Add SQLite support, fix un-thread-safe DB caches, small performance f… (#172)

* Add SQLite support, fix un-thread-safe DB caches, small performance fixes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add SQLite licenses to README

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* appease the linter, and fix my dumbass-ery

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* make requested changes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add back comment

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

13 files changed, 694 insertions, 0 deletions

diff --git a/vendor/golang.org/x/xerrors/LICENSE b/vendor/golang.org/x/xerrors/LICENSE
new file mode 100644
index 000000000..e4a47e17f
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2019 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/vendor/golang.org/x/xerrors/PATENTS b/vendor/golang.org/x/xerrors/PATENTS
new file mode 100644
index 000000000..733099041
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/PATENTS
@@ -0,0 +1,22 @@
+Additional IP Rights Grant (Patents)
+
+"This implementation" means the copyrightable works distributed by
+Google as part of the Go project.
+
+Google hereby grants to You a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this section)
+patent license to make, have made, use, offer to sell, sell, import,
+transfer and otherwise run, modify and propagate the contents of this
+implementation of Go, where such license applies only to those patent
+claims, both currently owned or controlled by Google and acquired in
+the future, licensable by Google that are necessarily infringed by this
+implementation of Go.  This grant does not include claims that would be
+infringed only as a consequence of further modification of this
+implementation.  If you or your agent or exclusive licensee institute or
+order or agree to the institution of patent litigation against any
+entity (including a cross-claim or counterclaim in a lawsuit) alleging
+that this implementation of Go or any code incorporated within this
+implementation of Go constitutes direct or contributory patent
+infringement, or inducement of patent infringement, then any patent
+rights granted to you under this License for this implementation of Go
+shall terminate as of the date such litigation is filed.
diff --git a/vendor/golang.org/x/xerrors/README b/vendor/golang.org/x/xerrors/README
new file mode 100644
index 000000000..aac7867a5
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/README
@@ -0,0 +1,2 @@
+This repository holds the transition packages for the new Go 1.13 error values.
+See golang.org/design/29934-error-values.
diff --git a/vendor/golang.org/x/xerrors/adaptor.go b/vendor/golang.org/x/xerrors/adaptor.go
new file mode 100644
index 000000000..4317f2483
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/adaptor.go
@@ -0,0 +1,193 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"reflect"
+	"strconv"
+)
+
+// FormatError calls the FormatError method of f with an errors.Printer
+// configured according to s and verb, and writes the result to s.
+func FormatError(f Formatter, s fmt.State, verb rune) {
+	// Assuming this function is only called from the Format method, and given
+	// that FormatError takes precedence over Format, it cannot be called from
+	// any package that supports errors.Formatter. It is therefore safe to
+	// disregard that State may be a specific printer implementation and use one
+	// of our choice instead.
+
+	// limitations: does not support printing error as Go struct.
+
+	var (
+		sep    = " " // separator before next error
+		p      = &state{State: s}
+		direct = true
+	)
+
+	var err error = f
+
+	switch verb {
+	// Note that this switch must match the preference order
+	// for ordinary string printing (%#v before %+v, and so on).
+
+	case 'v':
+		if s.Flag('#') {
+			if stringer, ok := err.(fmt.GoStringer); ok {
+				io.WriteString(&p.buf, stringer.GoString())
+				goto exit
+			}
+			// proceed as if it were %v
+		} else if s.Flag('+') {
+			p.printDetail = true
+			sep = "\n  - "
+		}
+	case 's':
+	case 'q', 'x', 'X':
+		// Use an intermediate buffer in the rare cases that precision,
+		// truncation, or one of the alternative verbs (q, x, and X) are
+		// specified.
+		direct = false
+
+	default:
+		p.buf.WriteString("%!")
+		p.buf.WriteRune(verb)
+		p.buf.WriteByte('(')
+		switch {
+		case err != nil:
+			p.buf.WriteString(reflect.TypeOf(f).String())
+		default:
+			p.buf.WriteString("<nil>")
+		}
+		p.buf.WriteByte(')')
+		io.Copy(s, &p.buf)
+		return
+	}
+
+loop:
+	for {
+		switch v := err.(type) {
+		case Formatter:
+			err = v.FormatError((*printer)(p))
+		case fmt.Formatter:
+			v.Format(p, 'v')
+			break loop
+		default:
+			io.WriteString(&p.buf, v.Error())
+			break loop
+		}
+		if err == nil {
+			break
+		}
+		if p.needColon || !p.printDetail {
+			p.buf.WriteByte(':')
+			p.needColon = false
+		}
+		p.buf.WriteString(sep)
+		p.inDetail = false
+		p.needNewline = false
+	}
+
+exit:
+	width, okW := s.Width()
+	prec, okP := s.Precision()
+
+	if !direct || (okW && width > 0) || okP {
+		// Construct format string from State s.
