[lttng-dev] UST app and lttng-tools compatibility

[Francis Giraldeau]

Hi,

I wanted to share my lttng-ust 2.1 update experience, maybe it will save
time for others.

I updated lttng-ust recently. After this change, the app would not
produce a trace anymore. No error message is displayed by the traced app
to indicate that something is wrong. Even when setting LTTNG_DEBUG_UST
to the app's environment variable, there is no error message. The debug
output suggests that probes are registered and everything is fine, while
it's not.

By running lttng-sessiond with -vvv --verbose-consumer, I finally got
this message:

DEBUG2: UST app PID 8112 is not compatible with major version 3
(supporting <= 2) [in ust_app_validate_version() at ust-app.c:2633]

Updating lttng-tools to 2.1 solved the issue. Seems that it's mandatory
to update lttng-tools to support latest lttng-ust. It may be obvious for
developers, but it should be clear for users that they must upgrade both.

IMHO, It would be nice if the app side log could tell if the
session/consumer refused the registration.

Cheers,

Francis Giraldeau

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