experimental: mutex protection for AppendGTIDInPlace

[shlomi-noach]

Description

#18196 seemed to have introduces a recurring CI failure in onlineddl_vrepl_suite where vttablet panics as follows:

fatal error: concurrent map read and map write

goroutine 364949 gp=0x1400034dc00 m=17 mp=0x140008d4808 [running]:
runtime.fatal({0x1021f9923, 0x21})
	runtime/panic.go:1116 +0x48 fp=0x1400481adc0 sp=0x1400481ad90 pc=0x1000e5c58
internal/runtime/maps.fatal({0x1021f9923?, 0x1000ad1d0?})
	runtime/panic.go:1046 +0x20 fp=0x1400481ade0 sp=0x1400481adc0 pc=0x10011eb50
runtime.mapaccess2(0x1031abf00, 0x14000d81770, 0x1400481ae78)
	internal/runtime/maps/runtime_swiss.go:136 +0x48 fp=0x1400481ae50 sp=0x1400481ade0 pc=0x1000b04b8
vitess.io/vitess/go/mysql/replication.Mysql56GTIDSet.Contains(0x14000d81770, {0x1032e6b40?, 0x14002e9e750?})
	vitess.io/vitess/go/mysql/replication/mysql56_gtid_set.go:248 +0xc8 fp=0x1400481af00 sp=0x1400481ae50 pc=0x100bfd1f8
vitess.io/vitess/go/mysql/replication.Position.AtLeast(...)
	vitess.io/vitess/go/mysql/replication/replication_position.go:83
vitess.io/vitess/go/vt/vttablet/tabletserver/schema.(*Engine).ReloadAtEx(0x140006b3e10, {0x1032c7df8, 0x1053a98e0}, {{}, {0x1032e6b40?, 0x14002e9e750?}}, 0x0)
	vitess.io/vitess/go/vt/vttablet/tabletserver/schema/engine.go:405 +0x148 fp=0x1400481af90 sp=0x1400481af00 pc=0x1012d6ec8
vitess.io/vitess/go/vt/vttablet/tabletserver/schema.(*Engine).ReloadAt(...)
	vitess.io/vitess/go/vt/vttablet/tabletserver/schema/engine.go:389
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*vstreamer).parseEvent(0x14001dd8360, {0x1032fb590, 0x140020783a0}, 0x14004fa3630)
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/vstreamer.go:572 +0xbcc fp=0x1400481b330 sp=0x1400481af90 pc=0x1014b2f3c
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*vstreamer).parseEvents(0x14001dd8360, {0x1032c7f70, 0x140018759a0}, 0x14002088e00, 0x14002088e70)
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/vstreamer.go:389 +0x5a8 fp=0x1400481b660 sp=0x1400481b330 pc=0x1014b1168
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*vstreamer).replicate(0x14001dd8360, {0x33?, 0x140018726d0?})
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/vstreamer.go:214 +0xdc fp=0x1400481b6e0 sp=0x1400481b660 pc=0x1014b0aac
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*vstreamer).Stream(0x14001dd8360)
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/vstreamer.go:193 +0x160 fp=0x1400481b780 sp=0x1400481b6e0 pc=0x1014b0840
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*uvstreamer).Stream(0x1400013ec80)
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/uvstreamer.go:460 +0x240 fp=0x1400481b850 sp=0x1400481b780 pc=0x1014af2f0
vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer.(*Engine).Stream(0x14000548580, {0x1032c7f38, 0x14002455170}, {0x14002d0b000, 0x33}, {0x0, 0x0, 0x0}, 0x14001c13560, {0x1021a330d, ...}, ...)
	vitess.io/vitess/go/vt/vttablet/tabletserver/vstreamer/engine.go:280 +0x114 fp=0x1400481b910 sp=0x1400481b850 pc=0x10149d8c4
vitess.io/vitess/go/vt/vttablet/tabletserver.(*TabletServer).VStream(0x14000044870, {0x1032c7f38, 0x14002455170}, 0x14001b28300, 0x14002b13638)
	vitess.io/vitess/go/vt/vttablet/tabletserver/tabletserver.go:1285 +0x80 fp=0x1400481b990 sp=0x1400481b910 pc=0x10155f450
vitess.io/vitess/go/vt/vttablet/grpcqueryservice.(*query).VStream(0x14000d05ba0, 0x14001b28300, {0x1032debf0, 0x14001504f60})
	vitess.io/vitess/go/vt/vttablet/grpcqueryservice/server.go:348 +0x100 fp=0x1400481ba30 sp=0x1400481b990 pc=0x101d49620

This is the reading code. The writing code must be AppendGTIDInPlace().
I'm not entire sure yet how come there's concurrency specifically in ReloadEx, since parseEvent runs Reload() and AppendGTIDInPlace() sequentially. In this PR I will try to resolve the matter.

Related Issue(s)

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.52%. Comparing base (39d6c69) to head (24e2a2e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18610   +/-   ##
=======================================
  Coverage   67.51%   67.52%           
=======================================
  Files        1607     1607           
  Lines      263328   263328           
=======================================
+ Hits       177796   177822   +26     
+ Misses      85532    85506   -26     

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