Debugging Go Code with GDB

by Shintaro Kaneko

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Debugging Go Code with GDB kaneshin Go Conference 2015 summer

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Shintaro Kaneko (kaneshin) - Senior Engineer @ eureka kaneshin kaneshinth shintaro.kaneko

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What is eureka?

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How do you Debug Go code?

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Print Debugging?

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func main() { v := map[string]string{"key": "value"} fmt.Println("DEBUG", v) for _, d := range v { .. } }

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func main() { v := map[string]string{"key": "value"} fmt.Println("DEBUG", v) for _, d := range v { .. } }

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The Disadvantages of Print Debugging fmt.Print

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Ad-hoc Code is temporarily added

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func main() { v := map[string]string{"key": "value"} for _, d := range v { .. } } If problems occur

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func main() { v := map[string]string{"key": "value"} fmt.Println("DEBUG", v) for _, d := range v { .. } fmt.Println("DEBUG", v) } Add code temporarily

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func main() { v := map[string]string{"key": "value"} // fmt.Println("DEBUG", v) for _, d := range v { .. } // fmt.Println("DEBUG", v) } Disable/Remove code as soon as solved

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Lost buffer in case of a crash

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r, w, _ := os.Pipe() os.Stdout = w oc := make(chan string) go func() { var buf bytes.Buffer io.Copy(&buf, r) oc <-‐ buf.String() }() Pipe STDOUT

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r, w, _ := os.Pipe() os.Stdout = w oc := make(chan string) go func() { var buf bytes.Buffer io.Copy(&buf, r) oc <-‐ buf.String() }() In case of io.Copy crash

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r, w, _ := os.Pipe() os.Stdout = w oc := make(chan string) go func() { var buf bytes.Buffer io.Copy(&buf, r) oc <-‐ buf.String() }() Maybe you add print code.. fmt.Println(buf) fmt.Println(buf)

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Can’t interrupt processing program

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func main() { v := map[string]string{"key": "value", ..} for _, d := range v { .. .. } } If problems occur

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func main() { v := map[string]string{"key": "value", ..} for _, d := range v { .. fmt.Println("DEBUG", d) .. } } Add code temporarily

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DEBUG value DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. DEBUG .. The Stream of the log..

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No more!

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Do you debug with GDB, don’t you?

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The Advantages of GDB over Print Debugging

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Printable Variables directly Doesn’t need to add temp code

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Interruptible/Attachable processing program

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Remote debugging

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How to execute GDB?

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How to execute GDB? ‣ It’s very easy to debug code ‣ See "Debugging Go Code with GDB" ‣ https://golang.org/doc/gdb ‣ GDB debugger ver 7.1+ ‣ To inspect a live process or a core dump from DWARF ‣ Go ver 1.4+ ‣ Go v1.2~1.3.3 are not working (DWARF has been missing)

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How to execute GDB? ‣ Compile Go Code to contain useful debugging information ‣ Pass `-gcflags "-N -l"` to disable optimizations of compile ‣ For example: `go build -gcflags "-N -l" /path/to/repo` ‣ Pass `-ldflags "-w"` to omit the debug information ‣ For example: `go build -ldflags "-w" /path/to/repo`

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$ go build -‐o fib github.com/kaneshin/go-‐fibonacci $ go build -‐gcflags "-‐N -‐l" -‐o fib-‐none \ github.com/kaneshin/go-‐fibonacci $ go build -‐ldflags "-‐w" -‐o fib-‐omit \ github.com/kaneshin/go-‐fibonacci $ ls -‐al | grep fib -‐rwxr-‐xr-‐x 1 root root 6452704 Jun 20 11:19 fib* -‐rwxr-‐xr-‐x 1 root root 6452744 Jun 20 11:24 fib-‐none* -‐rwxr-‐xr-‐x 1 root root 5128473 Jun 20 11:19 fib-‐omit*

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$ gdb main.go to execute GDB

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I’m not talking about How to USE gdb I will if there is a next time

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͜͜·Ͱத൫ ͪΐͬͱ͚ͩ೔ຊޠ͕ग़͖ͯ·͢

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Struggled with GDB

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1. Broken DWARF 2. An old GDB is in my GDB 3. Runtime Bug (runtime.py) 4. Web Application Framework: Revel

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1. Broken DWARF

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Broken DWARF ‣ Go 1.4+͸मਖ਼ࡁΈ ‣ Go 1.3.x Ͱ͸ਖ਼͍͠஋͕දࣔ͞Εͳ͍

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Broken DWARF ‣ Go 1.4+͸मਖ਼ࡁΈ ‣ Go 1.3.x Ͱ͸ਖ਼͍͠஋͕දࣔ͞Εͳ͍ num := r.URL.Query().Get("num") n, err := strconv.Atoi(num) > if err != nil { fmt.Fprintln(os.Stderr, err)

