concurrency: enable joint claims on the request scan path

This patch addresses a TODO that was blocking joint claims on a
request's scan path and adds a test to show joint claims work. In
particular, previously the lock mode of a queued request was hardcoded
to use locking strength `lock.Intent`. We now use the correct lock
mode by snapshotting an immutable copy of the request's lock strength
on to the queuedGuard when inserting a it into a lock's wait queue.

Informs cockroachdb#102275

Release note: None

pkg/kv/kvserver/concurrency/lock_table.go

@@ -764,23 +764,28 @@ func (g *lockTableGuardImpl) curStrength() lock.Strength {
 // request. The value returned by this method are mutable as the request's scan
 // of the lock table progresses from lock to lock.
 func (g *lockTableGuardImpl) curLockMode() lock.Mode {
-	switch g.curStrength() {
+	return makeLockMode(g.curStrength(), g.txn, g.ts)
+}
+
+// makeLockMode constructs and returns a lock mode.
+func makeLockMode(str lock.Strength, txn *roachpb.Transaction, ts hlc.Timestamp) lock.Mode {
+	switch accessStrength {
 	case lock.None:
 		iso := isolation.Serializable
-		if g.txn != nil {
-			iso = g.txn.IsoLevel
+		if txn != nil {
+			iso = txn.IsoLevel
 		}
-		return lock.MakeModeNone(g.ts, iso)
+		return lock.MakeModeNone(ts, iso)
 	case lock.Shared:
-		assert(g.txn != nil, "only transactional requests can acquire shared locks")
+		assert(txn != nil, "only transactional requests can acquire shared locks")
 		return lock.MakeModeShared()
 	case lock.Exclusive:
-		assert(g.txn != nil, "only transactional requests can acquire exclusive locks")
-		return lock.MakeModeExclusive(g.ts, g.txn.IsoLevel)
+		assert(txn != nil, "only transactional requests can acquire exclusive locks")
+		return lock.MakeModeExclusive(ts, txn.IsoLevel)
 	case lock.Intent:
-		return lock.MakeModeIntent(g.ts)
+		return lock.MakeModeIntent(ts)
 	default:
-		panic(fmt.Sprintf("unhandled request strength: %s", g.curStrength()))
+		panic(fmt.Sprintf("unhandled request strength: %s", accessStrength))
 	}
 }
 
@@ -919,7 +924,8 @@ func (g *lockTableGuardImpl) resumeScan(notify bool) {
 // and non-transactional requests.
 type queuedGuard struct {
 	guard  *lockTableGuardImpl
-	active bool // protected by lockState.mu
+	mode   lock.Mode // protected by lockState.mu
+	active bool      // protected by lockState.mu
 }
 
 // Information about a lock holder for unreplicated locks.
@@ -2201,6 +2207,7 @@ func (l *lockState) enqueueLockingRequest(g *lockTableGuardImpl) (maxQueueLength
 	}
 	qg := &queuedGuard{
 		guard:  g,
+		mode:   g.curLockMode(),
 		active: true,
 	}
 	// The request isn't in the queue. Add it in the correct position, based on
@@ -2281,13 +2288,7 @@ func (l *lockState) shouldRequestActivelyWait(g *lockTableGuardImpl) bool {
 			// conflicting waiters; no need to actively wait here.
 			return false
 		}
-		// TODO(arul): Inactive waiters will need to capture the strength at which
-		// they're trying to acquire a lock in their queuedGuard. We can't simply
-		// use the guard's curStrength (or curLockMode) -- inactive waiters may have
-		// mutated these values as they scan. For now, we can just use the intent
-		// lock mode as that's the only lock strength supported by the lock table.
-		waiterLockMode := lock.MakeModeIntent(qqg.guard.ts)
-		if lock.Conflicts(waiterLockMode, g.curLockMode(), &g.lt.settings.SV) {
+		if lock.Conflicts(qqg.mode, g.curLockMode(), &g.lt.settings.SV) {
 			return true
 		}
 	}
@@ -2691,6 +2692,7 @@ func (l *lockState) discoveredLock(
 			// Put self in queue as inactive waiter.
 			qg := &queuedGuard{
 				guard:  g,
+				mode:   makeLockMode(accessStrength, g.txn, g.ts),
 				active: false,
 			}
 			// g is not necessarily first in the queue in the (rare) case (a) above.

