failpoint

An implementation of failpoints for Golang.

Design principles

• Define failpoint in valid Golang code, not comments or anything else

• Failpoint does not have any extra cost

  • Will not take effect on regular logic
  • Will not cause regular code performance regression
  • Failpoint code will not appear in the final binary

• Failpoint routine is writable/readable and should be checked by a compiler

• Generated code by failpoint definition is easy to read

• Keep the line numbers same with the injecting codes(easier to debug)

• Support parallel tests with context.Context

Key concepts

• Failpoint

  Faillpoint is a code snippet that is only executed when the corresponding failpoint is active. The closure will never be executed if failpoint.Disable("failpoint-name-for-demo") is executed.

  var outerVar = "declare in outer scope"
  failpoint.Inject("failpoint-name-for-demo", func(val failpoint.Value) {
      fmt.Println("unit-test", val, outerVar)
  })

• Marker functions

  • It is just an empty function

    • To hint the rewriter to rewrite with an equality statement
    • To receive some parameters as the rewrite rule
    • It will be inline in the compiling time and emit nothing to binary (zero cost)
    • The variables in external scope can be accessed in closure by capturing, and the converted code is still legal because all the captured-variables location in outer scope of IF statement.

  • It is easy to write/read

  • Introduce a compiler check for failpoints which cannot compile in the regular mode if failpoint code is invalid

• Marker funtion list

  • func Inject(fpname string, fpblock func(val Value)) {}
  • func InjectContext(fpname string, ctx context.Context, fpblock func(val Value)) {}
  • func Break(label ...string) {}
  • func Goto(label string) {}
  • func Continue(label ...string) {}
  • func Fallthrough() {}
  • func Label(label string) {}

How to inject a failpoint to your program

• You can call failpoint.Inject to inject a failpoint to the call site, where failpoint-name is used to trigger the failpoint and failpoint-closure will be expanded as the body of the IF statement.

  failpoint.Inject("failpoint-name", func(val failpoint.Value) {
      fmt.Println("unit-test", val)
  })

  The converted code looks like:

  if ok, val := failpoint.Eval(_curpkg_("failpoint-name")); ok {
      fmt.Println("unit-test", val)
  }

• failpoint.Value is the value that passes by failpoint.Enable("failpoint-name", "return(5)") which can be ignored.

  failpoint.Inject("failpoint-name", func(_ failpoint.Value) {
      fmt.Println("unit-test")
  })

  OR

  failpoint.Inject("failpoint-name", func() {
      fmt.Println("unit-test")
  })

  And the converted code looks like:

  if ok, _ := failpoint.Eval(_curpkg_("failpoint-name")); ok {
      fmt.Println("unit-test")
  }

• Also, the failpoint closure can be a function which takes context.Context. You can do some customized things with context.Context like controlling whether a failpoint is active in parallel tests or other cases. For example,

  failpoint.InjectContext(ctx, "failpoint-name", func(val failpoint.Value) {
      fmt.Println("unit-test", val)
  })

  The converted code looks like:

  if ok, val := failpoint.EvalContext(ctx, _curpkg_("failpoint-name")); ok {
      fmt.Println("unit-test", val)
  }

• You can ignore context.Context, and this will generate the same code as above non-context version. For example,

  failpoint.InjectContext(nil, "failpoint-name", func(val failpoint.Value) {
      fmt.Println("unit-test", val)
  })

  Becomes

  if ok, val := failpoint.EvalContext(nil, _curpkg_("failpoint-name")); ok {
      fmt.Println("unit-test", val)
  }

