2013-06-21

[Technology] DOMException, its nature and appropriateness for GError reporting

I've had to recently study the nature of the various DOMExceptions specified by the DOM Level 3 Core spec, to better understand whether we want to use the GError reporting system of GLib in GXml, or whether we want to use something else.  I defaulted to using GError, because the spec presents them as exceptions to be raised, and because I was used to handling libgdata errors (which are legitimately run-time errors) with GError.

The GLib Error Reporting documentation is pretty clear about when it's appropriate to use GError: basically, only when it's a run-time issue that a programmer can't reasonably anticipate.
First and foremost: GError should only be used to report recoverable runtime errors, never to report programming errors. If the programmer has screwed up, then you should use g_warning(), g_return_if_fail(), g_assert(), g_error(), or some similar facility. (Incidentally, remember that the g_error() function should only be used for programming errors, it should not be used to print any error reportable via GError.)

Examples of recoverable runtime errors are "file not found" or "failed to parse input." Examples of programming errors are "NULL passed to strcmp()" or "attempted to free the same pointer twice." These two kinds of errors are fundamentally different: runtime errors should be handled or reported to the user, programming errors should be eliminated by fixing the bug in the program. This is why most functions in GLib and GTK+ do not use the GError facility.
Below are my feelings on which DOMExceptions constitute programming errors (and shouldn't use GError) or run-time errors (and should). I look at run-time errors as consequences of the environment in which GXml is used (e.g. is it the fault of a document we're reading in, the content of which we have little control), and programming errors the direct result of a programmed call that's supplying bad data or misusing the document.

I find most of the DOMExceptions to be programming errors.  If you believe that something is more properly a run-time error and deserves GError reporting, please comment and explain why!  I am not an expert.  Like, for invalid characters, GXml might be used by a GUI where a user is entering data, and enters characters that aren't supported.  I think input checking like that should be handled in the application instead of in the GXml library, though; it shouldn't get as far as trying to actually set an attribute with bad data.  Perhaps we could provide convenience validation methods to GXml, though.  Thoughts welcome.



R: Runtime, GError
P: Programming
N: N/A

Defined Constants

N    DOMSTRING_SIZE_ERR
        If the specified range of text does not fit into a DOMString.
  we don't have our own DOMString type, we just use gchar* arrays, which don't really have 
  a fixed limit, just as much as you can alloc in a single go.

P    HIERARCHY_REQUEST_ERR
        If any Node is inserted somewhere it doesn't belong.
  this would be a programmer error, like trying to append a node to be a child of itself.

P    INDEX_SIZE_ERR
        If index or size is negative, or greater than the allowed value.
  programmer error, they should check size before accessing non-existent members of lists

P    INUSE_ATTRIBUTE_ERR
        If an attempt is made to add an attribute that is already in use elsewhere.
  programmer error

?    INVALID_ACCESS_ERR, introduced in DOM Level 2.
        If a parameter or an operation is not supported by the underlying object.
  nclear as to when this might happen; property of a document or implementation or the DOM?

?P    INVALID_CHARACTER_ERR
        If an invalid or illegal character is specified, such as in an XML name.
  I'd almost think it's a run-time error, because it probably reflects an issue with the 
  document we're working on.  Invalid characters are related to the XML version of a document,
  and which features our implementation supports.  Right now, we basically don't deal with 
  this (!).  libxml2 has version information, of course, but I'm not sure how it handles 
  invalid characters yet.  TODO: figure this out.  
  However, I think of it more as programmer error, because almost all the methods its 
  associated with are where the programmer provides a string that might contain it, rather 
  than just accessing a string already in the Document.  The other ones involve bringing
  foreign nodes into documents, and the programmer has the option of checking version 
  information and characters beforehand to ensure compatibility
    *Document.adoptNode
    Document.createAttribute
    Document.createElement
    Document.createEntityReference
    Document.createProcessingInstruction
    *Document.importNode
    Document.renameNode
    Node.prefix.set
    Element.setAttribute
    DOMImplementation.createDocument
    DOMImplementation.createDocumentType

?    INVALID_MODIFICATION_ERR, introduced in DOM Level 2.
        If an attempt is made to modify the type of the underlying object.
  Not really specified for any methods

?    INVALID_STATE_ERR, introduced in DOM Level 2.
        If an attempt is made to use an object that is not, or is no longer, usable.
  Not really specified for any methods

P    NAMESPACE_ERR, introduced in DOM Level 2.
        If an attempt is made to create or change an object in a way which is incorrect 
        with regard to namespaces.
  The result of bad input from a programmer, for qualified names.
     DOMImplementation.createDocument
     DOMImplementation.createDocumentType
     DOMImplementation.createAttributeNS
     DOMImplementation.createElementNS
     DOMImplementation.renameNode
     Node.prefix.set

P    NOT_FOUND_ERR
        If an attempt is made to reference a Node in a context where it does not exist.
  Programmer error, they're specifying the node in reference, so like with insertBefore, 
  removeChild, etc.

?RP    NOT_SUPPORTED_ERR
        If the implementation does not support the requested type of object or operation.
  We couldn't don't deal with the features of our implementation or XML versions (that's bad, 
  but oh well); not exactly sure where this would fall.  Sometimes it seems like it would be 
  a programming error (trying to adopt a Document node into another Document with adoptNode) 
  but other times, like when creating a document, I could see that being a runtime error.

?    NO_DATA_ALLOWED_ERR
        If data is specified for a Node which does not support data.
  Currently nothing raised it according to the spec, and there aren't any obvious cases 
  where I want to.  
 
?PR    NO_MODIFICATION_ALLOWED_ERR
        If an attempt is made to modify an object where modifications are not allowed.
  We currently don't have a concept of a read only node, and thus no way to check. I'm not
  sure if libxml2 has a concept of readability either.  Its common for most nodes to want to 
  raise this, so perhaps it's a real use case we should support, but even then, it feels like
  something that isn't variable at run time, and that a programmer might be able to reasonably 
  anticipate and check for.  

P    SYNTAX_ERR, introduced in DOM Level 2.
        If an invalid or illegal string is specified.
  Probably a programmer error, if they're the ones specifying strings.

P    TYPE_MISMATCH_ERR, introduced in DOM Level 3.
        If the type of an object is incompatible with the expected type of the parameter 
        associated to the object. 
  Only used by DOMConfiguration.setParameter, which we don't support.  This seems like a 
  programmer thing, anyway
    DOMConfiguration.setParameter

P    VALIDATION_ERR, introduced in DOM Level 3.
        If a call to a method such as insertBefore or removeChild would make the Node invalid 
        with respect to "partial validity", this exception would be raised and the operation 
        would not be done. This code is used in [DOM Level 3 Validation]. Refer to this 
        specification for further information.

P    WRONG_DOCUMENT_ERR
        If a Node is used in a different document than the one that created it (that doesn't 
        support it).
  Definitely a programmer error.

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