[WIP] Fix UNC path on Windows

org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/uriresolver/CacheResourceFileNotFoundException.java

@@ -0,0 +1,9 @@
+package org.eclipse.lemminx.uriresolver;
+
+public class CacheResourceFileNotFoundException extends CacheResourceException {
+
+	public CacheResourceFileNotFoundException(String resourceURI, String message) {
+		super(resourceURI, message);
+	}
+
+}

org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/uriresolver/CacheResourcesManager.java

@@ -14,6 +14,7 @@
 
 import static org.eclipse.lemminx.utils.ExceptionUtils.getRootCause;
 
+import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -26,6 +27,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.text.MessageFormat;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -165,6 +167,16 @@ public Path getResource(final String resourceURI) throws IOException {
 					CacheResourceDownloadingError.DOWNLOAD_DISABLED, null, null);
 		}
 
+		URI resourceUriParsed = URI.create(resourceURI);
+		if ("file".equals(resourceUriParsed.getScheme())) {
+			// UNC path
+			Path path = Path.of(resourceUriParsed);
+			if (!Files.exists(path)) {
+				throw new CacheResourceFileNotFoundException(path.toString(), "The network-accessible file '" + path.toString() + "' doesn't exist");
+			}
+			return path;
+		}
+
 		if (!FilesUtils.isIncludedInDeployedPath(resourceCachePath)) {
 			throw new CacheResourceDownloadingException(resourceURI, resourceCachePath,
 					CacheResourceDownloadingError.RESOURCE_NOT_IN_DEPLOYED_PATH, null, null);
@@ -309,6 +321,7 @@ public static Path getResourceCachePath(String resourceURI) throws IOException {
 	}
 
 	public static Path getResourceCachePath(URI uri) throws IOException {
+
 		// Eliminate all path traversals
 		URI normalizedUri = uri.normalize();
 

org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLValidationExternalResourcesBasedOnXSDTest.java

@@ -180,6 +180,31 @@ public void schemaLocationDownloadDisabledUNC() throws Exception {
 						fileURI)));
 	}
 
+	@Test
+	public void schemaLocationFileDoesntExistUNC() throws Exception {
+
+		XMLValidationRootSettings validation = new XMLValidationRootSettings();
+		validation.setResolveExternalEntities(true);
+
+		XMLLanguageService ls = new XMLLanguageService();
+		ls.initializeIfNeeded();
+
+		String xml = "<?xml-model href=\"\\\\myserver\\myshare\\schema.xsd\"?>\r\n" + //
+				"<root-element>\r\n" + //
+				"	\r\n" + //
+				"</root-element>\r\n";
+
+		String fileURI = "test.xml";
+
+		Diagnostic d = new Diagnostic(r(0, 18, 0, 47), "The network-accessible file '\\\\myserver\\myshare\\schema.xsd' doesn't exist",
+				DiagnosticSeverity.Error, "xml", XMLSchemaErrorCode.schema_reference_4.getCode());
+
+		// Test diagnostics
+		XMLAssert.testPublishDiagnosticsFor(xml, fileURI, validation, ls, pd(fileURI, d, //
+				new Diagnostic(r(1, 1, 1, 13), "cvc-elt.1.a: Cannot find the declaration of element 'root-element'.",
+						DiagnosticSeverity.Error, "xml", XMLSchemaErrorCode.cvc_elt_1_a.getCode())));
+	}
+
 	@Test
 	public void schemaLocationDownloadDisabledUNC2() throws Exception {