fix: util/path: CheckSystemDriveAndRemoveDriveLetter to preserve /

The call to CheckSystemDriveAndRemoveDriveLetter() does not preserve
the trailing `/` or `\\`. This happens because `filepath.Clean()`
strips away any trailing slashes. For example `/sample/` will be
`\\sample` on Windows. This function was mainly written for Windows
scenarios, which have System Drive Letters like C:/, etc.

This was causing cases like `COPY testfile /testdir/` to
be intepreted as `COPY testfile /testdir`, and if `testdir` is
not explictly created before the call, it ends up being treated
as a destination file other than a directory.

Fix this by checking that if we have a trailing `/` or `\\`, we
preserve it after the call to `filepath.Clean()`.

Fixes #5249

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

frontend/dockerfile/dockerfile_test.go

@@ -152,6 +152,7 @@ var allTests = integration.TestFuncs(
 	testNamedMultiplatformInputContext,
 	testNamedFilteredContext,
 	testEmptyDestDir,
+	testPreserveDestDirSlash,
 	testCopyLinkDotDestDir,
 	testCopyLinkEmptyDestDir,
 	testCopyChownCreateDest,
@@ -547,6 +548,42 @@ RUN cmd /V:on /C "set /p tfcontent=<testfile \
 	require.NoError(t, err)
 }
 
+func testPreserveDestDirSlash(t *testing.T, sb integration.Sandbox) {
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(integration.UnixOrWindows(
+		`
+FROM busybox
+COPY testfile /sample/
+RUN [ "$(cat /sample/testfile)" == "contents0" ]
+`,
+		`
+FROM nanoserver
+COPY testfile /sample/
+RUN cmd /V:on /C "set /p tfcontent=<\sample\testfile \
+	& if !tfcontent! NEQ contents0 (exit 1)"
+`,
+	))
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("testfile", []byte("contents0"), 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+}
+
 func testCopyLinkDotDestDir(t *testing.T, sb integration.Sandbox) {
 	integration.SkipOnPlatform(t, "windows")
 	f := getFrontend(t, sb)

util/system/path.go

@@ -193,9 +193,11 @@ func CheckSystemDriveAndRemoveDriveLetter(path string, inputOS string) (string,
 	}
 
 	parts := strings.SplitN(path, ":", 2)
+
+	// we need to preserve the `/` or `\\` at the end because #5249
 	// Path does not have a drive letter. Just return it.
 	if len(parts) < 2 {
-		return ToSlash(filepath.Clean(path), inputOS), nil
+		return ToSlash(filepath.Clean(path), inputOS) + keepTrailingSlash(parts[0], inputOS), nil
 	}
 
 	// We expect all paths to be in C:
@@ -220,5 +222,19 @@ func CheckSystemDriveAndRemoveDriveLetter(path string, inputOS string) (string,
 	//
 	// We must return the second element of the split path, as is, without attempting to convert
 	// it to an absolute path. We have no knowledge of the CWD; that is treated elsewhere.
-	return ToSlash(filepath.Clean(parts[1]), inputOS), nil
+	return ToSlash(filepath.Clean(parts[1]), inputOS) + keepTrailingSlash(parts[1], inputOS), nil
+}
+
+// For Windows, checks if a path other than / or \\ has a trailing slash
+// and returns it to be retained after running the path
+// through the filepath.Clean() function which strips it off.
+// eg. "/sample/" -> "/sample". Using "/" since eventually the
+// path goes through the `ToSlash` function.
+func keepTrailingSlash(path, inputOS string) string {
+	if inputOS == "windows" &&
+		len(path) > 1 &&
+		(strings.HasSuffix(path, "/") || strings.HasSuffix(path, "\\")) {
+		return "/"
+	}
+	return ""
 }