IDEA-204822 [followup] indexing svg files can be slow

platform/platform-tests/testData/ui/myIcon_xml_20x10.svg

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- follow the white rabbit -->
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg xmlns="http://www.w3.org/2000/svg" width="20" height="10">
+  <rect fill="lime" width="100%" height="100%"/>
+</svg>

platform/platform-tests/testSrc/com/intellij/util/ui/SvgIconSizeTest.java

@@ -10,6 +10,7 @@ import org.junit.Test;
 import org.junit.rules.ExternalResource;
 
 import java.awt.*;
+import java.awt.geom.Dimension2D;
 import java.awt.image.BufferedImage;
 import java.io.File;
 import java.io.IOException;
@@ -50,6 +51,22 @@ public class SvgIconSizeTest {
     image = ImageUtil.toBufferedImage(image);
     assertEquals("wrong image width", pixScale * 25, (double)image.getWidth(null));
     assertEquals("wrong image height", pixScale * 15, (double)image.getHeight(null));
+
+    /*
+     * Test SVGLoader.getDocumentSize for SVG starting with <svg.
+     */
+    url = new File(getSvgIconPath("20x10")).toURI().toURL();
+    Dimension2D size = SVGLoader.getDocumentSize(url, url.openStream(), 1);
+    assertEquals("wrong svg doc width", 20d, size.getWidth());
+    assertEquals("wrong svg doc height", 10d, size.getHeight());
+
+    /*
+     * Test SVGLoader.getDocumentSize for SVG starting with <?xml.
+     */
+    url = new File(getSvgIconPath("xml_20x10")).toURI().toURL();
+    size = SVGLoader.getDocumentSize(url, url.openStream(), 1);
+    assertEquals("wrong svg doc width", 20d, size.getWidth());
+    assertEquals("wrong svg doc height", 10d, size.getHeight());
   }
 
   private static void test(ScaleContext ctx) {

platform/util/src/com/intellij/util/SVGLoader.java

@@ -31,6 +31,7 @@ import java.awt.image.BufferedImage;
 import java.io.*;
 import java.net.URISyntaxException;
 import java.net.URL;
+import java.util.Arrays;
 
 import static com.intellij.util.ui.JBUI.ScaleType.PIX_SCALE;
 
@@ -151,15 +152,25 @@ public class SVGLoader {
     return (T)ImageUtil.ensureHiDPI(image, ctx);
   }
 
+  @SuppressWarnings("SSBasedInspection")
   public static Dimension2D getDocumentSize(@Nullable URL url, @NotNull InputStream stream , double scale) throws IOException {
     // In order to get the size we parse the whole document and build a tree ("GVT"), what might be too expensive.
-    // So, to optimize we extract the svg header and parse only it.
+    // So, to optimize we extract the svg header (possibly prepended with <?xml> header) and parse only it.
     StringBuilder builder = new StringBuilder(100);
+    byte[] bytes = new byte[3];
+    boolean checkClosingBracket = false;
     int ch;
     while((ch = stream.read()) != -1) {
       builder.append((char)ch);
-      if (ch == '>') {
-        //noinspection SSBasedInspection
+      if (ch == '<') {
+        if (stream.read(bytes, 0, 3) == -1) {
+          break;
+        }
+        String str = new String(bytes);
+        builder.append(str);
+        checkClosingBracket = "svg".equals(str);
+      }
+      else if (checkClosingBracket && ch == '>') {
         return new SVGLoader(url, new ByteArrayInputStream(builder.append("</svg>").toString().getBytes()), scale).getDocumentSize();
       }
     }