Improve size limiting string message

docs/changelog/122427.yaml

@@ -0,0 +1,5 @@
+pr: 122427
+summary: Improve size limiting string message
+area: Infra/Core
+type: enhancement
+issues: []

modules/lang-mustache/src/test/java/org/elasticsearch/script/mustache/MustacheScriptEngineTests.java

@@ -423,7 +423,7 @@ public void testResultSizeLimit() throws IOException {
             ex.getCause().getCause(),
             allOf(
                 instanceOf(SizeLimitingStringWriter.SizeLimitExceededException.class),
-                transformedMatch(Throwable::getMessage, endsWith("has exceeded the size limit [1024]"))
+                transformedMatch(Throwable::getMessage, endsWith("has size [1030] which exceeds the size limit [1024]"))
             )
         );
     }

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]"));
+    }
 }