RFR: 8351110: ImageIO.write for JPEG can write corrupt JPEG for certain thumbnail dimensions [v2]

[Jeremy Wood]

> JPEG segments can only be 65535-bytes long. (The marker length is expressed as 2 bytes.) The problem in this ticket is that we were writing more than 65535 bytes to a segment, which later caused parsing errors when we tried to read the file back.
> 
> This includes 2 changes:
> 
> 1. We now clip the thumbnail width/height to fit within the available marker. We were already constraining the width & height to a max of 255. (Because the width & height are expressed as 1 byte.) So this new clipping is similar to something we were already doing, and we issue the same WARNING_THUMB_CLIPPED warning to the writer. (Does this require a CSR?)
> 
> 2. This adds a bounds check to `write2bytes`. IMO the bigger problem in this ticket was the `ImageIO.write(..)` caller thought they ended up with a valid JPEG. (We were violating the "do no harm / lose no data" doctrine.) Now if something like this ever comes up again: writing will fail with an IIOException. Technically you can argue this change is unnecessary because the new clipping avoids this error condition, but IMO including this safety net is an overall good thing to keep. If anyone disagrees I'm OK with removing it.
> 
> Separately:
> I included (and reversed) a commit in this branch that provides an alternate solution. That commit anticipates the overflow problem and switches to a JPEG-encoded thumbnail. From an end user perspective this "just works": they get a (slightly lossly) thumbnail of the original dimensions. But I assume it would be more controversial compared to the clipping approach, since the clipping approach has a strong precedent in this codebase.

Jeremy Wood has updated the pull request incrementally with one additional commit since the last revision:

  8351110: code cleanup
  
  This is in response to:
  https://github.com/openjdk/jdk/pull/23920#discussion_r1984465597
  https://github.com/openjdk/jdk/pull/23920#discussion_r1984466151
  https://github.com/openjdk/jdk/pull/23920#discussion_r1984466228
  https://github.com/openjdk/jdk/pull/23920#discussion_r1984466870
  
  This also updates the test summary, removes an unused method, and fixes a trivial compiler warning.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23920/files
  - new: https://git.openjdk.org/jdk/pull/23920/files/3b39df55..4d6afb6b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23920&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23920&range=00-01

  Stats: 19 lines in 1 file changed: 5 ins; 7 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/23920.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23920/head:pull/23920

PR: https://git.openjdk.org/jdk/pull/23920