err):
global errno
#print "handler(%s, %s)" % (ctxt, err)
errno = err
libvirt.registerErrorHandler(handler, 'context') </pre>
<p>the second argument to the registerErrorHandler function is passed as the
first argument of the callback like in the C version. The error is a tuple
containing the same field as a virError in C, but cast to Python.</p>
</body>
</html>

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docs/errors.rst Normal file
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@ -0,0 +1,109 @@
==================
Handling of errors
==================
The main goals of libvirt when it comes to error handling are:
- provide as much detail as possible
- provide the information as soon as possible
- dont force the library user into one style of error handling
As result the library provide both synchronous, callback based and asynchronous
error reporting. When an error happens in the library code the error is logged,
allowing to retrieve it later and if the user registered an error callback it
will be called synchronously. Once the call to libvirt ends the error can be
detected by the return value and the full information for the last logged error
can be retrieved.
To avoid as much as possible troubles with a global variable in a multithreaded
environment, libvirt will associate when possible the errors to the current
connection they are related to, that way the error is stored in a dynamic
structure which can be made thread specific. Error callback can be set
specifically to a connection with
So error handling in the code is the following:
#. if the error can be associated to a connection for example when failing to
look up a domain
#. if there is a callback associated to the connection set with
`virConnSetErrorFunc <html/libvirt-virterror.html#virConnSetErrorFunc>`__,
call it with the error information
#. otherwise if there is a global callback set with
`virSetErrorFunc <html/libvirt-virterror.html#virSetErrorFunc>`__, call it
with the error information
#. otherwise call
`virDefaultErrorFunc <html/libvirt-virterror.html#virDefaultErrorFunc>`__
which is the default error function of the library issuing the error on
stderr
#. save the error in the connection for later retrieval with
`virConnGetLastError <html/libvirt-virterror.html#virConnGetLastError>`__
#. otherwise like when failing to create a hypervisor connection:
#. if there is a global callback set with
`virSetErrorFunc <html/libvirt-virterror.html#virSetErrorFunc>`__, call it
with the error information
#. otherwise call
`virDefaultErrorFunc <html/libvirt-virterror.html#virDefaultErrorFunc>`__
which is the default error function of the library issuing the error on
stderr
#. save the error in the connection for later retrieval with
`virGetLastError <html/libvirt-virterror.html#virGetLastError>`__
In all cases the error information is provided as a
`virErrorPtr <html/libvirt-virterror.html#virErrorPtr>`__ pointer to read-only
structure `virError <html/libvirt-virterror.html#virError>`__ containing the
following fields:
- code: an error number from the
`virErrorNumber <html/libvirt-virterror.html#virErrorNumber>`__ enum
- domain: an enum indicating which part of libvirt raised the error see
`virErrorDomain <html/libvirt-virterror.html#virErrorDomain>`__
- level: the error level, usually VIR_ERR_ERROR, though there is room for
warnings like VIR_ERR_WARNING
- message: the full human-readable formatted string of the error
- conn: if available a pointer to the
`virConnectPtr <html/libvirt-libvirt-host.html#virConnectPtr>`__ connection
to the hypervisor where this happened
- dom: if available a pointer to the
`virDomainPtr <html/libvirt-libvirt-domain.html#virDomainPtr>`__ domain
targeted in the operation
and then extra raw information about the error which may be initialized to 0 or
NULL if unused
- str1, str2, str3: string information, usually str1 is the error message
format
- int1, int2: integer information
So usually, setting up specific error handling with libvirt consist of
registering a handler with
`virSetErrorFunc <html/libvirt-virterror.html#virSetErrorFunc>`__ or with
`virConnSetErrorFunc <html/libvirt-virterror.html#virConnSetErrorFunc>`__, check
the value of the code value, take appropriate action, if needed let libvirt
print the error on stderr by calling
`virDefaultErrorFunc <html/libvirt-virterror.html#virDefaultErrorFunc>`__. For
asynchronous error handing, set such a function doing nothing to avoid the error
being reported on stderr, and call virConnGetLastError or virGetLastError when
an API call returned an error value. It can be a good idea to use
`virResetError <html/libvirt-virterror.html#virResetLastError>`__ or
`virConnResetLastError <html/libvirt-virterror.html#virConnResetLastError>`__
once an error has been processed fully.
At the python level, there only a global reporting callback function at this
point, see the error.py example about it:
::
def handler(ctxt, err):
global errno
#print "handler(%s, %s)" % (ctxt, err)
errno = err
libvirt.registerErrorHandler(handler, 'context')
the second argument to the registerErrorHandler function is passed as the first
argument of the callback like in the C version. The error is a tuple containing
the same field as a virError in C, but cast to Python.

2
docs/meson.build
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@ -39,7 +39,6 @@ docs_html_in_files = [
'drvvirtuozzo', 'drvvirtuozzo',
'drvvmware', 'drvvmware',
'drvxen', 'drvxen',
'errors',
'firewall', 'firewall',
'formatcaps', 'formatcaps',
'formatdomaincaps', 'formatdomaincaps',
@ -93,6 +92,7 @@ docs_rst_files = [
'developer-tooling', 'developer-tooling',
'drvqemu', 'drvqemu',
'drvch', 'drvch',
'errors',
'formatbackup', 'formatbackup',
'formatcheckpoint', 'formatcheckpoint',
'formatdomain', 'formatdomain',