Split code into modules

This is a massive commit that restructures the code into modules:

db/
    All functions related to modifying the Database

types/
    All type definitions and methods that can be exclusivly used on
    these types without dependencies

policy/
    All Policy related code, now without dependencies on the Database.

policy/matcher/
    Dedicated code to match machines in a list of FilterRules

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/types/common.go

@@ -0,0 +1,108 @@
+package types
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/netip"
+
+	"tailscale.com/tailcfg"
+)
+
+var ErrCannotParsePrefix = errors.New("cannot parse prefix")
+
+// This is a "wrapper" type around tailscales
+// Hostinfo to allow us to add database "serialization"
+// methods. This allows us to use a typed values throughout
+// the code and not have to marshal/unmarshal and error
+// check all over the code.
+type HostInfo tailcfg.Hostinfo
+
+func (hi *HostInfo) Scan(destination interface{}) error {
+	switch value := destination.(type) {
+	case []byte:
+		return json.Unmarshal(value, hi)
+
+	case string:
+		return json.Unmarshal([]byte(value), hi)
+
+	default:
+		return fmt.Errorf("%w: unexpected data type %T", ErrMachineAddressesInvalid, destination)
+	}
+}
+
+// Value return json value, implement driver.Valuer interface.
+func (hi HostInfo) Value() (driver.Value, error) {
+	bytes, err := json.Marshal(hi)
+
+	return string(bytes), err
+}
+
+type IPPrefix netip.Prefix
+
+func (i *IPPrefix) Scan(destination interface{}) error {
+	switch value := destination.(type) {
+	case string:
+		prefix, err := netip.ParsePrefix(value)
+		if err != nil {
+			return err
+		}
+		*i = IPPrefix(prefix)
+
+		return nil
+	default:
+		return fmt.Errorf("%w: unexpected data type %T", ErrCannotParsePrefix, destination)
+	}
+}
+
+// Value return json value, implement driver.Valuer interface.
+func (i IPPrefix) Value() (driver.Value, error) {
+	prefixStr := netip.Prefix(i).String()
+
+	return prefixStr, nil
+}
+
+type IPPrefixes []netip.Prefix
+
+func (i *IPPrefixes) Scan(destination interface{}) error {
+	switch value := destination.(type) {
+	case []byte:
+		return json.Unmarshal(value, i)
+
+	case string:
+		return json.Unmarshal([]byte(value), i)
+
+	default:
+		return fmt.Errorf("%w: unexpected data type %T", ErrMachineAddressesInvalid, destination)
+	}
+}
+
+// Value return json value, implement driver.Valuer interface.
+func (i IPPrefixes) Value() (driver.Value, error) {
+	bytes, err := json.Marshal(i)
+
+	return string(bytes), err
+}
+
+type StringList []string
+
+func (i *StringList) Scan(destination interface{}) error {
+	switch value := destination.(type) {
+	case []byte:
+		return json.Unmarshal(value, i)
+
+	case string:
+		return json.Unmarshal([]byte(value), i)
+
+	default:
+		return fmt.Errorf("%w: unexpected data type %T", ErrMachineAddressesInvalid, destination)
+	}
+}
+
+// Value return json value, implement driver.Valuer interface.
+func (i StringList) Value() (driver.Value, error) {
+	bytes, err := json.Marshal(i)
+
+	return string(bytes), err
+}