ratelimits: improve disabled limit handling (#7803)

In the FailedAuthorizations limits, there was code that intentionally
ignored errLimitDisabled errors (`errors.Is(err, errLimitDisabled)`).
However, that that resulted in those functions later using a returned
`limit` value that was invalid (i.e. its zero value). That happened to
trigger some later checks in validateTransaction. Specifically this
check failed:

    	if txn.cost > txn.limit.Burst {
        // error

When txt.limit.Burst is zero, this will always fail.

This problem doesn't really show up in prod, where all the limits are
configured. But it showed up in tests, specifically
TestPerformValidation_FailedValidationsTriggerPauseIdentifiersRatelimit,
where the limits are constructed using a simplified config that leaves
most of them disabled.

In this change, I tried to make handling of errLimitDisabled more
consistent, and always return an allow-only transaction as early as
possible instead of falling through the error condition.

Where that wasn't possible, I used a boolean to record whether the
result of `builder.getLimit()` was valid before referencing any of its
fields.

I also added some "shouldn't happen" errors to catch this problem
earlier if it recurs.

I removed some "skip disabled limit" comments because those say "what
the code does" (which the code also says), not "why the code does it".

Fixes the test failures in #7797.

Author: jsha

ratelimits/limit.go

@@ -15,6 +15,10 @@ import (
 // currently configured.
 var errLimitDisabled = errors.New("limit disabled")
 
+// limit defines the configuration for a rate limit or a rate limit override.
+//
+// The zero value of this struct is invalid, because some of the fields must
+// be greater than zero.
 type limit struct {
 	// Burst specifies maximum concurrent allowed requests at any given time. It
 	// must be greater than zero.

ratelimits/transaction.go

@@ -102,6 +102,9 @@ func newFQDNSetBucketKey(name Name, orderNames []string) (string, error) { //nol
 //   - allow-only: when neither check nor spend are true, the transaction will
 //     be considered "allowed" regardless of the bucket's capacity. This is
 //     useful for limits that are disabled.
+//
+// The zero value of Transaction is an allow-only transaction and is valid even if
+// it would fail validateTransaction (for instance because cost and burst are zero).
 type Transaction struct {
 	bucketKey string
 	limit     limit
@@ -126,6 +129,14 @@ func validateTransaction(txn Transaction) (Transaction, error) {
 	if txn.cost < 0 {
 		return Transaction{}, ErrInvalidCost
 	}
+	if txn.limit.Burst == 0 {
+		// This should never happen. If the limit was loaded from a file,
+		// Burst was validated then. If this is a zero-valued Transaction
+		// (that is, an allow-only transaction), then validateTransaction
+		// shouldn't be called because zero-valued transactions are automatically
+		// valid.
+		return Transaction{}, fmt.Errorf("invalid limit, burst must be > 0")
+	}
 	if txn.cost > txn.limit.Burst {
 		return Transaction{}, ErrInvalidCostOverLimit
 	}
@@ -160,9 +171,9 @@ func newSpendOnlyTransaction(limit limit, bucketKey string, cost int64) (Transac
 	})
 }
 
-func newAllowOnlyTransaction() (Transaction, error) {
+func newAllowOnlyTransaction() Transaction {
 	// Zero values are sufficient.
-	return validateTransaction(Transaction{})
+	return Transaction{}
 }
 
