Did you get any deadlock or race conditions in practice? This structure is used in the context of one connection and processed only by two goroutines working sequentially. Due to sequential processing and no shared data between several connections, we don't use extra locks. We have next flow:
[new connection] -> [new protocol & connection related structs] -> [2 new gorotuines: client & db connection processing] -> [end of processing in 2 goroutines]

Due to database protocols being synchronous and sequential (except extra cases of notifications that out of scope of processing by Acra), at first we send request, and then we get the response. So, two goroutines that handle client & db connections work sequentially and wouldn't do anything simultaneously due to protocols that don't support it.

So, we don't need here any mutes that will only slow down processing

I don't know that the "list" is thread safe by design, but in my test, i got this panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc6da19]

goroutine 22 [running]:
container/list.(*List).insert(...)
        /go/src/container/list/list.go:96
container/list.(*List).insertValue(...)
        /go/src/container/list/list.go:104
container/list.(*List).PushBack(...)
        /go/src/container/list/list.go:152
github.com/cossacklabs/acra/decryptor/postgresql.(*pendingPacketsList).Add(0xc0000142b0, {0x10fd560?, 0xc0007c45d0?})
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pending_packets.go:53 +0x359
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).handleClientPacket(0xc00020a000, {0x1337408?, 0xc000560e40?}, 0x117d9aa?, 0x6?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:360 +0x605
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyClientConnection(0xc00020a000, {0x1337408?, 0xc0001fedb0?}, 0xc00004c120?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:299 +0xab9
created by github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyDatabaseConnection
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:780 +0xe7e
exit status 2

Let's say your design is like that, maybe there is another issue in the implementation. Because i don't know the design, i just put mutex there

I don't know that the "list" is thread safe by design, but in my test, i got this panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc6da19]

goroutine 22 [running]:
container/list.(*List).insert(...)
        /go/src/container/list/list.go:96
container/list.(*List).insertValue(...)
        /go/src/container/list/list.go:104
container/list.(*List).PushBack(...)
        /go/src/container/list/list.go:152
github.com/cossacklabs/acra/decryptor/postgresql.(*pendingPacketsList).Add(0xc0000142b0, {0x10fd560?, 0xc0007c45d0?})
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pending_packets.go:53 +0x359
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).handleClientPacket(0xc00020a000, {0x1337408?, 0xc000560e40?}, 0x117d9aa?, 0x6?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:360 +0x605
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyClientConnection(0xc00020a000, {0x1337408?, 0xc0001fedb0?}, 0xc00004c120?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:299 +0xab9
created by github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyDatabaseConnection
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:780 +0xe7e
exit status 2

thanks for the traceback. can you tell which tool or language + library you used for PostgreSQL?

I don't know that the "list" is thread safe by design, but in my test, i got this panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc6da19]

goroutine 22 [running]:
container/list.(*List).insert(...)
        /go/src/container/list/list.go:96
container/list.(*List).insertValue(...)
        /go/src/container/list/list.go:104
container/list.(*List).PushBack(...)
        /go/src/container/list/list.go:152
github.com/cossacklabs/acra/decryptor/postgresql.(*pendingPacketsList).Add(0xc0000142b0, {0x10fd560?, 0xc0007c45d0?})
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pending_packets.go:53 +0x359
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).handleClientPacket(0xc00020a000, {0x1337408?, 0xc000560e40?}, 0x117d9aa?, 0x6?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:360 +0x605
github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyClientConnection(0xc00020a000, {0x1337408?, 0xc0001fedb0?}, 0xc00004c120?)
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:299 +0xab9
created by github.com/cossacklabs/acra/decryptor/postgresql.(*PgProxy).ProxyDatabaseConnection
        /go/src/github.com/cossacklabs/acra/decryptor/postgresql/pg_decryptor.go:780 +0xe7e
exit status 2

thanks for the traceback. can you tell which tool or language + library you used for PostgreSQL?

I use this library npgsql