RFR: 8347334: JimageDiffGenerator code clean-ups

[Archie Cobbs]

On Thu, 9 Jan 2025 17:32:39 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> During code review of [JDK-8346239](https://bugs.openjdk.org/browse/JDK-8346239) a few comments were made after the PR integrated. This follow-up patch cleans this up and adds a unit test for the `JimageDiffGenerator` class.
> 
> Testing:
> - [ ] GHA
> - [x] tools/jlink tests (including the new one).
> 
> Thoughts?

src/jdk.jlink/share/classes/jdk/tools/jlink/internal/runtimelink/JimageDiffGenerator.java line 131:

> 129:         int bytesRead1, bytesRead2;
> 130:         try (is1; is2) {
> 131:             while ((bytesRead1 = is1.readNBytes(buf1, 0, buf1.length)) != 0) {

If `in1` is exactly 1024 bytes, and `in2` is those same bytes plus one extra byte (1025 bytes), this method will return a false positive.

Probably just want `while (true)` here, then read both `in1` and `in2`, then check results.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23014#discussion_r1909219888