Move to an external snapshot file.

[LindirQuenya]

Improved variant on #11, using a read-only external state file with a read-only disk to achieve complete hash stability. All writes are performed to the in-memory tmpfs, and don't get written back to the state file. The state file can also be compressed, for extra space savings. The QEMU command line should be changed to: qemu-system-i386 -machine pc-i440fx-5.2 -m 128 -net nic,model=virtio-net-pci -net user,hostfwd=tcp:127.0.0.1:22500-:80 -qmp tcp:127.0.0.1:22501,server,nowait -qmp tcp:127.0.0.1:22502,server,nowait -drive file=alpine.qcow2,if=virtio,readonly=on -serial stdio -incoming "exec:7za x -so snapshot.7z" -display none
The corresponding arguments list in services.json would be:

[
  "-machine",
  "pc-i440fx-5.2",
  "-m",
  "128",
  "-net",
  "nic,model=virtio-net-pci",
  "-net",
  "user,hostfwd=tcp:127.0.0.1:22500-:80",
  "-qmp",
  "tcp:127.0.0.1:22501,server,nowait",
  "-qmp",
  "tcp:127.0.0.1:22502,server,nowait",
  "-drive",
  "file=alpine.qcow2,if=virtio,readonly=on",
  "-serial",
  "stdio",
  "-incoming",
  "exec:7za x -so snapshot.7z",
  "-display",
  "none"
]

A built image and snapshot may be found here. Note that Windows will require some extra steps: you'll need to copy 7za.exe to Server, and update QEMU to at least 7.2.0.

I'll mark this as ready for review after I've tested it on Windows.

[LindirQuenya]

Hm, Windows support doesn't seem to be as good as I thought. No matter what I put into the exec: argument (even things that should be found, like powershell), it always fails with "No such file or directory." Perhaps I will be able to look into this later.

[LindirQuenya]

Here's what I've found so far:

• The QEMU windows builds need to include GLib's gspawn-win32-helper.exe and gspawn-win32-helper-console.exe.
• migration/exec.c needs a two-line change, from /bin/sh -c → C:\Windows\System32\cmd.exe /c
• And there might be more, or I might just need to recompile it against my version of GLib.

I'll probably have a bit more time to investigate this evening.

[puigru]

@LindirQuenya Awesome work on this, it looks like you're almost there. This would be great to have.

P.S. Definitely submit the patch to QEMU when you do.

[puigru]

Oh, one small thing. Windows may not necessarily be on the C:\ drive. There are specific APIs to get the path to the system folder (a or b, c). That said, if exec'ing looks in PATH, then just cmd.exe should work.

[LindirQuenya]

With those changes, the patch became a bit more complicated. I'll try to submit it to QEMU tomorrow.

[LindirQuenya]

From my minimal testing of my debug build, it works.