Fix LTTng build for build environments with older liblttng-ust-dev

[sywhang]

This fixes https://github.com/dotnet/coreclr/issues/27024.

For all of our event sinks for LTTng, we wrap the call to fire events with tracepoint_enabled, which is a macro defined in a header file liblttng-ust-dev. This macro checks whether a particular probe point is enabled before we fire an event, so that we're not unnecessarily making the call to fire events through LTTng when nobody is listening to it.

For environments that don't want to build using LTTng, we have a macro definition for tracepoint_enabled, which used to be defined as:

#define tracepoint_enabled(provider, name) TRUE.

This meant that for custom CoreCLR builds that were produced without LTTng library enabled, the runtime will always try to fire all events only to find out nobody is listening. So this behavior was changed to

#define tracepoint_enabled(provider, name) FALSE.

Unfortunately this caused the compiler to optimize out the bodies for all the event sinks when such thing happens. This would be fine for custom builds that didn't want LTTng since they become no-ops. However, some build environments that we still use, we use old enough distributions that have old LTTng libraries, specifically CentOS 6 which uses liblttng-ust-dev version 2.4. This particular version of the library doesn't have the definition of tracepoint_enabled in its header, so we hit this bad behavior in builds produced in CentOS 6 container.

Even more unfortunately, CentOS 6 is the distro we use for our official SDK build containers for Linux, so this caused LTTng support in CoreCLR to regress in the official builds.

This fix changes the macro definition of tracepoint_enabled to use XplatEventLogger::IsEventLoggingEnabled method, which checks if the environment variable we require for using LTTng tracing (COMPlus_EnableEventLog) is enabled.

The long term fix for this would be to change the official build containers to build and install liblttng-ust-dev versions 2.5 or newer, but this is the short-term fix until that happens.

The fix will be ported to 3.0 and 3.1 once merged.

cc @tommcdon

[brianrob]

@sywhang, this looks good for 3.0 and 3.1 (modulo comment below), but for 5.0, we really should upgrade and prereq a UST that supports tracepoint_enabled. Then we can remove the #ifndef.

[sywhang]

but for 5.0, we really should upgrade and prereq a UST that supports tracepoint_enabled

I agree. Once I am finished with this and the backports, I will file an issue to keep track of this.

[noahfalk]

LGTM