Detect Windows absolute paths on non-Windows CLI

When deploying a stack using a relative path as bind-mount
source in the compose file, the CLI converts the relative
path to an absolute path, relative to the location of the
docker-compose file.

This causes a problem when deploying a stack that uses
an absolute Windows path, because a non-Windows client will
fail to detect that the path (e.g. `C:\somedir`) is an absolute
path (and not a relative directory named `C:\`).

The existing code did already take Windows clients deploying
a Linux stack into account (by checking if the path had a leading
slash). This patch adds the reverse, and adds detection for Windows
absolute paths on non-Windows clients.

The code used to detect Windows absolute paths is copied from the
Golang filepath package;
1d0e94b1e1/src/path/filepath/path_windows.go (L12-L65)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/compose/loader/loader.go

@@ -479,12 +479,13 @@ func resolveVolumePaths(volumes []types.ServiceVolumeConfig, workingDir string,
 		}
 
 		filePath := expandUser(volume.Source, lookupEnv)
-		// Check for a Unix absolute path first, to handle a Windows client
+		// Check if source is an absolute path (either Unix or Windows), to
-		// with a Unix daemon. This handles a Windows client connecting to a
+		// handle a Windows client with a Unix daemon or vice-versa.
-		// Unix daemon. Note that this is not required for Docker for Windows
+		//
-		// when specifying a local Windows path, because Docker for Windows
+		// Note that this is not required for Docker for Windows when specifying
-		// translates the Windows path into a valid path within the VM.
+		// a local Windows path, because Docker for Windows translates the Windows
-		if !path.IsAbs(filePath) {
+		// path into a valid path within the VM.
+		if !path.IsAbs(filePath) && !isAbs(filePath) {
 			filePath = absPath(workingDir, filePath)
 		}
 		volume.Source = filePath

cli/compose/loader/loader_test.go

@@ -985,6 +985,84 @@ services:
 	assert.Error(t, err, `invalid mount config for type "bind": field Source must not be empty`)
 }
 
+func TestLoadBindMountSourceIsWindowsAbsolute(t *testing.T) {
+	tests := []struct {
+		doc      string
+		yaml     string
+		expected types.ServiceVolumeConfig
+	}{
+		{
+			doc: "Z-drive lowercase",
+			yaml: `
+version: '3.3'
+
+services:
+  windows:
+    image: mcr.microsoft.com/windows/servercore/iis:windowsservercore-ltsc2019
+    volumes:
+      - type: bind
+        source: z:\
+        target: c:\data
+`,
+			expected: types.ServiceVolumeConfig{Type: "bind", Source: `z:\`, Target: `c:\data`},
+		},
+		{
+			doc: "Z-drive uppercase",
+			yaml: `
+version: '3.3'
+
+services:
+  windows:
+    image: mcr.microsoft.com/windows/servercore/iis:windowsservercore-ltsc2019
+    volumes:
+      - type: bind
+        source: Z:\
+        target: C:\data
+`,
+			expected: types.ServiceVolumeConfig{Type: "bind", Source: `Z:\`, Target: `C:\data`},
+		},
+		{
+			doc: "Z-drive subdirectory",
+			yaml: `
+version: '3.3'
+
+services:
+  windows:
+    image: mcr.microsoft.com/windows/servercore/iis:windowsservercore-ltsc2019
+    volumes:
+      - type: bind
+        source: Z:\some-dir
+        target: C:\data
+`,
+			expected: types.ServiceVolumeConfig{Type: "bind", Source: `Z:\some-dir`, Target: `C:\data`},
+		},
+		{
+			doc: "forward-slashes",
+			yaml: `
+version: '3.3'
+
+services:
+  app:
+    image: app:latest
+    volumes:
+      - type: bind
+        source: /z/some-dir
+        target: /c/data
+`,
+			expected: types.ServiceVolumeConfig{Type: "bind", Source: `/z/some-dir`, Target: `/c/data`},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.doc, func(t *testing.T) {
+			config, err := loadYAML(tc.yaml)
+			assert.NilError(t, err)
+			assert.Check(t, is.Len(config.Services[0].Volumes, 1))
+			assert.Check(t, is.DeepEqual(tc.expected, config.Services[0].Volumes[0]))
+		})
+	}
+}
+
 func TestLoadBindMountWithSource(t *testing.T) {
 	config, err := loadYAML(`
 version: "3.5"