+		format := []byte{'%'}
+		if s.Flag('-') {
+			format = append(format, '-')
+		}
+		if s.Flag('+') {
+			format = append(format, '+')
+		}
+		if s.Flag(' ') {
+			format = append(format, ' ')
+		}
+		if okW {
+			format = strconv.AppendInt(format, int64(width), 10)
+		}
+		if okP {
+			format = append(format, '.')
+			format = strconv.AppendInt(format, int64(prec), 10)
+		}
+		format = append(format, string(verb)...)
+		fmt.Fprintf(s, string(format), p.buf.String())
+	} else {
+		io.Copy(s, &p.buf)
+	}
+}
+
+var detailSep = []byte("\n    ")
+
+// state tracks error printing state. It implements fmt.State.
+type state struct {
+	fmt.State
+	buf bytes.Buffer
+
+	printDetail bool
+	inDetail    bool
+	needColon   bool
+	needNewline bool
+}
+
+func (s *state) Write(b []byte) (n int, err error) {
+	if s.printDetail {
+		if len(b) == 0 {
+			return 0, nil
+		}
+		if s.inDetail && s.needColon {
+			s.needNewline = true
+			if b[0] == '\n' {
+				b = b[1:]
+			}
+		}
+		k := 0
+		for i, c := range b {
+			if s.needNewline {
+				if s.inDetail && s.needColon {
+					s.buf.WriteByte(':')
+					s.needColon = false
+				}
+				s.buf.Write(detailSep)
+				s.needNewline = false
+			}
+			if c == '\n' {
+				s.buf.Write(b[k:i])
+				k = i + 1
+				s.needNewline = true
+			}
+		}
+		s.buf.Write(b[k:])
+		if !s.inDetail {
+			s.needColon = true
+		}
+	} else if !s.inDetail {
+		s.buf.Write(b)
+	}
+	return len(b), nil
+}
+
+// printer wraps a state to implement an xerrors.Printer.
+type printer state
+
+func (s *printer) Print(args ...interface{}) {
+	if !s.inDetail || s.printDetail {
+		fmt.Fprint((*state)(s), args...)
+	}
+}
+
+func (s *printer) Printf(format string, args ...interface{}) {
+	if !s.inDetail || s.printDetail {
+		fmt.Fprintf((*state)(s), format, args...)
+	}
+}
+
+func (s *printer) Detail() bool {
+	s.inDetail = true
+	return s.printDetail
+}
diff --git a/vendor/golang.org/x/xerrors/codereview.cfg b/vendor/golang.org/x/xerrors/codereview.cfg
new file mode 100644
index 000000000..3f8b14b64
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/codereview.cfg
@@ -0,0 +1 @@
+issuerepo: golang/go
diff --git a/vendor/golang.org/x/xerrors/doc.go b/vendor/golang.org/x/xerrors/doc.go
new file mode 100644
index 000000000..eef99d9d5
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/doc.go
@@ -0,0 +1,22 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package xerrors implements functions to manipulate errors.
+//
+// This package is based on the Go 2 proposal for error values:
+//   https://golang.org/design/29934-error-values
+//
+// These functions were incorporated into the standard library's errors package
+// in Go 1.13:
+// - Is
+// - As
+// - Unwrap
+//
+// Also, Errorf's %w verb was incorporated into fmt.Errorf.
+//
+// Use this package to get equivalent behavior in all supported Go versions.
+//
+// No other features of this package were included in Go 1.13, and at present
+// there are no plans to include any of them.
+package xerrors // import "golang.org/x/xerrors"
diff --git a/vendor/golang.org/x/xerrors/errors.go b/vendor/golang.org/x/xerrors/errors.go
new file mode 100644
index 000000000..e88d3772d
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/errors.go
@@ -0,0 +1,33 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+import "fmt"
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s     string
+	frame Frame
+}
+
+// New returns an error that formats as the given text.
