[#737] Fix cn=changelog search failing when aliases are dereferenced

[vharseko]

Fixes #737.

Problem

Any search based at cn=changelog fails with result code 80 when the client dereferences aliases:

error code 80 - An unexpected error was encountered while processing a search in one of the
Directory Server backends: RuntimeException(Not implemented)

ChangelogBackend.getEntry threw RuntimeException("Not implemented") rather than honouring the LocalBackend.getEntry contract of returning null for a missing entry. That violation was harmless until f771315 ("[#287] ADD alias dereferencing for search requests", #369), which made the server read the search base entry before handing the search over to the backend. The lookup now throws before ChangelogBackend.search() — which handles these searches fine — is ever reached.

This breaks every client that dereferences aliases by default. JNDI sends derefAliases: always unless told otherwise, so the OpenICF LDAP connector's sync() cannot read the external change log at all.

Fix

Return the base changelog entry by DN, and null for change records. Change records need the search filter and change number range to position their cursors, so they cannot be looked up individually — and none of them is ever an alias, so a caller dereferencing aliases gets the answer it needs and the search proceeds into search() as before.

This also fixes entryExists on cn=changelog, which inherits the default getEntry(dn) != null implementation and threw the same way.

Testing

ChangelogBackendTestCase passes in full (30/30). The new tests are parameterized over the three deref policies that trigger the base entry lookup (ALWAYS, FINDING_BASE, IN_SEARCHING):

• searchInChangeNumberModeWhenDereferencingAliases — the reported reproducer.
• searchWhenNoChangesShouldReturnRootEntryOnlyWhenDereferencingAliases — base entry on an empty RS.
• getEntryShouldReturnBaseEntryOnly — the contract itself.

Verified they catch the regression rather than merely passing: reverting the ChangelogBackend change fails 7 of the 8 with the exact RuntimeException(Not implemented) from the report. The 8th is the pre-existing searchWhenNoChangesShouldReturnRootEntryOnly, which uses deref NEVER and passes either way — the control case.

Related

Two further defects in the same f771315 feature were found while analysing this one and are filed separately; neither is addressed here:

• Dereferencing an alias that points into another backend searches the wrong backend #738 — dereferencing an alias that points into another backend searches the wrong backend.
• Alias dereferencing accumulates every returned entry DN, growing without bound in persistent searches #739 — alias dereferencing accumulates every returned entry DN, growing without bound in persistent searches.