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Broken DWARF ‣ Go 1.4+͸मਖ਼ࡁΈ ‣ Go 1.3.x Ͱ͸ਖ਼͍͠஋͕දࣔ͞Εͳ͍ (gdb) info locals val = 833358128335 n = 3 num = 0xc20803e4d3 "1000" err = {tab = 0xc20802e170, data = 0xc20802e160} (gdb) num := r.URL.Query().Get("num") n, err := strconv.Atoi(num) > if err != nil { fmt.Fprintln(os.Stderr, err)

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2. An old GDB is in my MBA

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2. An old GDB is in my MBA ‣ Actually it was my fault. ‣ Be careful if you will install GDB

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2. An old GDB is in my MBA ‣ Actually it was my fault. ‣ Be careful if you will install GDB How to Install GDB on OS X (Needs to sign your certiﬁcation) $ brew install homebrew/dupes/gdb

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2. An old GDB is in my MBA ‣ Actually it was my fault. ‣ Be careful if you will install GDB How to Install GDB on OS X (Needs to sign your certiﬁcation) $ brew install homebrew/dupes/gdb $ gdb -‐v

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2. An old GDB is in my MBA $ which gdb /usr/bin/gdb

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2. An old GDB is in my MBA $ which gdb /usr/bin/gdb $ `brew -‐-‐prefix gdb`/bin/gdb -‐v GNU gdb (GDB) 7.9.1

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3. Runtime Bug

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3. Runtime Bug ‣ See "gdb: No struct type named runtime.rtype #9326" ‣ https://github.com/golang/go/issues/9326 gdb main.go -‐d $GOROOT ... (gdb) source /usr/local/go/src/runtime/runtime-‐gdb.py Loading Go Runtime support. Traceback (most recent call last): File "/usr/local/go/src/runtime/runtime-‐gdb.py", line 205, in _rctp_type = gdb.lookup_type("struct runtime.rtype").pointer() gdb.error: No struct type named runtime.rtype.

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3. Runtime Bug ‣ See "gdb: No struct type named runtime.rtype #9326" ‣ https://github.com/golang/go/issues/9326 gdb main.go -‐d $GOROOT ... (gdb) source /usr/local/go/src/runtime/runtime-‐gdb.py Loading Go Runtime support. Traceback (most recent call last): File "/usr/local/go/src/runtime/runtime-‐gdb.py", line 205, in _rctp_type = gdb.lookup_type("struct reflect.rtype").pointer() gdb.error: No struct type named runtime.rtype.

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4. WAF: Revel

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4. WAF: Revel ‣ Revel is kind of Web Application Framework ‣ See "How to debug app with GDB?" #921 ‣ https://github.com/revel/revel/issues/921

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GDBͱͷ૬ੑѱ͘ͳ͍Ͱ͔͢ʁ

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GDBͱͷ૬ੑѱ͘ͳ͍Ͱ͔͢ʁ ͦ΋ͦ΋ɺGoͷGDBαϙʔτ͕ऑ͍͔ΒͶ

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GDBͱͷ૬ੑѱ͘ͳ͍Ͱ͔͢ʁ ͦ΋ͦ΋ɺGoͷGDBαϙʔτ͕ऑ͍͔ΒͶ revelʹ͸ revel.DEBUG.Printf ͱ͍͏ૉ੖Β͍͠ Printؔ਺͕͋Δ͔Β

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Printσόοάͯ͠Ͷˑ

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OK, RevelͰGDBఘΊΔΘ

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·ͱΊ ‣ Print Debugging͸΍Ίͨ΄͏͕͍͍͚Ͳ… ‣ GDBͱGo͸·ͩൃల్্ ‣ ਖ਼௚Ϗϧυ͕଎͍ ‣ Web Application ͱ૬ੑ͕ѱ͍

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·ͱΊ ‣ Print Debugging͸΍Ίͨ΄͏͕͍͍͚Ͳ… ‣ GDBͱGo͸·ͩൃల్্ ‣ ਖ਼௚Ϗϧυ͕଎͍ ‣ Web Application ͱ૬ੑ͕ѱ͍ ‣ RevelΛ૬౰͍͡Γ·ͨ͠ɻ͕ɺRevelͷ։ൃ͕ਐΉͷ͕ਏ͍

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·ͱΊ ‣ Print Debugging͸΍Ίͨ΄͏͕͍͍͚Ͳ… ‣ GDBͱGo͸·ͩൃల్্ ‣ ਖ਼௚Ϗϧυ͕଎͍ ‣ Web Application ͱ૬ੑ͕ѱ͍ ‣ RevelΛ૬౰͍͡Γ·ͨ͠ɻ͕ɺRevelͷ։ൃ͕ਐΉͷ͕ਏ͍ ‣ GDB࢖͏ʹͯ͠΋೜଱ڧ͘ͳ͍ͱਏ͍