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On X11, query for higher Xrandr version

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This appears to be the solution for the elusive #3335 issue. Previously,
in the Xlib backend, we used the "XRRQueryVersion" function to query for
the Xrandr version, and used that to determine whether we should use the
"GetScreenResources" call or the "GetScreenResourcesCurrent" call.

However, we passed the version "0, 0" into "XRRQueryVersion".
Previously with Xlib this wasn't a problem, as Xlib ignores the version
you pass in and substitutes it with the version of RandR it expects.

https://gitlab.freedesktop.org/xorg/lib/libxrandr/-/blob/master/src/Xrandr.c?ref_type=heads#L386-387

The way that "XRRQueryVersion" is implemented on the server end, it
compares the version passed into the request with the version supported
by the server. If the server's version is greater than the client
version, it just returns the client version. If the client's version is
greater, it passes the server's version. Since we were passing in "0, 0"
this means that the server returned "0, 0".

https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/randr/rrdispatch.c?ref_type=heads#L50-59

To determine whether we use "GetScreenResources" or
"GetScreenResourcesCurrent", we compare the version returned by the
server against "1, 3". Since we got "0, 0"- a version of XRandR so old
it doesn't even exist- we use "GetScreenResources".

The problem manifests in that "GetScreenResources" can take several
seconds to query the screen state based on the current hardware
configuration. On the other hand, "GetScreenResourcesCurrent" is fast;
it uses the server's hardware cache if it is available.

This problem is visible in XTrace. On the latest `master`:

```
000:<:00c2: 12: RANDR-Request(140,0): QueryVersion major-version=0 minor-version=0
000:>:00c2:32: Reply to QueryVersion: major-version=0 minor-version=0
000:<:00c3:  8: RANDR-Request(140,8): GetScreenResources window=0x0000076e
000:>:00c3:1600: Reply to GetScreenResources:
```

On the `v0.28.0` tag:

```
000:<:0019: 12: RANDR-Request(140,0): QueryVersion major-version=1 minor-version=6
000:>:0019:32: Reply to QueryVersion: major-version=1 minor-version=6
...later
000:<:002d:  8: RANDR-Request(140,25): GetScreenResourcesCurrent window=0x0000076e
000:>:002d:1600: Reply to GetScreenResourcesCurrent
```

This commit fixes this issue by requesting "1, 3" instead. This returns
the version we expect, where we can now use "GetScreenResourcesCurrent"
properly.

Fixes #3335

Signed-off-by: John Nunley <dev@notgull.net>
This commit is contained in:
John Nunley 2023-12-29 08:13:06 -08:00 • committed by GitHub
parent ca1674519a
commit ad1843aea6
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2 changed files with 2 additions and 1 deletions
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1
CHANGELOG.md
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@ -11,6 +11,7 @@ Unreleased` header.
# Unreleased
- On X11, reduce the amount of time spent fetching screen resources.
- On Windows, macOS, X11, Wayland and Web, implement setting images as cursors. See the `custom_cursors.rs` example.
- **Breaking:** Remove `Window::set_cursor_icon`
- Add `WindowBuilder::with_cursor` and `Window::set_cursor` which takes a `CursorIcon` or `CustomCursor`

2
src/platform_impl/linux/x11/monitor.rs
View file

@ -344,7 +344,7 @@ impl ScreenResources {
conn: &impl x11rb::connection::Connection,
root: &x11rb::protocol::xproto::Screen,
) -> Result<Self, X11Error> {
let version = conn.randr_query_version(0, 0)?.reply()?;
let version = conn.randr_query_version(1, 3)?.reply()?;
if (version.major_version == 1 && version.minor_version >= 3) || version.major_version > 1 {
let reply = conn