feat: make not-found errors discoverable

Now we just need a common place to put:

func IsNotFound(e error) bool {
  for e != nil {
    if ne, ok := e.(interface { NotFound() bool }); ok {
      return ne.NotFound()
    }
    if ue, ok := e.(interface { Unwrap() error }); ok {
      e = ue.Unwrap()
    } else {
      return false
    }
  }
  return false
}

datastore.go

@@ -192,14 +192,18 @@ type TxnDatastore interface {
 
 // Errors
 
+type dsError struct {
+	error
+	isNotFound bool
+}
+
+func (e *dsError) NotFound() bool {
+	return e.isNotFound
+}
+
 // ErrNotFound is returned by Get and GetSize when a datastore does not map the
 // given key to a value.
-var ErrNotFound = errors.New("datastore: key not found")
-
-// ErrInvalidType is returned by Put when a given value is incopatible with
-// the type the datastore supports. This means a conversion (or serialization)
-// is needed beforehand.
-var ErrInvalidType = errors.New("datastore: invalid type error")
+var ErrNotFound error = &dsError{error: errors.New("datastore: key not found"), isNotFound: true}
 
 // GetBackedHas provides a default Datastore.Has implementation.
 // It exists so Datastore.Has implementations can use it, like so:

go.mod

@@ -6,5 +6,6 @@ require (
 	github.com/ipfs/go-ipfs-delay v0.0.0-20181109222059-70721b86a9a8
 	github.com/jbenet/goprocess v0.0.0-20160826012719-b497e2f366b8
 	github.com/kr/pretty v0.1.0 // indirect
+	golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7
 	gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 // indirect
 )

go.sum

@@ -11,5 +11,7 @@ github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORN
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7 h1:9zdDQZ7Thm29KFXgAX/+yaf3eVbP7djjWp/dXAppNCc=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

mount/mount.go

@@ -5,13 +5,13 @@ package mount
 import (
 	"container/heap"
 	"errors"
-	"fmt"
 	"sort"
 	"strings"
 	"sync"
 
 	ds "github.com/ipfs/go-datastore"
 	"github.com/ipfs/go-datastore/query"
+	xerrors "golang.org/x/xerrors"
 )
 
 var (
@@ -370,7 +370,7 @@ func (d *Datastore) Check() error {
 	for _, m := range d.mounts {
 		if c, ok := m.Datastore.(ds.CheckedDatastore); ok {
 			if err := c.Check(); err != nil {
-				return fmt.Errorf("checking datastore at %s: %s", m.Prefix.String(), err.Error())
+				return xerrors.Errorf("checking datastore at %s: %w", m.Prefix.String(), err)
 			}
 		}
 	}
@@ -381,7 +381,7 @@ func (d *Datastore) Scrub() error {
 	for _, m := range d.mounts {
 		if c, ok := m.Datastore.(ds.ScrubbedDatastore); ok {
 			if err := c.Scrub(); err != nil {
-				return fmt.Errorf("scrubbing datastore at %s: %s", m.Prefix.String(), err.Error())
+				return xerrors.Errorf("scrubbing datastore at %s: %w", m.Prefix.String(), err)
 			}
 		}
 	}
@@ -392,7 +392,7 @@ func (d *Datastore) CollectGarbage() error {
 	for _, m := range d.mounts {
 		if c, ok := m.Datastore.(ds.GCDatastore); ok {
 			if err := c.CollectGarbage(); err != nil {
-				return fmt.Errorf("gc on datastore at %s: %s", m.Prefix.String(), err.Error())
+				return xerrors.Errorf("gc on datastore at %s: %w", m.Prefix.String(), err)
 			}
 		}
 	}

retrystore/retrystore.go

@@ -3,10 +3,10 @@
 package retrystore
 
 import (
-	"fmt"
 	"time"
 
 	ds "github.com/ipfs/go-datastore"
+	xerrors "golang.org/x/xerrors"
 )
 
 // Datastore wraps a Batching datastore with a
@@ -23,7 +23,7 @@ type Datastore struct {
 	ds.Batching
 }
 
-var errFmtString = "ran out of retries trying to get past temporary error: %s"
+var errFmtString = "ran out of retries trying to get past temporary error: %w"
 
 func (d *Datastore) runOp(op func() error) error {
 	err := op()
@@ -40,7 +40,7 @@ func (d *Datastore) runOp(op func() error) error {
 		}
 	}
 
-	return fmt.Errorf(errFmtString, err)
+	return xerrors.Errorf(errFmtString, err)
 }
 
 // DiskUsage implements the PersistentDatastore interface.