pkg/kv/kvserver/concurrency/testdata/lock_table/shared_locks

@@ -38,31 +38,41 @@ scan r=req3
 ----
 start-waiting: false
 
+# Another shared locking request should be able to acquire a joint claim as
+# well.
+new-request r=req4 txn=txn2 ts=10 spans=shared@a
+----
+
+scan r=req4
+----
+start-waiting: false
+
 print
 ----
 num=1
  lock: "a"
   holder: txn: 00000000-0000-0000-0000-000000000001 epoch: 0, iso: Serializable, ts: 10.000000000,0, info: unrepl [(str: Shared seq: 0)]
    queued writers:
     active: false req: 2, txn: 00000000-0000-0000-0000-000000000002
+    active: false req: 3, txn: 00000000-0000-0000-0000-000000000002
 
 # TODO(arul): This is currently a limitation of the lock table, as it doesn't
 # allow multiple locks on a single key from different transactions.
 acquire r=req3 k=a durability=u strength=shared
 ----
 existing lock cannot be acquired by different transaction
 
-new-request r=req4 txn=txn2 ts=10 spans=exclusive@a
+new-request r=req5 txn=txn2 ts=10 spans=exclusive@a
 ----
 
-scan r=req4
+scan r=req5
 ----
 start-waiting: true
 
-new-request r=req5 txn=txn2 ts=10 spans=intent@a
+new-request r=req6 txn=txn2 ts=10 spans=intent@a
 ----
 
-scan r=req5
+scan r=req6
 ----
 start-waiting: true
 
@@ -73,20 +83,21 @@ num=1
   holder: txn: 00000000-0000-0000-0000-000000000001 epoch: 0, iso: Serializable, ts: 10.000000000,0, info: unrepl [(str: Shared seq: 0)]
    queued writers:
     active: false req: 2, txn: 00000000-0000-0000-0000-000000000002
-    active: true req: 3, txn: 00000000-0000-0000-0000-000000000002
+    active: false req: 3, txn: 00000000-0000-0000-0000-000000000002
     active: true req: 4, txn: 00000000-0000-0000-0000-000000000002
-   distinguished req: 3
+    active: true req: 5, txn: 00000000-0000-0000-0000-000000000002
+   distinguished req: 4
 
 # ------------------------------------------------------------------------------
 # Ensure requests with locking strength shared actively wait if there are active
 # waiters with conflicting lock strengths (even though the lock itself is
 # compatible with the shared lock request).
 # ------------------------------------------------------------------------------
 
-new-request r=req6 txn=txn2 ts=10 spans=shared@a
+new-request r=req7 txn=txn2 ts=10 spans=shared@a
 ----
 
-scan r=req6
+scan r=req7
 ----
 start-waiting: true
 
@@ -97,7 +108,8 @@ num=1
   holder: txn: 00000000-0000-0000-0000-000000000001 epoch: 0, iso: Serializable, ts: 10.000000000,0, info: unrepl [(str: Shared seq: 0)]
    queued writers:
     active: false req: 2, txn: 00000000-0000-0000-0000-000000000002
-    active: true req: 3, txn: 00000000-0000-0000-0000-000000000002
+    active: false req: 3, txn: 00000000-0000-0000-0000-000000000002
     active: true req: 4, txn: 00000000-0000-0000-0000-000000000002
     active: true req: 5, txn: 00000000-0000-0000-0000-000000000002
-   distinguished req: 3
+    active: true req: 6, txn: 00000000-0000-0000-0000-000000000002
+   distinguished req: 4