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