• You can control a failpoint by failpoint.WithHook

  func (s *dmlSuite) TestCRUDParallel() {
      sctx := failpoint.WithHook(context.Backgroud(), func(ctx context.Context, fpname string) bool {
          return ctx.Value(fpname) != nil // Determine by ctx key
      })
      insertFailpoints = map[string]struct{} {
          "insert-record-fp": {},
          "insert-index-fp": {},
          "on-duplicate-fp": {},
      }
      ictx := failpoint.WithHook(context.Backgroud(), func(ctx context.Context, fpname string) bool {
          _, found := insertFailpoints[fpname] // Only enables some failpoints.
          return found
      })
      deleteFailpoints = map[string]struct{} {
          "tikv-is-busy-fp": {},
          "fetch-tso-timeout": {},
      }
      dctx := failpoint.WithHook(context.Backgroud(), func(ctx context.Context, fpname string) bool {
          _, found := deleteFailpoints[fpname] // Only disables failpoints. 
          return !found
      })
      // other DML parallel test cases.
      s.RunParallel(buildSelectTests(sctx))
      s.RunParallel(buildInsertTests(ictx))
      s.RunParallel(buildDeleteTests(dctx))
  }

• If you use a failpoint in the loop context, maybe you will use other marker functions.

  failpoint.Label("outer")
  for i := 0; i < 100; i++ {
      inner:
          for j := 0; j < 1000; j++ {
              switch rand.Intn(j) + i {
              case j / 5:
                  failpoint.Break()
              case j / 7:
                  failpoint.Continue("outer")
              case j / 9:
                  failpoint.Fallthrough()
              case j / 10:
                  failpoint.Goto("outer")
              default:
                  failpoint.Inject("failpoint-name", func(val failpoint.Value) {
                      fmt.Println("unit-test", val.(int))
                      if val == j/11 {
                          failpoint.Break("inner")
                      } else {
                          failpoint.Goto("outer")
                      }
                  })
          }
      }
  }

  The above code block will generate the following code:

  outer:
      for i := 0; i < 100; i++ {
      inner:
          for j := 0; j < 1000; j++ {
              switch rand.Intn(j) + i {
              case j / 5:
                  break
              case j / 7:
                  continue outer
              case j / 9:
                  fallthrough
              case j / 10:
                  goto outer
              default:
                  if ok, val := failpoint.Eval(_curpkg_("failpoint-name")); ok {
                      fmt.Println("unit-test", val.(int))
                      if val == j/11 {
                          break inner
                      } else {
                          goto outer
                      }
                  }
              }
          }
      }

• You may doubt why we do not use label, break, continue, and fallthrough directly instead of using failpoint marker functions.

  • Any unused symbol like an ident or a label is not permitted in Golang. It will be invalid if some label is only used in the failpoint closure. For example,

    label1: // compiler error: unused label1
        failpoint.Inject("failpoint-name", func(val failpoint.Value) {
            if val.(int) == 1000 {
                goto label1 // illegal to use goto here
            }
            fmt.Println("unit-test", val)
        })

  • break and continue can only be used in the loop context, which is not legal in the Golang code if we use them in closure directly.

Some complicated failpoints demo

• Inject a failpoint to the IF INITIAL statement or CONDITIONAL expression

  if a, b := func() {
      failpoint.Inject("failpoint-name", func(val failpoint.Value) {
          fmt.Println("unit-test", val)
      })
  }, func() int { return rand.Intn(200) }(); b > func() int {
      failpoint.Inject("failpoint-name", func(val failpoint.Value) int {
          return val.(int)
      })
      return rand.Intn(3000)
  }() && b < func() int {
      failpoint.Inject("failpoint-name-2", func(val failpoint.Value) {
          return rand.Intn(val.(int))
      })
      return rand.Intn(6000)
  }() {
      a()
      failpoint.Inject("failpoint-name-3", func(val failpoint.Value) {
          fmt.Println("unit-test", val)
      })
  }

  The above code block will generate something like this:

  if a, b := func() {
      if ok, val := failpoint.Eval(_curpkg_("failpoint-name")); ok {
          fmt.Println("unit-test", val)
      }
  }, func() int { return rand.Intn(200) }(); b > func() int {
      if ok, val := failpoint.Eval(_curpkg_("failpoint-name")); ok {
          return val.(int)
      }
      return rand.Intn(3000)
  }() && b < func() int {
      if ok, val := failpoint.Eval(_curpkg_("failpoint-name-2")); ok {
          return rand.Intn(val.(int))
      }
      return rand.Intn(6000)
  }() {
      a()
      if ok, val := failpoint.Eval(_curpkg_("failpoint-name-3")); ok {
          fmt.Println("unit-test", val)
      }
  }

