1 files changed, 145 insertions, 0 deletions
diff --git a/pkg/p9/version.go b/pkg/p9/version.go
new file mode 100644
index 000000000..8783eaa7e
--- /dev/null
+++ b/pkg/p9/version.go
@@ -0,0 +1,145 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package p9
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+const (
+	// highestSupportedVersion is the highest supported version X in a
+	// version string of the format 9P2000.L.Google.X.
+	//
+	// Clients are expected to start requesting this version number and
+	// to continuously decrement it until a Tversion request succeeds.
+	highestSupportedVersion uint32 = 6
+
+	// lowestSupportedVersion is the lowest supported version X in a
+	// version string of the format 9P2000.L.Google.X.
+	//
+	// Clients are free to send a Tversion request at a version below this
+	// value but are expected to encounter an Rlerror in response.
+	lowestSupportedVersion uint32 = 0
+
+	// baseVersion is the base version of 9P that this package must always
+	// support.  It is equivalent to 9P2000.L.Google.0.
+	baseVersion = "9P2000.L"
+)
+
+// HighestVersionString returns the highest possible version string that a client
+// may request or a server may support.
+func HighestVersionString() string {
+	return versionString(highestSupportedVersion)
+}
+
+// parseVersion parses a Tversion version string into a numeric version number
+// if the version string is supported by p9.  Otherwise returns (0, false).
+//
+// From Tversion(9P): "Version strings are defined such that, if the client string
+// contains one or more period characters, the initial substring up to but not
+// including any single period in the version string defines a version of the protocol."
+//
+// p9 intentionally diverges from this and always requires that the version string
+// start with 9P2000.L to express that it is always compatible with 9P2000.L.  The
+// only supported versions extensions are of the format 9p2000.L.Google.X where X
+// is an ever increasing version counter.
+//
+// Version 9P2000.L.Google.0 implies 9P2000.L.
+//
+// New versions must always be a strict superset of 9P2000.L. A version increase must
+// define a predicate representing the feature extension introduced by that version. The
+// predicate must be commented and should take the format:
+//
+// // VersionSupportsX returns true if version v supports X and must be checked when ...
+// func VersionSupportsX(v int32) bool {
+//	...
+// )
+func parseVersion(str string) (uint32, bool) {
+	// Special case the base version which lacks the ".Google.X" suffix.  This
+	// version always means version 0.
+	if str == baseVersion {
+		return 0, true
+	}
+	substr := strings.Split(str, ".")
+	if len(substr) != 4 {
+		return 0, false
+	}
+	if substr[0] != "9P2000" || substr[1] != "L" || substr[2] != "Google" || len(substr[3]) == 0 {
+		return 0, false
+	}
+	version, err := strconv.ParseUint(substr[3], 10, 32)
+	if err != nil {
+		return 0, false
+	}
+	return uint32(version), true
+}
+
+// versionString formats a p9 version number into a Tversion version string.
+func versionString(version uint32) string {
+	// Special case the base version so that clients expecting this string
+	// instead of the 9P2000.L.Google.0 equivalent get it.  This is important
+	// for backwards compatibility with legacy servers that check for exactly
+	// the baseVersion and allow nothing else.
+	if version == 0 {
+		return baseVersion
+	}
+	return fmt.Sprintf("9P2000.L.Google.%d", version)
+}
+
+// VersionSupportsTflushf returns true if version v supports the Tflushf message.
+// This predicate must be checked by clients before attempting to make a Tflushf
+// request.  If this predicate returns false, then clients may safely no-op.
+func VersionSupportsTflushf(v uint32) bool {
+	return v >= 1
+}
+
+// versionSupportsTwalkgetattr returns true if version v supports the
+// Twalkgetattr message. This predicate must be checked by clients before
+// attempting to make a Twalkgetattr request.
+func versionSupportsTwalkgetattr(v uint32) bool {
+	return v >= 2
+}
+
+// versionSupportsTucreation returns true if version v supports the Tucreation
+// messages (Tucreate, Tusymlink, Tumkdir, Tumknod). This predicate must be
+// checked by clients before attempting to make a Tucreation request.
+// If Tucreation messages are not supported, their non-UID supporting
+// counterparts (Tlcreate, Tsymlink, Tmkdir, Tmknod) should be used.
+func versionSupportsTucreation(v uint32) bool {
+	return v >= 3
+}
+
+// VersionSupportsConnect returns true if version v supports the Tlconnect
+// message. This predicate must be checked by clients
+// before attempting to make a Tlconnect request. If Tlconnect messages are not
+// supported, Tlopen should be used.
+func VersionSupportsConnect(v uint32) bool {
+	return v >= 4
+}
+
+// VersionSupportsAnonymous returns true if version v supports Tlconnect
+// with the AnonymousSocket mode. This predicate must be checked by clients
+// before attempting to use the AnonymousSocket Tlconnect mode.
+func VersionSupportsAnonymous(v uint32) bool {
+	return v >= 5
+}
+
+// VersionSupportsMultiUser returns true if version v supports multi-user fake
+// directory permissions and ID values.
+func VersionSupportsMultiUser(v uint32) bool {
+	return v >= 6
+}