Allow backends to see taint status.

This can be seen via System(). In the PKI backend, if the CA is
reconfigured but not fully (e.g. an intermediate CSR is generated but no
corresponding cert set) and there are already leases (issued certs), the
CRL is unable to be built. As a result revocation fails. But in this
case we don't actually need revocation to be successful since the CRL is
useless after unmounting. By checking taint status we know if we can
simply fast-path out of revocation with a success in this case.

Fixes #946

builtin/logical/pki/crl_util.go

@@ -18,6 +18,15 @@ type revocationInfo struct {
 
 // Revokes a cert, and tries to be smart about error recovery
 func revokeCert(b *backend, req *logical.Request, serial string) (*logical.Response, error) {
+	// As this backend is self-contained and this function does not hook into
+	// third parties to manage users or resources, if the mount is tainted,
+	// revocation doesn't matter anyways -- the CRL that would be written will
+	// be immediately blown away by the view being cleared. So we can simply
+	// fast path a successful exit.
+	if b.System().Tainted() {
+		return nil, nil
+	}
+
 	alreadyRevoked := false
 	var revInfo revocationInfo
 

logical/system_view.go

@@ -16,12 +16,23 @@ type SystemView interface {
 	// SudoPrivilege returns true if given path has sudo privileges
 	// for the given client token
 	SudoPrivilege(path string, token string) bool
+
+	// Returns true if the mount is tainted. A mount is tainted if it is in the
+	// process of being unmounted. This should only be used in special
+	// circumstances; a primary use-case is as a guard in revocation functions.
+	// If revocation of a backend's leases fails it can keep the unmounting
+	// process from being successful. If the reason for this failure is not
+	// relevant when the mount is tainted (for instance, saving a CRL to disk
+	// when the stored CRL will be removed during the unmounting process
+	// anyways), we can ignore the errors to allow unmounting to complete.
+	Tainted() bool
 }
 
 type StaticSystemView struct {
 	DefaultLeaseTTLVal time.Duration
 	MaxLeaseTTLVal     time.Duration
 	SudoPrivilegeVal   bool
+	TaintedVal         bool
 }
 
 func (d StaticSystemView) DefaultLeaseTTL() time.Duration {
@@ -35,3 +46,7 @@ func (d StaticSystemView) MaxLeaseTTL() time.Duration {
 func (d StaticSystemView) SudoPrivilege(path string, token string) bool {
 	return d.SudoPrivilegeVal
 }
+
+func (d StaticSystemView) Tainted() bool {
+	return d.TaintedVal
+}

vault/dynamic_system_view.go

@@ -64,3 +64,8 @@ func (d dynamicSystemView) fetchTTLs() (def, max time.Duration) {
 
 	return
 }
+
+// Tainted indicates that the mount is in the process of being removed
+func (d dynamicSystemView) Tainted() bool {
+	return d.mountEntry.Tainted
+}