[Win32] Ensure and document thread-safety of SWTFontProvider

The SWTFontProvider is Win32-internal for accessing font registries that
manage the fonts for a device at different zoom levels. Currently, this
class does not consider it's accesses' thread-safety.

This change makes the SWTFontProvider thread-safe under current usage
contexts:
- Make the internal map thread-safe as it may be modified from different
threads (used for the different devices/displays) concurrently. Using a
ConcurrentHashMap ensures that read access stays fast. Write access only
happens once per device creation.
- Document non-thread-safety of the provider's operations and the
necessity to always call it from the same thread for the same device.
All current callers fulfill this requirement, thus no race conditions
may occur.

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/internal/SWTFontProvider.java

@@ -14,6 +14,7 @@
 package org.eclipse.swt.internal;
 
 import java.util.*;
+import java.util.concurrent.*;
 
 import org.eclipse.swt.graphics.*;
 import org.eclipse.swt.widgets.*;
@@ -26,20 +27,46 @@
  * take the provided values for the zoom into consideration and return scaled variant of a font if necessary.
  */
 public class SWTFontProvider {
-	private static Map<Device, SWTFontRegistry> fontRegistries = new HashMap<>();
+	private static final Map<Device, SWTFontRegistry> fontRegistries = new ConcurrentHashMap<>();
 
 	private static SWTFontRegistry getFontRegistry(Device device) {
 		return fontRegistries.computeIfAbsent(device, SWTFontProvider::newFontRegistry);
 	}
 
+	/**
+	 * Returns the system font for the given device at the specified zoom.
+	 *
+	 * <b>Note:</b> This operation is not thread-safe. It must thus always be called
+	 * from the same thread for the same device, such as the display's UI thread.
+	 *
+	 * @param device the device to retrieve the font for, must not be {@code null}
+	 * @param zoom   the zoom for which the font shall be scaled
+	 */
 	public static Font getSystemFont(Device device, int zoom) {
 		return getFontRegistry(device).getSystemFont(zoom);
 	}
 
+	/**
+	 * Returns the font with the given font data for the given device at the
+	 * specified zoom.
+	 *
+	 * <b>Note:</b> This operation is not thread-safe. It must thus always be called
+	 * from the same thread for the same device, such as the display's UI thread.
+	 *
+	 * @param device   the device to retrieve the font for, must not be {@code null}
+	 * @param fontData the data for the font to retrieve, must not be {@code null}
+	 * @param zoom     the zoom for which the font shall be scaled
+	 */
 	public static Font getFont(Device device, FontData fontData, int zoom) {
 		return getFontRegistry(device).getFont(fontData, zoom);
 	}
 
+	/**
+	 * Disposes the font registry for the given device, if one exists.
+	 *
+	 * @param device the device to dispose the font registry for, must not be
+	 *               {@code null}
+	 */
 	public static void disposeFontRegistry(Device device) {
 		SWTFontRegistry fontRegistry = fontRegistries.remove(device);
 		if (fontRegistry != null) {