Elektra  0.9.9
Ensure

Problem

Applications want to ensure that some functionality (=global plugin) is present in Elektra.

Constraints

Assumptions

Considered Alternatives

  • Keep kdbEnsure (Rejected because it is too flexible and can be called many times. Furthermore, kdbOpen would build up configurations that get removed afterwards.)
    • reduce for only global plugins, as the partial other functionality is confusing and not needed
    • find solution that list plugin is not needed
    • only as implementation detail below libraries (e.g. like done for notification)
  • Specific APIs per plugin, Rejected because:
    • difficult for application developers
    • every plugin would need to design new APIs
  • Have a new API: KDB * kdbOpen (Key * parent); int kdbConfigure (KDB * handle, KeySet * contract, Key * parentKey); KDB * kdbOpenDefault (Key * parent); The new kdbOpen only does the absolute minimum work, in particular it doesn't set up any global plugins. If you use kdbOpen you must call kdbConfigure otherwise kdbGet will fail. kdbConfigure configures global plugins (basically just a renamed kdbEnsure). Lastly, kdbOpenDefault does more or less what the old kdbOpen does. It sets up the default case and you can call kdbGet immediately. But you cannot call kdbConfigure after kdbOpenDefault. Rejected because of API bloat and introduction of further state in kdb.

Decision

Integrate kdbEnsure in kdbOpen(Key *errorKey, KeySet *contract) but only allow global plugins.

Rationale

  • can immediately build up correct plugin positioning
  • does not allow starting applications if the contract cannot be fulfilled
  • simplest and minimalistic solution

Implications

elektraNotificationOpen will be renamed and only return a contract KeySet:

KeySet * errorKey = keyNew ("/", KEY_END);
KeySet * contract = ksNew (0, KS_END);
elektraNotificationContract (contract, iobinding, errorKey);
keyNew
Key * keyNew(const char *name,...)
A practical way to fully create a Key object in one step.
Definition: key.c:138
KEY_END
@ KEY_END
Definition: kdbenum.c:95
ksNew
KeySet * ksNew(size_t alloc,...)
Allocate, initialize and return a new KeySet object.
Definition: keyset.c:229
KS_END
#define KS_END
End of a list of keys.
Definition: kdbenum.c:156
elektraNotificationContract
int elektraNotificationContract(KeySet *contract)
Creates a contract for use with kdbOpen() that sets up notifications.
Definition: notification.c:34

The same for gopts:

elektraGOptsContract (contract, argc, argv, environ, errorKey);
elektraGOptsContract
int elektraGOptsContract(KeySet *contract, int argc, const char *const *argv, const char *const *envp, const Key *parentKey, KeySet *goptsConfig)
Sets up a contract for use with kdbOpen() that configures the gopts plugin.
Definition: contracts.c:41

Finally, we create KDB with the contracts we got before:

KDB * kdb = kdbOpen (contract, parentKey);
kdbOpen
KDB * kdbOpen(const KeySet *contract, Key *errorKey)
Opens the session with the Key database.
Definition: kdb.c:417
kdb
This is the main namespace for the C++ binding and libraries.
Definition: backend.hpp:31

Opening KDB will fail if any of the contracts cannot be ensured.

As the contract gets copied, at any point after kdbOpen the contract can be safely deleted:

ksDel (contract);

The cleanup of the global plugins happens within:

kdbClose (kdb, errorKey);
kdbClose
int kdbClose(KDB *handle, Key *errorKey)
Closes the session with the Key database.
Definition: kdb.c:590

It is safe to use the contract KeySet also for kdbGet and kdbSet invocations. Contract KeySets only contain Keys below system:/elektra/contract. Therefore, normal KeySets should not interfere.

Related Decisions

Notes

  • Issue #2764
  • Implemented in #3651
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