The LSP log replayer takes a log from a gopls session, starts up an instance of gopls, and tries to replay the session. It produces a log from the replayed session and reports some comparative statistics of the two logs.
replay -log <logfile>
The logfile should be the log produced by gopls. It will have a name like /tmp/gopls-89775 or, on a Mac, $TMPDIR/gopls-29388.
If replay cannot find a copy of gopls to execute, use -cmd <path to gopls>. It looks in the same places where go install would put its output, namely $GOBIN/gopls, $GOPATH/bin/gopls, $HOME/go/bin/gopls.
The log for the replayed session is saved in /tmp/seen.
There is aloso a boolean argument -cmp which compares the log file with /tmp/seen without invoking gopls and rerunning the session.
The output is fairly cryptic, and generated by logging. Ideas for better output would be welcome. Here's an example, with intermingled comments:
main.go:50: old 1856, hist:[10:177 30:1 100:0 300:3 1000:4 ]
This says that the original log had 1856 records in it. The histogram is counting how long RPCs took, in milliseconds. In this case 177 took no more than 10ms, and 4 took between 300ms and 1000ms.
main.go:53: calling mimic main.go:293: mimic 1856
This is a reminder that it's replaying in a new session, with a log file containing 1856 records
main.go:61: new 1846, hist:[10:181 30:1 100:1 300:1 1000:1 ]
The new session produced 1846 log records (that's 10 fewer), and a vaguely similar histogram.
main.go:96: old: clrequest:578 clresponse:185 svrequest:2 svresponse:2 toserver:244 toclient:460 reporterr:385 main.go:96: new: clrequest:571 clresponse:185 svrequest:2 svresponse:2 toserver:241 toclient:460 reporterr:385
The first line is for the original log, the second for the new log. The new log has 7 fewer RPC requests from the client clrequest (578 vs 571), the same number of client responses clresponse, 3 fewer notifications toserver from the client, the same number from the server toclient to the client, and the same number of errors reporterr. (That‘s mysterious, but a look at the ends of the log files shows that the original session ended with several RPCs that don’t show up, for whatever reason, in the new session.)
Finally, there are counts of the various notifications seen, in the new log and the old log, and which direction they went. (The 3 fewer notifications in the summary above can be seen here to be from cancels and a didChange.)
main.go:107: counts of notifications main.go:110: '$/cancelRequest'. new toserver 1 main.go:110: '$/cancelRequest'. old toserver 3 main.go:110: 'initialized'. new toserver 1 main.go:110: 'initialized'. old toserver 1 main.go:110: 'textDocument/didChange'. new toserver 231 main.go:110: 'textDocument/didChange'. old toserver 232 main.go:110: 'textDocument/didOpen'. new toserver 1 main.go:110: 'textDocument/didOpen'. old toserver 1 main.go:110: 'textDocument/didSave'. new toserver 7 main.go:110: 'textDocument/didSave'. old toserver 7 main.go:110: 'textDocument/publishDiagnostics'. new toclient 182 main.go:110: 'textDocument/publishDiagnostics'. old toclient 182 main.go:110: 'window/logMessage'. new toclient 278 main.go:110: 'window/logMessage'. old toclient 278
Replay cannot restore the exact environment gopls saw for the original session. For instance, the first didOpen message in the new session will see the file as it was left by the original session.
Gopls invokes various tools, and the environment they see could have changed too.
Replay will use the gopls it finds (or is given). It has no way of using the same version that created the original session.