• Inject a failpoint to the SELECT statement to make it block one CASE if the failpoint is active

  func (s *StoreService) ExecuteStoreTask() {
      select {
      case <-func() chan *StoreTask {
          failpoint.Inject("priority-fp", func(_ failpoint.Value) {
              return make(chan *StoreTask)
          })
          return s.priorityHighCh
      }():
          fmt.Println("execute high priority task")
  
      case <- s.priorityNormalCh:
          fmt.Println("execute normal priority task")
  
      case <- s.priorityLowCh:
          fmt.Println("execute normal low task")
      }
  }

  The above code block will generate something like this:

  func (s *StoreService) ExecuteStoreTask() {
      select {
      case <-func() chan *StoreTask {
          if ok, _ := failpoint.Eval(_curpkg_("priority-fp")); ok {
              return make(chan *StoreTask)
          })
          return s.priorityHighCh
      }():
          fmt.Println("execute high priority task")
  
      case <- s.priorityNormalCh:
          fmt.Println("execute normal priority task")
  
      case <- s.priorityLowCh:
          fmt.Println("execute normal low task")
      }
  }

• Inject a failpoint to dynamically extend SWITCH CASE arms

  switch opType := operator.Type(); {
  case opType == "balance-leader":
      fmt.Println("create balance leader steps")
  
  case opType == "balance-region":
      fmt.Println("create balance region steps")
  
  case opType == "scatter-region":
      fmt.Println("create scatter region steps")
  
  case func() bool {
      failpoint.Inject("dynamic-op-type", func(val failpoint.Value) bool {
          return strings.Contains(val.(string), opType)
      })
      return false
  }():
      fmt.Println("do something")
  
  default:
      panic("unsupported operator type")
  }

  The above code block will generate something like this:

  switch opType := operator.Type(); {
  case opType == "balance-leader":
      fmt.Println("create balance leader steps")
  
  case opType == "balance-region":
      fmt.Println("create balance region steps")
  
  case opType == "scatter-region":
      fmt.Println("create scatter region steps")
  
  case func() bool {
      if ok, val := failpoint.Eval(_curpkg_("dynamic-op-type")); ok {
          return strings.Contains(val.(string), opType)
      }
      return false
  }():
      fmt.Println("do something")
  
  default:
      panic("unsupported operator type")
  }

• More complicated failpoints

  • There are more complicated failpoint sites that can be injected to
    • for the loop INITIAL statement, CONDITIONAL expression and POST statement
    • for the RANGE statement
    • SWITCH INITIAL statement
    • …
  • Anywhere you can call a function

Failpoint name best practice

As you see above, _curpkg_ will automatically wrap the original failpoint name in failpoint.Eval call. You can think of _curpkg_ as a macro that automatically prepends the current package path to the failpoint name. For example,

package ddl // which parent package is `github.com/pingcap/tidb`

func demo() {
	// _curpkg_("the-original-failpoint-name") will be expanded as `github.com/pingcap/tidb/ddl/the-original-failpoint-name`
	if ok, val := failpoint.Eval(_curpkg_("the-original-failpoint-name")); ok {...}
}

You do not need to care about _curpkg_ in your application. It is automatically generated after running failpoint-ctl enable and is deleted with failpoint-ctl disable.

Because all failpoints in a package share the same namespace, we need to be careful to avoid name conflict. There are some recommended naming rules to improve this situation.

• Keep name unique in current subpackage

• Use a self-explanatory name for the failpoint

  You can enable failpoints by environment variables

  GO_FAILPOINTS="github.com/pingcap/tidb/ddl/renameTableErr=return(100);github.com/pingcap/tidb/planner/core/illegalPushDown=return(true);github.com/pingcap/pd/server/schedulers/balanceLeaderFailed=return(true)"

Implementation details

1. Define a group of marker functions
2. Parse imports and prune a source file which does not import a failpoint
3. Traverse AST to find marker function calls
4. Marker function calls will be rewritten with an IF statement, which calls failpoint.Eval to determine whether a failpoint is active and executes failpoint code if the failpoint is enabled

Acknowledgments

• Thanks gofail to provide initial implementation.