From 42baf977ba31748e89a7bad1149270433e6953d9 Mon Sep 17 00:00:00 2001
From: Severin Gehwolf <sgehwolf@redhat.com>
Date: Thu, 9 Jan 2025 19:16:45 +0100
Subject: [PATCH] Review feedback

---
 .../runtimelink/JimageDiffGenerator.java      | 16 +++++----
 .../runtimeImage/JimageDiffGeneratorTest.java | 36 +++++++++++++++++++
 2 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/runtimelink/JimageDiffGenerator.java b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/runtimelink/JimageDiffGenerator.java
index 4fe2527de9a75..9ec1fa55175af 100644
--- a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/runtimelink/JimageDiffGenerator.java
+++ b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/runtimelink/JimageDiffGenerator.java
@@ -128,12 +128,11 @@ private boolean compareStreams(InputStream is1, InputStream is2) {
         byte[] buf2 = new byte[1024];
         int bytesRead1, bytesRead2;
         try (is1; is2) {
-            while ((bytesRead1 = is1.readNBytes(buf1, 0, buf1.length)) != 0) {
+            while (true) {
+                bytesRead1 = is1.readNBytes(buf1, 0, buf1.length);
                 bytesRead2 = is2.readNBytes(buf2, 0, buf2.length);
-                if (bytesRead2 == 0) {
-                    // first input stream didn't return the end of stream
-                    // but the second did?
-                    return false;
+                if (bytesRead1 == 0 || bytesRead2 == 0) {
+                    break; // no more bytes for a stream
                 }
                 if (bytesRead1 != bytesRead2) {
                     return false;
@@ -143,10 +142,15 @@ private boolean compareStreams(InputStream is1, InputStream is2) {
                     return false;
                 }
             }
+            // Streams are equal only if we've read them both to the end
+            if (bytesRead1 == 0 &&
+                bytesRead2 == 0) {
+                return true;
+            }
         } catch (IOException e) {
             throw new UncheckedIOException("IO exception when comparing bytes", e);
         }
-        return true;
+        return false;
     }
 
 }
diff --git a/test/jdk/tools/jlink/runtimeImage/JimageDiffGeneratorTest.java b/test/jdk/tools/jlink/runtimeImage/JimageDiffGeneratorTest.java
index 95fc781c3a420..0176e6d07a19d 100644
--- a/test/jdk/tools/jlink/runtimeImage/JimageDiffGeneratorTest.java
+++ b/test/jdk/tools/jlink/runtimeImage/JimageDiffGeneratorTest.java
@@ -241,8 +241,44 @@ public void testNotEqualLength() throws Exception {
         assertEquals(result.get(0).getResourceBytes(), bytesBase[0]);
     }
 
+    /*
+     * Expect a difference since entry 'a' on the optimized version is
+     * one byte longer.
+     */
+    @Test
+    public void testBytesDifferExactBufferSize() throws Exception {
+        List<String> entriesBase = List.of("a", "b", "c", "d");
+        byte[][] bytesBase = new byte[][] {
+            { }, /* a */
+            { 0x08, 0x04, 0x04 }, /* b */
+            { 0x09, 0x11, 0x11 }, /* c */
+            { 0x11, 0x12, 0x31 }, /* d */
+        };
+        byte[][] bytesOpt = new byte[][] {
+            { }, /* a */
+            { 0x08, 0x04, 0x04 }, /* b */
+            { 0x09, 0x11, 0x11 }, /* c */
+            { 0x11, 0x12, 0x31 }, /* d */
+        };
+        bytesBase[0] = genBytesOfSize(1024);    // exact buffer size
+        bytesOpt[0] = genBytesOfSize(1024 + 1); // buffer size + 1
+
+        ImageResource base = new BasicImageResource(entriesBase, bytesBase);
+        ImageResource opt = new BasicImageResource(entriesBase, bytesOpt);
+        JimageDiffGenerator gen = new JimageDiffGenerator();
+        List<ResourceDiff> result = gen.generateDiff(base, opt);
+        assertEquals(result.size(), 1);
+        assertEquals(result.get(0).getKind(), ResourceDiff.Kind.MODIFIED);
+        assertEquals(result.get(0).getName(), "a");
+        assertEquals(result.get(0).getResourceBytes(), bytesBase[0]);
+    }
+
     private byte[] generateBytes() {
         int size = 1024 + 254;
+        return genBytesOfSize(size);
+    }
+
+    private byte[] genBytesOfSize(int size) {
         byte[] result = new byte[size];
         for (int i = 0; i < size; i++) {
             result[i] = (byte)(i % Byte.MAX_VALUE);