 // TransactionBuilder is used to build Transactions for various rate limits.
@@ -194,7 +205,7 @@ func (builder *TransactionBuilder) registrationsPerIPAddressTransaction(ip net.I
 	limit, err := builder.getLimit(NewRegistrationsPerIPAddress, bucketKey)
 	if err != nil {
 		if errors.Is(err, errLimitDisabled) {
-			return newAllowOnlyTransaction()
+			return newAllowOnlyTransaction(), nil
 		}
 		return Transaction{}, err
 	}
@@ -212,7 +223,7 @@ func (builder *TransactionBuilder) registrationsPerIPv6RangeTransaction(ip net.I
 	limit, err := builder.getLimit(NewRegistrationsPerIPv6Range, bucketKey)
 	if err != nil {
 		if errors.Is(err, errLimitDisabled) {
-			return newAllowOnlyTransaction()
+			return newAllowOnlyTransaction(), nil
 		}
 		return Transaction{}, err
 	}
@@ -229,7 +240,7 @@ func (builder *TransactionBuilder) ordersPerAccountTransaction(regId int64) (Tra
 	limit, err := builder.getLimit(NewOrdersPerAccount, bucketKey)
 	if err != nil {
 		if errors.Is(err, errLimitDisabled) {
-			return newAllowOnlyTransaction()
+			return newAllowOnlyTransaction(), nil
 		}
 		return Transaction{}, err
 	}
@@ -250,7 +261,10 @@ func (builder *TransactionBuilder) FailedAuthorizationsPerDomainPerAccountCheckO
 		return nil, err
 	}
 	limit, err := builder.getLimit(FailedAuthorizationsPerDomainPerAccount, perAccountBucketKey)
-	if err != nil && !errors.Is(err, errLimitDisabled) {
+	if err != nil {
+		if errors.Is(err, errLimitDisabled) {
+			return []Transaction{newAllowOnlyTransaction()}, nil
+		}
 		return nil, err
 	}
 
@@ -287,7 +301,10 @@ func (builder *TransactionBuilder) FailedAuthorizationsPerDomainPerAccountSpendO
 		return Transaction{}, err
 	}
 	limit, err := builder.getLimit(FailedAuthorizationsPerDomainPerAccount, perAccountBucketKey)
-	if err != nil && !errors.Is(err, errLimitDisabled) {
+	if err != nil {
+		if errors.Is(err, errLimitDisabled) {
+			return newAllowOnlyTransaction(), nil
+		}
 		return Transaction{}, err
 	}
 
@@ -317,7 +334,10 @@ func (builder *TransactionBuilder) FailedAuthorizationsForPausingPerDomainPerAcc
 		return Transaction{}, err
 	}
 	limit, err := builder.getLimit(FailedAuthorizationsForPausingPerDomainPerAccount, perAccountBucketKey)
-	if err != nil && !errors.Is(err, errLimitDisabled) {
+	if err != nil {
+		if errors.Is(err, errLimitDisabled) {
+			return newAllowOnlyTransaction(), nil
+		}
 		return Transaction{}, err
 	}
 
@@ -351,9 +371,18 @@ func (builder *TransactionBuilder) certificatesPerDomainCheckOnlyTransactions(re
 	if err != nil {
 		return nil, err
 	}
+	accountOverride := true
 	perAccountLimit, err := builder.getLimit(CertificatesPerDomainPerAccount, perAccountLimitBucketKey)
-	if err != nil && !errors.Is(err, errLimitDisabled) {
-		return nil, err
+	if err != nil {
+		// The CertificatesPerDomainPerAccount limit never has a default. If there is an override for it,
+		// the above call will return the override. But if there is none, it will return errLimitDisabled.
+		// In that case we want to continue, but make sure we don't reference `perAccountLimit` because it
+		// is not a valid limit.
+		if errors.Is(err, errLimitDisabled) {
+			accountOverride = false
+		} else {
+			return nil, err
+		}
 	}
 