cli/compose/loader/windows_path.go

@@ -0,0 +1,66 @@
+package loader
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+// https://github.com/golang/go/blob/master/LICENSE
+
+// This file contains utilities to check for Windows absolute paths on Linux.
+// The code in this file was largely copied from the Golang filepath package
+// https://github.com/golang/go/blob/1d0e94b1e13d5e8a323a63cd1cc1ef95290c9c36/src/path/filepath/path_windows.go#L12-L65
+
+func isSlash(c uint8) bool {
+	return c == '\\' || c == '/'
+}
+
+// isAbs reports whether the path is a Windows absolute path.
+func isAbs(path string) (b bool) {
+	l := volumeNameLen(path)
+	if l == 0 {
+		return false
+	}
+	path = path[l:]
+	if path == "" {
+		return false
+	}
+	return isSlash(path[0])
+}
+
+// volumeNameLen returns length of the leading volume name on Windows.
+// It returns 0 elsewhere.
+// nolint: gocyclo
+func volumeNameLen(path string) int {
+	if len(path) < 2 {
+		return 0
+	}
+	// with drive letter
+	c := path[0]
+	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
+		return 2
+	}
+	// is it UNC? https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
+	if l := len(path); l >= 5 && isSlash(path[0]) && isSlash(path[1]) &&
+		!isSlash(path[2]) && path[2] != '.' {
+		// first, leading `\\` and next shouldn't be `\`. its server name.
+		for n := 3; n < l-1; n++ {
+			// second, next '\' shouldn't be repeated.
+			if isSlash(path[n]) {
+				n++
+				// third, following something characters. its share name.
+				if !isSlash(path[n]) {
+					if path[n] == '.' {
+						break
+					}
+					for ; n < l; n++ {
+						if isSlash(path[n]) {
+							break
+						}
+					}
+					return n
+				}
+				break
+			}
+		}
+	}
+	return 0
+}

cli/compose/loader/windows_path_test.go

@@ -0,0 +1,61 @@
+package loader
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+// https://github.com/golang/go/blob/master/LICENSE
+
+// The code in this file was copied from the Golang filepath package with some
+// small modifications to run it on non-Windows platforms.
+// https://github.com/golang/go/blob/1d0e94b1e13d5e8a323a63cd1cc1ef95290c9c36/src/path/filepath/path_test.go#L711-L763
+
+import "testing"
+
+type IsAbsTest struct {
+	path  string
+	isAbs bool
+}
+
+var isabstests = []IsAbsTest{
+	{"", false},
+	{"/", true},
+	{"/usr/bin/gcc", true},
+	{"..", false},
+	{"/a/../bb", true},
+	{".", false},
+	{"./", false},
+	{"lala", false},
+}
+
+var winisabstests = []IsAbsTest{
+	{`C:\`, true},
+	{`c\`, false},
+	{`c::`, false},
+	{`c:`, false},
+	{`/`, false},
+	{`\`, false},
+	{`\Windows`, false},
+	{`c:a\b`, false},
+	{`c:\a\b`, true},
+	{`c:/a/b`, true},
+	{`\\host\share\foo`, true},
+	{`//host/share/foo/bar`, true},
+}
+
+func TestIsAbs(t *testing.T) {
+	tests := append(isabstests, winisabstests...)
+	// All non-windows tests should fail, because they have no volume letter.
+	for _, test := range isabstests {
+		tests = append(tests, IsAbsTest{test.path, false})
+	}
+	// All non-windows test should work as intended if prefixed with volume letter.
+	for _, test := range isabstests {
+		tests = append(tests, IsAbsTest{"c:" + test.path, test.isAbs})
+	}
+
+	for _, test := range winisabstests {
+		if r := isAbs(test.path); r != test.isAbs {
+			t.Errorf("IsAbs(%q) = %v, want %v", test.path, r, test.isAbs)
+		}
+	}
+}