1 files changed, 187 insertions, 0 deletions
diff --git a/pkg/fspath/fspath.go b/pkg/fspath/fspath.go
new file mode 100644
index 000000000..4c983d5fd
--- /dev/null
+++ b/pkg/fspath/fspath.go
@@ -0,0 +1,187 @@
+// Copyright 2019 The gVisor Authors.
+//
+// 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 fspath provides efficient tools for working with file paths in
+// Linux-compatible filesystem implementations.
+package fspath
+
+import (
+	"strings"
+)
+
+const pathSep = '/'
+
+// Parse parses a pathname as described by path_resolution(7), except that
+// empty pathnames will be parsed successfully to a Path for which
+// Path.Absolute == Path.Dir == Path.HasComponents() == false. (This is
+// necessary to support AT_EMPTY_PATH.)
+func Parse(pathname string) Path {
+	if len(pathname) == 0 {
+		return Path{}
+	}
+	// Skip leading path separators.
+	i := 0
+	for pathname[i] == pathSep {
+		i++
+		if i == len(pathname) {
+			// pathname consists entirely of path separators.
+			return Path{
+				Absolute: true,
+				Dir:      true,
+			}
+		}
+	}
+	// Skip trailing path separators. This is required by Iterator.Next. This
+	// loop is guaranteed to terminate with j >= 0 because otherwise the
+	// pathname would consist entirely of path separators, so we would have
+	// returned above.
+	j := len(pathname) - 1
+	for pathname[j] == pathSep {
+		j--
+	}
+	// Find the end of the first path component.
+	firstEnd := i + 1
+	for firstEnd != len(pathname) && pathname[firstEnd] != pathSep {
+		firstEnd++
+	}
+	return Path{
+		Begin: Iterator{
+			partialPathname: pathname[i : j+1],
+			end:             firstEnd - i,
+		},
+		Absolute: i != 0,
+		Dir:      j != len(pathname)-1,
+	}
+}
+
+// Path contains the information contained in a pathname string.
+//
+// Path is copyable by value. The zero value for Path is equivalent to
+// fspath.Parse(""), i.e. the empty path.
+type Path struct {
+	// Begin is an iterator to the first path component in the relative part of
+	// the path.
+	//
+	// Path doesn't store information about path components after the first
+	// since this would require allocation.
+	Begin Iterator
+
+	// If true, the path is absolute, such that lookup should begin at the
+	// filesystem root. If false, the path is relative, such that where lookup
+	// begins is unspecified.
+	Absolute bool
+
+	// If true, the pathname contains trailing path separators, so the last
+	// path component must exist and resolve to a directory.
+	Dir bool
+}
+
+// String returns a pathname string equivalent to p. Note that the returned
+// string is not necessarily equal to the string p was parsed from; in
+// particular, redundant path separators will not be present.
+func (p Path) String() string {
+	var b strings.Builder
+	if p.Absolute {
+		b.WriteByte(pathSep)
+	}
+	sep := false
+	for pit := p.Begin; pit.Ok(); pit = pit.Next() {
+		if sep {
+			b.WriteByte(pathSep)
+		}
+		b.WriteString(pit.String())
+		sep = true
+	}
+	// Don't return "//" for Parse("/").
+	if p.Dir && p.Begin.Ok() {
+		b.WriteByte(pathSep)
+	}
+	return b.String()
+}
+
+// HasComponents returns true if p contains a non-zero number of path
+// components.
+func (p Path) HasComponents() bool {
+	return p.Begin.Ok()
+}
+
+// An Iterator represents either a path component in a Path or a terminal
+// iterator indicating that the end of the path has been reached.
+//
+// Iterator is immutable and copyable by value. The zero value of Iterator is
+// valid, and represents a terminal iterator.
+type Iterator struct {
+	// partialPathname is a substring of the original pathname beginning at the
+	// start of the represented path component and ending immediately after the
+	// end of the last path component in the pathname. If partialPathname is
+	// empty, the PathnameIterator is terminal.
+	//
+	// See TestParseIteratorPartialPathnames in fspath_test.go for a worked
+	// example.
+	partialPathname string
+
+	// end is the offset into partialPathname of the first byte after the end
+	// of the represented path component.
+	end int
+}
+
+// Ok returns true if it is not terminal.
+func (it Iterator) Ok() bool {
+	return len(it.partialPathname) != 0
+}
+
+// String returns the path component represented by it.
+//
+// Preconditions: it.Ok().
+func (it Iterator) String() string {
+	return it.partialPathname[:it.end]
+}
+
+// Next returns an iterator to the path component after it. If it is the last
+// component in the path, Next returns a terminal iterator.
+//
+// Preconditions: it.Ok().
+func (it Iterator) Next() Iterator {
+	if it.end == len(it.partialPathname) {
+		// End of the path.
+		return Iterator{}
+	}
+	// Skip path separators. Since Parse trims trailing path separators, if we
+	// aren't at the end of the path, there is definitely another path
+	// component.
+	i := it.end + 1
+	for {
+		if it.partialPathname[i] != pathSep {
+			break
+		}
+		i++
+	}
+	nextPartialPathname := it.partialPathname[i:]
+	// Find the end of this path component.
+	nextEnd := 1
+	for nextEnd < len(nextPartialPathname) && nextPartialPathname[nextEnd] != pathSep {
+		nextEnd++
+	}
+	return Iterator{
+		partialPathname: nextPartialPathname,
+		end:             nextEnd,
+	}
+}
+
+// NextOk is equivalent to it.Next().Ok(), but is faster.
+//
+// Preconditions: it.Ok().
+func (it Iterator) NextOk() bool {
+	return it.end != len(it.partialPathname)
+}