 	var txns []Transaction
@@ -362,9 +391,10 @@ func (builder *TransactionBuilder) certificatesPerDomainCheckOnlyTransactions(re
 		if err != nil {
 			return nil, err
 		}
-		if perAccountLimit.isOverride() {
-			// An override is configured for the CertificatesPerDomainPerAccount
-			// limit.
+		if accountOverride {
+			if !perAccountLimit.isOverride() {
+				return nil, fmt.Errorf("shouldn't happen: CertificatesPerDomainPerAccount limit is not an override")
+			}
 			perAccountPerDomainKey, err := NewRegIdDomainBucketKey(CertificatesPerDomainPerAccount, regId, name)
 			if err != nil {
 				return nil, err
@@ -373,18 +403,20 @@ func (builder *TransactionBuilder) certificatesPerDomainCheckOnlyTransactions(re
 			// bucket.
 			txn, err := newCheckOnlyTransaction(perAccountLimit, perAccountPerDomainKey, 1)
 			if err != nil {
+				if errors.Is(err, errLimitDisabled) {
+					continue
+				}
 				return nil, err
 			}
 			txns = append(txns, txn)
 		} else {
 			// Use the per domain bucket key when no per account per domain override
 			// is configured.
 			perDomainLimit, err := builder.getLimit(CertificatesPerDomain, perDomainBucketKey)
-			if errors.Is(err, errLimitDisabled) {
-				// Skip disabled limit.
-				continue
-			}
 			if err != nil {
+				if errors.Is(err, errLimitDisabled) {
+					continue
+				}
 				return nil, err
 			}
 			// Add a check-only transaction for each per domain bucket.
@@ -417,9 +449,18 @@ func (builder *TransactionBuilder) CertificatesPerDomainSpendOnlyTransactions(re
 	if err != nil {
 		return nil, err
 	}
+	accountOverride := true
 	perAccountLimit, err := builder.getLimit(CertificatesPerDomainPerAccount, perAccountLimitBucketKey)
-	if err != nil && !errors.Is(err, errLimitDisabled) {
-		return nil, err
+	if err != nil {
+		// The CertificatesPerDomainPerAccount limit never has a default. If there is an override for it,
+		// the above call will return the override. But if there is none, it will return errLimitDisabled.
+		// In that case we want to continue, but make sure we don't reference `perAccountLimit` because it
+		// is not a valid limit.
+		if errors.Is(err, errLimitDisabled) {
+			accountOverride = false
+		} else {
+			return nil, err
+		}
 	}
 
 	var txns []Transaction
@@ -428,9 +469,10 @@ func (builder *TransactionBuilder) CertificatesPerDomainSpendOnlyTransactions(re
 		if err != nil {
 			return nil, err
 		}
-		if perAccountLimit.isOverride() {
-			// An override is configured for the CertificatesPerDomainPerAccount
-			// limit.
+		if accountOverride {
+			if !perAccountLimit.isOverride() {
+				return nil, fmt.Errorf("shouldn't happen: CertificatesPerDomainPerAccount limit is not an override")
+			}
 			perAccountPerDomainKey, err := NewRegIdDomainBucketKey(CertificatesPerDomainPerAccount, regId, name)
 			if err != nil {
 				return nil, err
@@ -444,11 +486,10 @@ func (builder *TransactionBuilder) CertificatesPerDomainSpendOnlyTransactions(re
 			txns = append(txns, txn)
 
 			perDomainLimit, err := builder.getLimit(CertificatesPerDomain, perDomainBucketKey)
-			if errors.Is(err, errLimitDisabled) {
-				// Skip disabled limit.
-				continue
-			}
 			if err != nil {
+				if errors.Is(err, errLimitDisabled) {
+					continue
+				}
 				return nil, err
 			}
 
@@ -462,11 +503,10 @@ func (builder *TransactionBuilder) CertificatesPerDomainSpendOnlyTransactions(re
 			// Use the per domain bucket key when no per account per domain
 			// override is configured.
 			perDomainLimit, err := builder.getLimit(CertificatesPerDomain, perDomainBucketKey)
-			if errors.Is(err, errLimitDisabled) {
-				// Skip disabled limit.
-				continue
-			}
 			if err != nil {
+				if errors.Is(err, errLimitDisabled) {
+					continue
+				}
 				return nil, err
 			}
 			// Add a spend-only transaction for each per domain bucket.
@@ -491,7 +531,7 @@ func (builder *TransactionBuilder) certificatesPerFQDNSetCheckOnlyTransaction(or
 	limit, err := builder.getLimit(CertificatesPerFQDNSet, bucketKey)
 	if err != nil {
 		if errors.Is(err, errLimitDisabled) {
-			return newAllowOnlyTransaction()
+			return newAllowOnlyTransaction(), nil
 		}
 		return Transaction{}, err
 	}
@@ -509,7 +549,7 @@ func (builder *TransactionBuilder) CertificatesPerFQDNSetSpendOnlyTransaction(or
 	limit, err := builder.getLimit(CertificatesPerFQDNSet, bucketKey)
 	if err != nil {
 		if errors.Is(err, errLimitDisabled) {
-			return newAllowOnlyTransaction()
+			return newAllowOnlyTransaction(), nil
 		}
 		return Transaction{}, err
 	}