Logger kept its rendered prefix in a plain string field that was written by
AddPrefix, AppendPrefix and ResetPrefixes and read without synchronization by
Tracef, Spawn and the other logging methods. In frpc the race is reachable on
the service-level logger, which every component of a session shares through
its context: adding the run ID after a login writes the prefix while control,
proxy manager, connector and visitor goroutines keep logging through the same
logger, and a reload that creates proxies calls Spawn on it. A torn string
header can pair a nil data pointer with a non-zero length, and formatting such
a prefix can crash the process when the log call's level is enabled.
Serialize the writers with a per-Logger mutex and publish the rendered prefix
as an immutable string through an atomic pointer, so the log path takes no
lock. Spawn copies the parent prefix list under the same mutex, keeping
children independent snapshots. Logging output is unchanged.
In AddPrefix, the loop `for _, p := range l.prefixes` creates a copy
of each element. Assignments to p.Value and p.Priority only modify
the local copy, not the original slice element, causing updates to
existing prefixes to be silently lost.
This affects client/service.go where AddPrefix is called with
Name:"runID" on reconnection — the old runID value would persist
in log output instead of being updated to the new one.
Fix by using index-based access `l.prefixes[i]` to modify the
original slice element, and add break since prefix names are unique.
fatedierandGitHub
Gabriel MarinandGitHub