Fix potential deadlock in rawdb.(*freezer).Ancient

[jotto]

This appears to fix an issue where geth appears to lock up while running random transactions through the EVM, specifically transactions that make use of the blockhash opcode (typically as a source of randomness).

My limited understanding is:

• run ethapi.DoCall with tx with blockhash opcode
• meanwhile, something else is holding a lock on something in the freezer (insert chain?)

[jotto]

Disregard, this fix (lock on Ancient lookups) worked longer than my previous attempts, but ultimately failed. Here's what the goroutine in question is doing:

syscall.Syscall6(0x11, 0x10, 0xc084e3878a, 0x6, 0x3d96c98, 0x0, 0x0, 0x6, 0x6, 0x0)
        syscall/asm_linux_amd64.s:43 +0x5
syscall.Pread(0x10, 0xc084e3878a, 0x6, 0x6, 0x3d96c98, 0x6, 0x0, 0x0)
        syscall/zsyscall_linux_amd64.go:1211 +0x72
internal/poll.(*FD).Pread(0xc0001ce720, 0xc084e3878a, 0x6, 0x6, 0x3d96c98, 0x6, 0x0, 0x0)
        internal/poll/fd_unix.go:192 +0xdf
os.(*File).pread(...)
        os/file_posix.go:40
os.(*File).ReadAt(0xc0002945e8, 0xc084e3878a, 0x6, 0x6, 0x3d96c98, 0x6, 0x0, 0x0)
        os/file.go:135 +0x114
github.com/ethereum/go-ethereum/core/rawdb.(*freezerTable).getBounds(0xc00016c2c0, 0xa43cc4, 0x10, 0x119c220, 0xc07ea55620, 0x0)
        github.com/ethereum/go-ethereum/core/rawdb/freezer_table.go:536 +0x105
github.com/ethereum/go-ethereum/core/rawdb.(*freezerTable).Retrieve(0xc00016c2c0, 0xa43cc4, 0x12e1610, 0x7, 0xc000566b18, 0x0, 0x0)
        github.com/ethereum/go-ethereum/core/rawdb/freezer_table.go:575 +0x175
github.com/ethereum/go-ethereum/core/rawdb.(*freezer).Ancient(0xc0001ce5a0, 0x12e1610, 0x7, 0xa43cc4, 0x0, 0x0, 0x0, 0x0, 0x0)
        github.com/ethereum/go-ethereum/core/rawdb/freezer.go:172 +0x125
github.com/ethereum/go-ethereum/core/rawdb.ReadHeaderRLP(0x7f193c0e5e98, 0xc00000e2d0, 0x968a634452a09b79, 0x5ab761ab0cd8f0cd, 0xde041bbec8af6056, 0xbce91dcbf59cba79, 0xa43cc4, 0xc084e38ac8, 0x7cb41d, 0x11577e0)
        github.com/ethereum/go-ethereum/core/rawdb/accessors_chain.go:282 +0xe6
github.com/ethereum/go-ethereum/core/rawdb.ReadHeader(0x7f193c0e5e98, 0xc00000e2d0, 0x968a634452a09b79, 0x5ab761ab0cd8f0cd, 0xde041bbec8af6056, 0xbce91dcbf59cba79, 0xa43cc4, 0xc026a38b40)
        github.com/ethereum/go-ethereum/core/rawdb/accessors_chain.go:311 +0x74
github.com/ethereum/go-ethereum/core.(*HeaderChain).GetHeader(0xc000138090, 0x968a634452a09b79, 0x5ab761ab0cd8f0cd, 0xde041bbec8af6056, 0xbce91dcbf59cba79, 0xa43cc4, 0xc026a38b40)
        github.com/ethereum/go-ethereum/core/headerchain.go:433 +0x125
github.com/ethereum/go-ethereum/core.(*BlockChain).GetHeader(0xc000147900, 0x968a634452a09b79, 0x5ab761ab0cd8f0cd, 0xde041bbec8af6056, 0xbce91dcbf59cba79, 0xa43cc4, 0xc026a38b40)
        github.com/ethereum/go-ethereum/core/blockchain.go:2466 +0x4d
github.com/ethereum/go-ethereum/core.GetHashFn.func1(0xb66c9b, 0x0, 0x0, 0x0, 0x0)
        github.com/ethereum/go-ethereum/core/evm.go:80 +0x177
github.com/ethereum/go-ethereum/core/vm.opBlockhash(0xc079043b18, 0xc068064000, 0xc082125600, 0x0, 0x0, 0x0, 0x0, 0x0)
        github.com/ethereum/go-ethereum/core/vm/instructions.go:448 +0x10f
github.com/ethereum/go-ethereum/core/vm.(*EVMInterpreter).Run(0xc068064000, 0xc052632b40, 0xc0613ab220, 0x44, 0x44, 0xc091e3ed00, 0x0, 0x0, 0x0, 0x0, ...)
        github.com/ethereum/go-ethereum/core/vm/interpreter.go:287 +0x594
github.com/ethereum/go-ethereum/core/vm.run(0xc066cfd500, 0xc052632b40, 0xc0613ab220, 0x44, 0x44, 0xa83b419d7ee39100, 0x0, 0x0, 0x0, 0x0, ...)
        github.com/ethereum/go-ethereum/core/vm/evm.go:88 +0x224
github.com/ethereum/go-ethereum/core/vm.(*EVM).Call(0xc066cfd500, 0x14e9c80, 0xc090d83b78, 0xcf8b3fe8eeeeced1, 0x15b6027243aded3b, 0x5549f4e, 0xc0613ab220, 0x44, 0x44, 0xa7a0, ...)
        github.com/ethereum/go-ethereum/core/vm/evm.go:260 +0x6cb
github.com/ethereum/go-ethereum/core.(*StateTransition).TransitionDb(0xc06396fc70, 0x150c5b0, 0xc066499860, 0xc079043ae8)
        github.com/ethereum/go-ethereum/core/state_transition.go:260 +0x3e9
github.com/ethereum/go-ethereum/core.ApplyMessage(0xc066cfd500, 0x150c5b0, 0xc066499860, 0xc079043ae8, 0xc066cfd500, 0xc08b7dc300, 0xc066cc0d80)
        github.com/ethereum/go-ethereum/core/state_transition.go:164 +0x57

[jotto]

This is looking less like a deadlock, and more like blockhash opcode simply causing slow freezer lookups.