+//
+// The returned error contains a Frame set to the caller's location and
+// implements Formatter to show this information when printed with details.
+func New(text string) error {
+	return &errorString{text, Caller(1)}
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
+
+func (e *errorString) Format(s fmt.State, v rune) { FormatError(e, s, v) }
+
+func (e *errorString) FormatError(p Printer) (next error) {
+	p.Print(e.s)
+	e.frame.Format(p)
+	return nil
+}
diff --git a/vendor/golang.org/x/xerrors/fmt.go b/vendor/golang.org/x/xerrors/fmt.go
new file mode 100644
index 000000000..829862ddf
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/fmt.go
@@ -0,0 +1,187 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/xerrors/internal"
+)
+
+const percentBangString = "%!"
+
+// Errorf formats according to a format specifier and returns the string as a
+// value that satisfies error.
+//
+// The returned error includes the file and line number of the caller when
+// formatted with additional detail enabled. If the last argument is an error
+// the returned error's Format method will return it if the format string ends
+// with ": %s", ": %v", or ": %w". If the last argument is an error and the
+// format string ends with ": %w", the returned error implements an Unwrap
+// method returning it.
+//
+// If the format specifier includes a %w verb with an error operand in a
+// position other than at the end, the returned error will still implement an
+// Unwrap method returning the operand, but the error's Format method will not
+// return the wrapped error.
+//
+// It is invalid to include more than one %w verb or to supply it with an
+// operand that does not implement the error interface. The %w verb is otherwise
+// a synonym for %v.
+func Errorf(format string, a ...interface{}) error {
+	format = formatPlusW(format)
+	// Support a ": %[wsv]" suffix, which works well with xerrors.Formatter.
+	wrap := strings.HasSuffix(format, ": %w")
+	idx, format2, ok := parsePercentW(format)
+	percentWElsewhere := !wrap && idx >= 0
+	if !percentWElsewhere && (wrap || strings.HasSuffix(format, ": %s") || strings.HasSuffix(format, ": %v")) {
+		err := errorAt(a, len(a)-1)
+		if err == nil {
+			return &noWrapError{fmt.Sprintf(format, a...), nil, Caller(1)}
+		}
+		// TODO: this is not entirely correct. The error value could be
+		// printed elsewhere in format if it mixes numbered with unnumbered
+		// substitutions. With relatively small changes to doPrintf we can
+		// have it optionally ignore extra arguments and pass the argument
+		// list in its entirety.
+		msg := fmt.Sprintf(format[:len(format)-len(": %s")], a[:len(a)-1]...)
+		frame := Frame{}
+		if internal.EnableTrace {
+			frame = Caller(1)
+		}
+		if wrap {
+			return &wrapError{msg, err, frame}
+		}
+		return &noWrapError{msg, err, frame}
+	}
+	// Support %w anywhere.
+	// TODO: don't repeat the wrapped error's message when %w occurs in the middle.
+	msg := fmt.Sprintf(format2, a...)
+	if idx < 0 {
+		return &noWrapError{msg, nil, Caller(1)}
+	}
+	err := errorAt(a, idx)
+	if !ok || err == nil {
+		// Too many %ws or argument of %w is not an error. Approximate the Go
+		// 1.13 fmt.Errorf message.
+		return &noWrapError{fmt.Sprintf("%sw(%s)", percentBangString, msg), nil, Caller(1)}
+	}
+	frame := Frame{}
+	if internal.EnableTrace {
+		frame = Caller(1)
+	}
+	return &wrapError{msg, err, frame}
+}
+
+func errorAt(args []interface{}, i int) error {
+	if i < 0 || i >= len(args) {
+		return nil
+	}
+	err, ok := args[i].(error)
+	if !ok {
+		return nil
+	}
+	return err
+}
+
+// formatPlusW is used to avoid the vet check that will barf at %w.
+func formatPlusW(s string) string {
+	return s
+}
+
+// Return the index of the only %w in format, or -1 if none.
+// Also return a rewritten format string with %w replaced by %v, and
+// false if there is more than one %w.
+// TODO: handle "%[N]w".
+func parsePercentW(format string) (idx int, newFormat string, ok bool) {
+	// Loosely copied from golang.org/x/tools/go/analysis/passes/printf/printf.go.
