Add ensure-file parser package.

cipd/client/cipd/ensure/package_def.go

Index: cipd/client/cipd/ensure/package_def.go
diff --git a/cipd/client/cipd/ensure/package_def.go b/cipd/client/cipd/ensure/package_def.go
new file mode 100644
index 0000000000000000000000000000000000000000..11f6749e5ea0b3ba554e497d60408d18b806ac07
--- /dev/null
+++ b/cipd/client/cipd/ensure/package_def.go
@@ -0,0 +1,71 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package ensure
+
+import (
+	"fmt"
+
+	"github.com/luci/luci-go/cipd/client/cipd/common"
+	"github.com/luci/luci-go/common/errors"
+)
+
+// PackageDef defines a package line parsed out of an ensure file.
+type PackageDef struct {
+	PackageTemplate   string
+	UnresolvedVersion string
+
+	// LineNo is set while parsing an ensure file by the ParseFile method. It is
+	// used by File.Resolve to give additional context if an error occurs.
+	LineNo int
+}
+
+func (p *PackageDef) String() string {
+	return fmt.Sprintf("%s %s", p.PackageTemplate, p.UnresolvedVersion)
+}
+
+// VersionResolver is expected to tranform a {PackageName, Version} tuple into
+// a resolved instance ID.
+//
+//  - `pkg` is guaranteed to pass common.ValidatePackageName
+//  - `vers` is guaranteed to pass common.ValidateInstanceVersion
+//
+// The returned `instID` will NOT be checked (so that you
+// can provide a pass-through resolver, if you like).
+type VersionResolver func(pkg, vers string) (common.Pin, error)
+
+// Resolve takes a Package definition containing a possibly templated package
+// name, and a possibly unresolved version string and attempts to resolve them
+// into a Pin.
+func (p *PackageDef) Resolve(plat, arch string, rslv VersionResolver) (pin common.Pin, err error) {
+	pin.PackageName, err = expandTemplate(p.PackageTemplate, plat, arch)
+	if err == errSkipTemplate {
+		return
+	}
+	if err != nil {
+		err = errors.Annotate(err).
+			Reason("failed to resolve package template (line %(line)d)").
+			D("line", p.LineNo).
+			Err()
+		return
+	}
+
+	pin, err = rslv(pin.PackageName, p.UnresolvedVersion)
+	if err != nil {
+		err = errors.Annotate(err).
+			Reason("failed to resolve package version (line %(line)d)").
+			D("line", p.LineNo).
+			Err()
+		return
+	}
+
+	return
+}
+
+// PackageSlice is a sortable slice of PackageDef
+type PackageSlice []PackageDef
+
+func (ps PackageSlice) Len() int           { return len(ps) }
+func (ps PackageSlice) Less(i, j int) bool { return ps[i].PackageTemplate < ps[j].PackageTemplate }
+func (ps PackageSlice) Swap(i, j int)      { ps[i], ps[j] = ps[j], ps[i] }