8357249: Compiler task keeps --system files open

[david-beaumont]

Version of #2374 which rewrites the initialization to clarify the way buffers are used. This class is historically very complex and, in mainline, contains issues around whether files should or should not be memory mapped, and how much of the file exists in the "memoryMap" field (which may not even be a memory mapped buffer).

Now, if non-null, the memoryMap field only contains the entire buffer that's mapped.
Under all other circumstances, the channel exists and is used to read buffer entries piecemeal.

This makes reasoning about who owns the buffer and how its lifecycle is managed much easier and makes it clear that non-runtime (target) images are only ever loaded piecemeal to avoid opening a memory mapped file.

---

• I confirm that I make this contribution in accordance with the OpenJDK Interim AI Policy.

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed (1 review required, with at least 1 Committer)

Issue

• JDK-8357249: Compiler task keeps --system files open (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/valhalla.git pull/2377/head:pull/2377
$ git checkout pull/2377

Update a local copy of the PR:
$ git checkout pull/2377
$ git pull https://git.openjdk.org/valhalla.git pull/2377/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 2377

View PR using the GUI difftool:
$ git pr show -t 2377

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/valhalla/pull/2377.diff

[bridgekeeper]

👋 Welcome back dbeaumont! A progress list of the required criteria for merging this PR into lworld will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@david-beaumont this pull request can not be integrated into lworld due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout JDK_8357249_refactor
git fetch https://git.openjdk.org/valhalla.git lworld
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge lworld"
git push