Add check for valid Dart SDK to preference page (#99)

closes #83

Signed-off-by: Jonas Hungershausen <jonas.hungershausen@vogella.com>

org.eclipse.dartboard/src/org/eclipse/dartboard/preference/DartPreferencePage.java

@@ -13,11 +13,12 @@
  *******************************************************************************/
 package org.eclipse.dartboard.preference;
 
+import java.io.File;
 import java.io.IOException;
-import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 
+import org.eclipse.core.runtime.Platform;
 import org.eclipse.core.runtime.preferences.InstanceScope;
 import org.eclipse.dartboard.Constants;
 import org.eclipse.dartboard.Messages;
@@ -107,32 +108,18 @@ public boolean performOk() {
 	 * @return
 	 */
 	private Path getPath(String location) {
+
 		if (dartSDKLocationEditor.isValid()) {
 			Path path = null;
 			try {
-				path = Paths.get(location);
-				path = path.toRealPath();
+				boolean isWindows = Platform.OS_WIN32.equals(Platform.getOS());
+				path = Paths.get(location + "bin" + File.separator + (isWindows ? "dart.exe" : "dart")); //$NON-NLS-1$ //$NON-NLS-2$ //$NON-NLS-3$
+				// Since we append /bin/dart to resolve the symbolic links, we need to get 2
+				// levels up here.
+				path = path.toRealPath().toAbsolutePath().getParent().getParent();
 			} catch (IOException e) {
 				LOG.error("Couldn't follow symlink", e); //$NON-NLS-1$
 			}
-
-			if (path == null) {
-				return null;
-			}
-
-			// Sometimes users put in the path to the Dart executable directly, instead of
-			// the directory of the installation. Here we use the parent first (which should
-			// be /bin)
-			if (path.endsWith("dart") && !Files.isDirectory(path)) { //$NON-NLS-1$
-				path = path.getParent();
-			}
-			// Sometimes users put in the path when it still contains the /bin portion.
-			// Since we only want the root of the Dart SDK installation we use the parent if
-			// /bin was supplied.
-			if (path.endsWith("bin")) {//$NON-NLS-1$
-				path = path.getParent();
-			}
-
 			return path;
 		} else {
 			return null;

org.eclipse.dartboard/src/org/eclipse/dartboard/preference/DartSDKLocationFieldEditor.java

@@ -1,14 +1,26 @@
 package org.eclipse.dartboard.preference;
 
+import java.io.BufferedReader;
 import java.io.File;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.nio.file.Files;
+import java.nio.file.InvalidPathException;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 
+import org.eclipse.core.runtime.Platform;
 import org.eclipse.dartboard.Messages;
 import org.eclipse.jface.preference.DirectoryFieldEditor;
 import org.eclipse.swt.events.ModifyListener;
 import org.eclipse.swt.widgets.Composite;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class DartSDKLocationFieldEditor extends DirectoryFieldEditor {
 
+	private static final Logger LOG = LoggerFactory.getLogger(DartSDKLocationFieldEditor.class);
+
 	public DartSDKLocationFieldEditor(String preferencesKey, String label, Composite parent) {
 		super(preferencesKey, label, parent);
 		setValidateStrategy(VALIDATE_ON_KEY_STROKE);
@@ -17,15 +29,85 @@ public DartSDKLocationFieldEditor(String preferencesKey, String label, Composite
 	@Override
 	protected boolean doCheckState() {
 		String location = getTextControl().getText();
-		File file = new File(location);
-		boolean isValid = file.exists();
+		boolean isValid = isValidDartSDK(location);
 		if (!isValid) {
 			setErrorMessage(Messages.Preference_SDKNotFound_Message);
 			showErrorMessage();
 		}
 		return isValid;
 	}
 
+	/**
+	 * Checks if a given path is the root directory of a Dart SDK installation.
+	 * 
+	 * Returns false if the path does not exist or the given location can not be
+	 * converted to a {@link Path}.
+	 * 
+	 * Similarly if the Path is not a directory, false is returned.
+	 * 
+	 * If the location is a symbolic link but it can not be resolved, false is
+	 * returned.
+	 * 
+	 * If the process to test the version string returned by the Dart executable can
+	 * not be executed, false is returned.
+	 * 
+	 * Finally, if the returned version string does not start with "Dart VM
+	 * version", false is returned.
+	 * 
+	 * @param location - A {@link String} that should be checked to be a Dart SDK
+	 *                 root directory.
+	 * @return <code>false</code> if the location is not a Dart SDK root directory,
+	 *         <code>true</code> otherwise.
+	 */
+	@SuppressWarnings("nls")
+	private boolean isValidDartSDK(String location) {
+		boolean isWindows = Platform.OS_WIN32.equals(Platform.getOS());
+		Path path = null;
+		// On Windows if a certain wrong combination of characters are entered a
+		// InvalidPathException is thrown. In that case we can assume that the location
+		// entered is not a valid Dart SDK directory either.
+		try {
+			path = Paths.get(location).resolve("bin" + File.separator + (isWindows ? "dart.exe" : "dart"));
+		} catch (InvalidPathException e) {
+			return false;
+		}
+		// If the entered file doesn't exist, there is no need to run it
+		// Similarly if the file is a directory it can't be the dart executable
+		if (!Files.exists(path) || Files.isDirectory(path)) {
+			return false;
+		}
+		// Follow symbolic links
+		try {
+			path = path.toRealPath();
+		} catch (IOException e1) {
+			LOG.error("Couldn't follow symlink", e1);
+			return false;
+		}
+
+
+		String executablePath = path.toAbsolutePath().toString();
+
+		String[] commands;
+		if (isWindows) {
+			commands = new String[] { "cmd", "/c", executablePath, "--version" };
+		} else {
+			commands = new String[] { "/bin/bash", "-c", executablePath + " --version" };
+		}
+
+		ProcessBuilder processBuilder = new ProcessBuilder(commands);
+
+		processBuilder.redirectErrorStream(true);
+		String version = null;
+		try (BufferedReader reader = new BufferedReader(
+				new InputStreamReader(processBuilder.start().getInputStream()))) {
+			version = reader.readLine();
+		} catch (IOException e) {
+			return false;
+		}
+
+		return version.startsWith("Dart VM version");
+	}
+
 	protected void addModifyListener(ModifyListener listener) {
 		getTextControl().addModifyListener(listener);
 	}