Debugging Go Code with GDB kaneshin Go Conference 2015 summer

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

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What is eureka?

How do you Debug Go code?

Print Debugging?

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

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

Ad-hoc Code is temporarily added

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

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

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

  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

  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

  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

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

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

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

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No more!

Do you debug with GDB, don’t you?

The Advantages of GDB over Print Debugging

Printable Variables directly Doesn’t need to add temp code

Interruptible/Attachable processing program

Remote debugging

How to execute GDB?

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)

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`

$  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*

$ gdb main.go to execute GDB

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

͜͜·Ͱத൫ ͪΐͬͱ͚ͩ೔ຊޠ͕ग़͖ͯ·͢

Struggled with GDB

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

1. Broken DWARF

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

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)

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)

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)

2. An old GDB is in my MBA

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

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

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  

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      GNU  gdb  6.3.50-­‐20050815  (Apple  version  gdb-­‐1824)      (Wed  Feb    6  22:51:23  UTC  2013)  

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      GNU  gdb  6.3.50-­‐20050815  (Apple  version  gdb-­‐1824)      (Wed  Feb    6  22:51:23  UTC  2013)  

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

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  

3. Runtime Bug

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.

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.

4. WAF: Revel

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

GDBͱͷ૬ੑѱ͘ͳ͍Ͱ͔͢ʁ

GDBͱͷ૬ੑѱ͘ͳ͍Ͱ͔͢ʁ ͦ΋ͦ΋ɺGoͷGDBαϙʔτ͕ऑ͍͔ΒͶ

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

Printσόοάͯ͠Ͷˑ

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OK, RevelͰGDBఘΊΔΘ

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

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

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

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