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Type:
Bug
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Status: Closed
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Priority:
Critical
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Resolution: Fixed
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Affects Version/s: 1.9.4
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Fix Version/s: 2.0
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Component/s: Roles / Access
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Labels:None
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Affected Branches:MOODLE_19_STABLE
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Fixed Branches:MOODLE_20_STABLE
Sam reported in MDL-19038:
Tim - the function get_u_b_c doesn't actually return all half-million users - but when you make query at CONTEXT_SYSTEM level like this, it really DOES retrieve all half-million users into php, then it filters them. This is about line 4670 in accesslib. Code below.
I think your lazy generation patch is correct (although it would be nice if somebody could make get_u_b_c faster for this case, but that's likely to be an awful lot harder to figure out).
if ($context->contextlevel == CONTEXT_SYSTEM
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|| $isfrontpage
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|| $defaultroleinteresting) {
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// Handle system / sitecourse / defaultrole-with-perhaps-neg-overrides
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// with a SELECT FROM user LEFT OUTER JOIN against ra -
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// This is expensive on the SQL and PHP sides -
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// moves a ton of data across the wire.
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$ss = "SELECT u.id as userid, ra.roleid,
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ctx.depth
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FROM {$CFG->prefix}user u
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LEFT OUTER JOIN {$CFG->prefix}role_assignments ra
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ON (ra.userid = u.id
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AND ra.contextid IN ($ctxids)
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AND ra.roleid IN (".implode(',',$roleids) .")
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$condhiddenra)
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LEFT OUTER JOIN {$CFG->prefix}context ctx
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ON ra.contextid=ctx.id
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WHERE u.deleted=0";
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Clearly this is silly. The CONTEXT_SYSTEM case must be easy to do in pure SQL.
Note that there are quite extensive unit tests for get_users_by_capability in HEAD. If you plan to work on this, you probably should use them to reduce the chance of regressions.