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guestfs_readdir(): rewrite with FileOut transfer, to lift protocol limit

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Currently the guestfs_readdir() API can not list long directories, due to
it sending back the whole directory listing in a single guestfs protocol
response, which is limited to GUESTFS_MESSAGE_MAX (approx. 4MB) in size.

Introduce the "internal_readdir" action, for transferring the directory
listing from the daemon to the library through a FileOut parameter.
Rewrite guestfs_readdir() on top of this new internal function:

- The new "internal_readdir" action is a daemon action. Do not repurpose
  the "readdir" proc_nr (138) for "internal_readdir", as some distros ship
  the binary appliance to their users, and reusing the proc_nr could
  create a mismatch between library & appliance with obscure symptoms.
  Replace the old proc_nr (138) with a new proc_nr (511) instead; a
  mismatch would then produce a clear error message. Assume the new action
  will first be released in libguestfs-1.48.2.

- Turn "readdir" from a daemon action into a non-daemon one. Call the
  daemon action guestfs_internal_readdir() manually, receive the FileOut
  parameter into a temp file, then deserialize the dirents array from the
  temp file.

This patch sneakily fixes an independent bug, too. In the pre-patch
do_readdir() function [daemon/readdir.c], when readdir() returns NULL, we
don't distinguish "end of directory stream" from "readdir() failed". This
rewrite fixes this problem -- I didn't see much value separating out the
fix for the original do_readdir().

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1674392
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20220502085601.15012-2-lersek@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
This commit is contained in:
Laszlo Ersek
2022-05-02 10:56:00 +02:00
parent ac00e603f8
commit 45b7f1736b
7 changed files with 267 additions and 136 deletions
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TODO
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@@ -484,14 +484,6 @@ this approach works, it doesn't solve the MBR problem, so likely we'd
have to write a library for that (or perhaps go back to sfdisk but
using a very abstracted interface over sfdisk).
Reimplement some APIs to avoid protocol limits
----------------------------------------------
Mostly this item was done (eg. commits a69f44f56f and before). The
most notable API with a protocol limit remaining is:
- guestfs_readdir
hivex
-----