no panic on closed connection reuse

[arnehormann]

Closes #142

I'm not sure if this is good to go yet. The test should probably be expanded, it doesn't test all cases I fixed.
Let's take it as a basis for a discussion.

I didn't see a central place to add the checks and I'm not sure if I missed some relevant ones.

How this was created:
I located the places to fix by searching for mysql(Stmt|Conn|Tx|Rows)\) [A-Z] in all go files. Those are the only structs containing (or beeing) mysqlConn, the exported functions as entry points should suffice for the check and make sure it's not "overchecked".

I changed the mc.netConn checks to mc.buf in existing code to keep the checked condition the same everywhere. I did not check the disassembled object code for this being faster than comparing mc.netConn == nil or writing a small func which is or is not inlined.

[julienschmidt]

http://play.golang.org/p/tGf3r4UtFS

BenchmarkPerfCheckBuf   2000000000               0.59 ns/op
BenchmarkPerfCheckConn  2000000000               0.59 ns/op
BenchmarkPerfCheckCfg   2000000000               0.59 ns/op

--- prog list "BenchmarkPerfCheckBuf" ---
0000 (perf_test.go:19) TEXT    BenchmarkPerfCheckBuf+0(SB),$0-8
0001 (perf_test.go:19) MOVQ    b+0(FP),SI
0002 (perf_test.go:19) MOVQ    someConn+0(SB),DX
0003 (perf_test.go:19) FUNCDATA $0,gcargs┬À0+0(SB)
0004 (perf_test.go:19) FUNCDATA $1,gclocals┬À0+0(SB)
0005 (perf_test.go:19) TYPE    b+0(FP){*testing.B},$8
0006 (perf_test.go:20) MOVQ    $0,AX
0007 (perf_test.go:20) JMP     ,9
0008 (perf_test.go:20) INCQ    ,AX
0009 (perf_test.go:20) NOP     ,
0010 (perf_test.go:20) MOVQ    112(SI),BX
0011 (perf_test.go:20) CMPQ    BX,AX
0012 (perf_test.go:20) JLE     $0,20
0013 (perf_test.go:21) NOP     ,
0014 (perf_test.go:21) MOVQ    32(DX),BX
0015 (perf_test.go:21) MOVQ    $0,BP
0016 (perf_test.go:21) CMPQ    BX,BP
0017 (perf_test.go:21) JEQ     ,8
0018 (perf_test.go:22) MOVQ    $1,CX
0019 (perf_test.go:21) JMP     ,8
0020 (perf_test.go:25) RET     ,

--- prog list "BenchmarkPerfCheckConn" ---
0021 (perf_test.go:27) TEXT    BenchmarkPerfCheckConn+0(SB),$0-8
0022 (perf_test.go:27) MOVQ    b+0(FP),SI
0023 (perf_test.go:27) MOVQ    someConn+0(SB),DX
0024 (perf_test.go:27) FUNCDATA $0,gcargs┬À1+0(SB)
0025 (perf_test.go:27) FUNCDATA $1,gclocals┬À1+0(SB)
0026 (perf_test.go:27) TYPE    b+0(FP){*testing.B},$8
0027 (perf_test.go:28) MOVQ    $0,AX
0028 (perf_test.go:28) JMP     ,30
0029 (perf_test.go:28) INCQ    ,AX
0030 (perf_test.go:28) NOP     ,
0031 (perf_test.go:28) MOVQ    112(SI),BX
0032 (perf_test.go:28) CMPQ    BX,AX
0033 (perf_test.go:28) JLE     $0,39
0034 (perf_test.go:29) NOP     ,
0035 (perf_test.go:29) CMPQ    16(DX),$0
0036 (perf_test.go:29) JEQ     ,29
0037 (perf_test.go:30) MOVQ    $1,CX
0038 (perf_test.go:29) JMP     ,29
0039 (perf_test.go:33) RET     ,

--- prog list "BenchmarkPerfCheckCfg" ---
0040 (perf_test.go:35) TEXT    BenchmarkPerfCheckCfg+0(SB),$0-8
0041 (perf_test.go:35) MOVQ    b+0(FP),SI
0042 (perf_test.go:35) MOVQ    someConn+0(SB),DX
0043 (perf_test.go:35) FUNCDATA $0,gcargs┬À2+0(SB)
0044 (perf_test.go:35) FUNCDATA $1,gclocals┬À2+0(SB)
0045 (perf_test.go:35) TYPE    b+0(FP){*testing.B},$8
0046 (perf_test.go:36) MOVQ    $0,AX
0047 (perf_test.go:36) JMP     ,49
0048 (perf_test.go:36) INCQ    ,AX
0049 (perf_test.go:36) NOP     ,
0050 (perf_test.go:36) MOVQ    112(SI),BX
0051 (perf_test.go:36) CMPQ    BX,AX
0052 (perf_test.go:36) JLE     $0,60
0053 (perf_test.go:37) NOP     ,
0054 (perf_test.go:37) MOVQ    (DX),BX
0055 (perf_test.go:37) MOVQ    $0,BP
0056 (perf_test.go:37) CMPQ    BX,BP
0057 (perf_test.go:37) JEQ     ,48
0058 (perf_test.go:38) MOVQ    $1,CX
0059 (perf_test.go:37) JMP     ,48
0060 (perf_test.go:41) RET     ,

[julienschmidt]

TestReuseClosedConnection

[arnehormann]

I won't merge until you LGTM it again because I don't handle err == nil separately.
PTAL

[julienschmidt]

LGTM