+	idx = -1
+	ok = true
+	n := 0
+	sz := 0
+	var isW bool
+	for i := 0; i < len(format); i += sz {
+		if format[i] != '%' {
+			sz = 1
+			continue
+		}
+		// "%%" is not a format directive.
+		if i+1 < len(format) && format[i+1] == '%' {
+			sz = 2
+			continue
+		}
+		sz, isW = parsePrintfVerb(format[i:])
+		if isW {
+			if idx >= 0 {
+				ok = false
+			} else {
+				idx = n
+			}
+			// "Replace" the last character, the 'w', with a 'v'.
+			p := i + sz - 1
+			format = format[:p] + "v" + format[p+1:]
+		}
+		n++
+	}
+	return idx, format, ok
+}
+
+// Parse the printf verb starting with a % at s[0].
+// Return how many bytes it occupies and whether the verb is 'w'.
+func parsePrintfVerb(s string) (int, bool) {
+	// Assume only that the directive is a sequence of non-letters followed by a single letter.
+	sz := 0
+	var r rune
+	for i := 1; i < len(s); i += sz {
+		r, sz = utf8.DecodeRuneInString(s[i:])
+		if unicode.IsLetter(r) {
+			return i + sz, r == 'w'
+		}
+	}
+	return len(s), false
+}
+
+type noWrapError struct {
+	msg   string
+	err   error
+	frame Frame
+}
+
+func (e *noWrapError) Error() string {
+	return fmt.Sprint(e)
+}
+
+func (e *noWrapError) Format(s fmt.State, v rune) { FormatError(e, s, v) }
+
+func (e *noWrapError) FormatError(p Printer) (next error) {
+	p.Print(e.msg)
+	e.frame.Format(p)
+	return e.err
+}
+
+type wrapError struct {
+	msg   string
+	err   error
+	frame Frame
+}
+
+func (e *wrapError) Error() string {
+	return fmt.Sprint(e)
+}
+
+func (e *wrapError) Format(s fmt.State, v rune) { FormatError(e, s, v) }
+
+func (e *wrapError) FormatError(p Printer) (next error) {
+	p.Print(e.msg)
+	e.frame.Format(p)
+	return e.err
+}
+
+func (e *wrapError) Unwrap() error {
+	return e.err
+}
diff --git a/vendor/golang.org/x/xerrors/format.go b/vendor/golang.org/x/xerrors/format.go
new file mode 100644
index 000000000..1bc9c26b9
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/format.go
@@ -0,0 +1,34 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+// A Formatter formats error messages.
+type Formatter interface {
+	error
+
+	// FormatError prints the receiver's first error and returns the next error in
+	// the error chain, if any.
+	FormatError(p Printer) (next error)
+}
+
+// A Printer formats error messages.
+//
+// The most common implementation of Printer is the one provided by package fmt
+// during Printf (as of Go 1.13). Localization packages such as golang.org/x/text/message
+// typically provide their own implementations.
+type Printer interface {
+	// Print appends args to the message output.
+	Print(args ...interface{})
+
+	// Printf writes a formatted string.
+	Printf(format string, args ...interface{})
+
+	// Detail reports whether error detail is requested.
+	// After the first call to Detail, all text written to the Printer
+	// is formatted as additional detail, or ignored when
+	// detail has not been requested.
+	// If Detail returns false, the caller can avoid printing the detail at all.
+	Detail() bool
+}
diff --git a/vendor/golang.org/x/xerrors/frame.go b/vendor/golang.org/x/xerrors/frame.go
new file mode 100644
index 000000000..0de628ec5
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/frame.go
@@ -0,0 +1,56 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+import (
+	"runtime"
+)
+
+// A Frame contains part of a call stack.
+type Frame struct {
+	// Make room for three PCs: the one we were asked for, what it called,
+	// and possibly a PC for skipPleaseUseCallersFrames. See:
+	// https://go.googlesource.com/go/+/032678e0fb/src/runtime/extern.go#169
+	frames [3]uintptr
+}
+
+// Caller returns a Frame that describes a frame on the caller's stack.
+// The argument skip is the number of frames to skip over.
+// Caller(0) returns the frame for the caller of Caller.
+func Caller(skip int) Frame {
+	var s Frame
+	runtime.Callers(skip+1, s.frames[:])
+	return s
+}
+
+// location reports the file, line, and function of a frame.
+//
+// The returned function may be "" even if file and line are not.
