Aristo+Kvt: Let destructor crash when nil argument is given (#2080)

why:
  Ignoring `nil` objects was handy for a while but eventually led to
  lazy programming which in turn led to double destructor calls for
  the rocks-db.

nimbus/db/aristo/aristo_init/memory_only.nim

@@ -98,10 +98,9 @@ proc finish*(db: AristoDbRef; flush = false) =
   ##
   ## This distructor may be used on already *destructed* descriptors.
   ##
-  if not db.isNil:
+  if not db.backend.isNil:
-    if not db.backend.isNil:
+    db.backend.closeFn flush
-      db.backend.closeFn flush
+  discard db.getCentre.forgetOthers()
-    discard db.getCentre.forgetOthers()
 
 # ------------------------------------------------------------------------------
 # End

nimbus/db/kvt/kvt_init/memory_only.nim

@@ -73,10 +73,9 @@ proc finish*(db: KvtDbRef; flush = false) =
   ##
   ## This distructor may be used on already *destructed* descriptors.
   ##
-  if not db.isNil:
+  if not db.backend.isNil:
-    if not db.backend.isNil:
+    db.backend.closeFn flush
-      db.backend.closeFn flush
+  discard db.getCentre.forgetOthers()
-    discard db.getCentre.forgetOthers()
 
 # ------------------------------------------------------------------------------
 # End

tests/test_aristo.nim

@@ -183,8 +183,8 @@ proc storagesRunner(
 
 proc aristoMain*(noisy = defined(debug)) =
   noisy.miscRunner()
-  noisy.accountsRunner(persistent=false)
+  noisy.accountsRunner()
-  noisy.storagesRunner(persistent=false)
+  noisy.storagesRunner()
 
 when isMainModule:
   const

tests/test_aristo/test_filter.nim

@@ -202,8 +202,10 @@ proc dbTriplet(w: LeafQuartet; rdbPath: string): Result[DbTriplet,AristoError] =
 
 # ----------------------
 
-proc cleanUp(dx: DbTriplet) =
+proc cleanUp(dx: var DbTriplet) =
-  dx[0].finish(flush=true)
+  if not dx[0].isNil:
+    dx[0].finish(flush=true)
+    dx.reset
 
 proc isDbEq(a, b: FilterRef; db: AristoDbRef; noisy = true): bool =
   ## Verify that argument filter `a` has the same effect on the
@@ -559,7 +561,7 @@ proc testDistributedAccess*(
     block:
 
       # Clause (8) from `aristo/README.md` example
-      let
+      var
         dx = block:
           let rc = dbTriplet(w, rdbPath)
           xCheckRc rc.error == 0
@@ -612,7 +614,7 @@ proc testDistributedAccess*(
 
     # Work through clauses (12)..(15) from `aristo/README.md` example
     block:
-      let
+      var
         dy = block:
           let rc = dbTriplet(w, rdbPath)
           xCheckRc rc.error == 0

tests/test_aristo/test_tx.nim

@@ -108,13 +108,15 @@ proc randomisedLeafs(
       lvp2[n].swap lvp2[r]
   ok lvp2
 
-proc innerCleanUp(db: AristoDbRef): bool {.discardable.}  =
+proc innerCleanUp(db: var AristoDbRef): bool {.discardable.}  =
   ## Defer action
-  let rx = db.txTop()
+  if not db.isNil:
-  if rx.isOk:
+    let rx = db.txTop()
-    let rc = rx.value.collapse(commit=false)
+    if rx.isOk:
-    xCheckRc rc.error == 0
+      let rc = rx.value.collapse(commit=false)
-  db.finish(flush=true)
+      xCheckRc rc.error == 0
+    db.finish(flush=true)
+    db = AristoDbRef(nil)
 
 proc schedStow(
     db: AristoDbRef;                  # Database
@@ -334,10 +336,11 @@ proc testTxMergeAndDeleteOneByOne*(
        ): bool =
   var
     prng = PrngDesc.init 42
-    db = AristoDbRef()
+    db = AristoDbRef(nil)
     fwdRevVfyToggle = true
   defer:
-    db.finish(flush=true)
+    if not db.isNil:
+      db.finish(flush=true)
 
   for n,w in list:
     # Start with brand new persistent database.
@@ -439,10 +442,10 @@ proc testTxMergeAndDeleteSubTree*(
        ): bool =
   var
     prng = PrngDesc.init 42
-    db = AristoDbRef()
+    db = AristoDbRef(nil)
-
   defer:
-    db.finish(flush=true)
+    if not db.isNil:
+      db.finish(flush=true)
 
   for n,w in list:
     # Start with brand new persistent database.
@@ -524,7 +527,7 @@ proc testTxMergeProofAndKvpList*(
   let
     oopsTab = oops.toTable
   var
-    db = AristoDbRef()
+    db = AristoDbRef(nil)
     tx = AristoTxRef(nil)
     rootKey: Hash256
     count = 0