Improve size limiting string message

SizeLimitingStringWriter is used to limit the output size of mustache
scripts. When the size is limited, the resulting exception lacks detail
needed to identify the problem. In particular, the actual size that
would result is not given. Additionally, the excerpt lacks any of the
new string being added. This commit tweaks the exception to include both
of these details.

Author: rjernst

server/src/main/java/org/elasticsearch/common/text/SizeLimitingStringWriter.java

@@ -30,39 +30,59 @@ public SizeLimitingStringWriter(int sizeLimit) {
         this.sizeLimit = sizeLimit;
     }
 
-    private void checkSizeLimit(int additionalChars) {
-        int bufLen = getBuffer().length();
-        if (bufLen + additionalChars > sizeLimit) {
-            throw new SizeLimitExceededException(
-                Strings.format("String [%s...] has exceeded the size limit [%s]", getBuffer().substring(0, Math.min(bufLen, 20)), sizeLimit)
-            );
+    private int limitSize(int additionalChars) {
+        int neededSize = getBuffer().length() + additionalChars;
+        if (neededSize > sizeLimit) {
+            return additionalChars - (neededSize - sizeLimit);
         }
+        return additionalChars;
+    }
+
+    private void throwSizeLimitExceeded(int limitedChars, int requestedChars) {
+        assert limitedChars < requestedChars;
+        int bufLen = getBuffer().length();
+        int foundSize = bufLen - limitedChars + requestedChars; // reconstitute original
+        String selection = getBuffer().substring(0, Math.min(bufLen, 20));
+        throw new SizeLimitExceededException(
+            Strings.format("String [%s...] has size [%d] which exceeds the size limit [%d]", selection, foundSize, sizeLimit)
+        );
     }
 
     @Override
     public void write(int c) {
-        checkSizeLimit(1);
+        if (limitSize(1) != 1) {
+            throwSizeLimitExceeded(0, 1);
+        }
         super.write(c);
     }
 
     // write(char[]) delegates to write(char[], int, int)
 
     @Override
     public void write(char[] cbuf, int off, int len) {
-        checkSizeLimit(len);
-        super.write(cbuf, off, len);
+        int limitedLen = limitSize(len);
+        if (limitedLen > 0) {
+            super.write(cbuf, off, limitedLen);
+        }
+        if (limitedLen != len) {
+            throwSizeLimitExceeded(limitedLen, len);
+        }
     }
 
     @Override
     public void write(String str) {
-        checkSizeLimit(str.length());
-        super.write(str);
+        this.write(str, 0, str.length());
     }
 
     @Override
     public void write(String str, int off, int len) {
-        checkSizeLimit(len);
-        super.write(str, off, len);
+        int limitedLen = limitSize(len);
+        if (limitedLen > 0) {
+            super.write(str, off, limitedLen);
+        }
+        if (limitedLen != len) {
+            throwSizeLimitExceeded(limitedLen, len);
+        }
     }
 
     // append(...) delegates to write(...) methods

server/src/test/java/org/elasticsearch/common/text/SizeLimitingStringWriterTests.java

@@ -11,6 +11,8 @@
 
 import org.elasticsearch.test.ESTestCase;
 
+import static org.hamcrest.Matchers.equalTo;
+
 public class SizeLimitingStringWriterTests extends ESTestCase {
     public void testSizeIsLimited() {
         SizeLimitingStringWriter writer = new SizeLimitingStringWriter(10);
@@ -26,4 +28,11 @@ public void testSizeIsLimited() {
         expectThrows(SizeLimitingStringWriter.SizeLimitExceededException.class, () -> writer.append("a"));
         expectThrows(SizeLimitingStringWriter.SizeLimitExceededException.class, () -> writer.append("a", 0, 1));
     }
+
+    public void testLimitMessage() {
+        SizeLimitingStringWriter writer = new SizeLimitingStringWriter(3);
+
+        var e = expectThrows(SizeLimitingStringWriter.SizeLimitExceededException.class, () -> writer.write("abcdefgh"));
+        assertThat(e.getMessage(), equalTo("String [abc...] has size [8] which exceeds the size limit [3]"));
+    }
 }