+func (f Frame) location() (function, file string, line int) {
+	frames := runtime.CallersFrames(f.frames[:])
+	if _, ok := frames.Next(); !ok {
+		return "", "", 0
+	}
+	fr, ok := frames.Next()
+	if !ok {
+		return "", "", 0
+	}
+	return fr.Function, fr.File, fr.Line
+}
+
+// Format prints the stack as error detail.
+// It should be called from an error's Format implementation
+// after printing any other error detail.
+func (f Frame) Format(p Printer) {
+	if p.Detail() {
+		function, file, line := f.location()
+		if function != "" {
+			p.Printf("%s\n    ", function)
+		}
+		if file != "" {
+			p.Printf("%s:%d\n", file, line)
+		}
+	}
+}
diff --git a/vendor/golang.org/x/xerrors/go.mod b/vendor/golang.org/x/xerrors/go.mod
new file mode 100644
index 000000000..870d4f612
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/go.mod
@@ -0,0 +1,3 @@
+module golang.org/x/xerrors
+
+go 1.11
diff --git a/vendor/golang.org/x/xerrors/internal/internal.go b/vendor/golang.org/x/xerrors/internal/internal.go
new file mode 100644
index 000000000..89f4eca5d
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/internal/internal.go
@@ -0,0 +1,8 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internal
+
+// EnableTrace indicates whether stack information should be recorded in errors.
+var EnableTrace = true
diff --git a/vendor/golang.org/x/xerrors/wrap.go b/vendor/golang.org/x/xerrors/wrap.go
new file mode 100644
index 000000000..9a3b51037
--- /dev/null
+++ b/vendor/golang.org/x/xerrors/wrap.go
@@ -0,0 +1,106 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xerrors
+
+import (
+	"reflect"
+)
+
+// A Wrapper provides context around another error.
+type Wrapper interface {
+	// Unwrap returns the next error in the error chain.
+	// If there is no next error, Unwrap returns nil.
+	Unwrap() error
+}
+
+// Opaque returns an error with the same error formatting as err
+// but that does not match err and cannot be unwrapped.
+func Opaque(err error) error {
+	return noWrapper{err}
+}
+
+type noWrapper struct {
+	error
+}
+
+func (e noWrapper) FormatError(p Printer) (next error) {
+	if f, ok := e.error.(Formatter); ok {
+		return f.FormatError(p)
+	}
+	p.Print(e.error)
+	return nil
+}
+
+// Unwrap returns the result of calling the Unwrap method on err, if err implements
+// Unwrap. Otherwise, Unwrap returns nil.
+func Unwrap(err error) error {
+	u, ok := err.(Wrapper)
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}
+
+// Is reports whether any error in err's chain matches target.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+func Is(err, target error) bool {
+	if target == nil {
+		return err == target
+	}
+
+	isComparable := reflect.TypeOf(target).Comparable()
+	for {
+		if isComparable && err == target {
+			return true
+		}
+		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
+			return true
+		}
+		// TODO: consider supporing target.Is(err). This would allow
+		// user-definable predicates, but also may allow for coping with sloppy
+		// APIs, thereby making it easier to get away with them.
+		if err = Unwrap(err); err == nil {
+			return false
+		}
+	}
+}
+
+// As finds the first error in err's chain that matches the type to which target
+// points, and if so, sets the target to its value and returns true. An error
+// matches a type if it is assignable to the target type, or if it has a method
+// As(interface{}) bool such that As(target) returns true. As will panic if target
+// is not a non-nil pointer to a type which implements error or is of interface type.
+//
+// The As method should set the target to its value and return true if err
+// matches the type to which target points.
+func As(err error, target interface{}) bool {
+	if target == nil {
+		panic("errors: target cannot be nil")
+	}
+	val := reflect.ValueOf(target)
+	typ := val.Type()
+	if typ.Kind() != reflect.Ptr || val.IsNil() {
+		panic("errors: target must be a non-nil pointer")
+	}
+	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
+		panic("errors: *target must be interface or implement error")
+	}
+	targetType := typ.Elem()
+	for err != nil {
+		if reflect.TypeOf(err).AssignableTo(targetType) {
+			val.Elem().Set(reflect.ValueOf(err))
+			return true
+		}
+		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
+			return true
+		}
+		err = Unwrap(err)
+	}
+	return false
+}
+
+var errorType = reflect.TypeOf((*error)(nil)).Elem()