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writer.go
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writer.go
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package chain
import (
"io"
)
// WriterBuilder lets you build a chain of io.Writers
// in a more natural way
type WriterBuilder struct {
wcs []WriteChain
}
// WriteChain represents a common pattern in go packages.
// For examples, see:
// https://pkg.go.dev/compress/gzip#NewWriter
// https://pkg.go.dev/golang.org/x/crypto/openpgp#Encrypt
//
// While this type only expects an io.Writer to be returned, it will
// detect if the result is a Closer and make sure the
// types Close function is run.
type WriteChain func(io.WriteCloser) (io.WriteCloser, error)
// NewWriteBuilder creates a new WriteBuilder with the
// given WriteChain being the first in the chain
func NewWriteBuilder(first WriteChain) *WriterBuilder {
return &WriterBuilder{
wcs: []WriteChain{first},
}
}
// Then adds the next WriteChain to the current builder chain.
// Returns self
func (wc *WriterBuilder) Then(next WriteChain) *WriterBuilder {
wc.wcs = append(wc.wcs, next)
return wc
}
// WritingTo builds the chain. The resulting data from the chain is
// written to the io.Writer provided.
//
// If w is a Closer type too, calling the returned writer's Close function
// will also close w.
func (wc *WriterBuilder) WritingTo(w io.WriteCloser) (io.WriteCloser, error) {
for i := len(wc.wcs) - 1; i >= 0; i-- {
newW, err := wc.wcs[i](w)
if err != nil {
w.Close()
return nil, err
}
w = newW
}
return w, nil
}
type WriterFileBuilder struct {
builder *WriterBuilder
name string
}
func (wc *WriterBuilder) Open(name string) *WriterFileBuilder {
return &WriterFileBuilder{
builder: wc,
name: name,
}
}
func (builder *WriterFileBuilder) InFS(next WriteFSChain) *WriterBuilder {
return builder.builder.Then(func(w io.WriteCloser) (io.WriteCloser, error) {
fs, err := next(w)
if err != nil {
return nil, err
}
f, err := fs.Create(builder.name)
if err != nil {
fs.Close()
return nil, err
}
return WriteCloser2{
WriteCloser: f,
Closer: fs,
}, nil
})
}
func (builder *WriterFileBuilder) WritingToFS(fs WriteFS) (io.WriteCloser, error) {
f, err := fs.Create(builder.name)
if err != nil {
fs.Close()
return nil, err
}
return builder.builder.WritingTo(WriteCloser2{
WriteCloser: f,
Closer: fs,
})
}
type WriteFSBuilder struct {
first *WriterBuilder
fs WriteFSChain
after *WriterBuilder
}
type WriteFSChain func(io.WriteCloser) (WriteFS, error)
func (wc *WriterBuilder) IntoFS(next WriteFSChain) *WriteFSBuilder {
return &WriteFSBuilder{
first: wc,
fs: next,
after: &WriterBuilder{},
}
}
func (wc *WriterBuilder) WritingToFS(fs WriteFS) WriteFS {
return &writeFs{
close: NopWriteCloser{},
first: wc,
fs: fs,
}
}
func (wc *WriteFSBuilder) Then(next WriteChain) *WriteFSBuilder {
wc.after.Then(next)
return wc
}
func (wc *WriteFSBuilder) WritingTo(w io.WriteCloser) (WriteFS, error) {
after, err := wc.after.WritingTo(w)
if err != nil {
return nil, err
}
fs, err := wc.fs(after)
if err != nil {
after.Close()
return nil, err
}
return &writeFs{
first: wc.first,
fs: fs,
close: after,
}, nil
}
type writeFs struct {
close io.Closer
fs WriteFS
first *WriterBuilder
}
func (fs *writeFs) Create(path string) (io.WriteCloser, error) {
w, err := fs.fs.Create(path)
if err != nil {
return nil, err
}
return fs.first.WritingTo(w)
}
func (fs *writeFs) Close() error {
err1 := fs.fs.Close()
err2 := fs.close.Close()
if err1 != nil {
return err1
}
return err2
}
type WriteFS interface {
Create(path string) (io.WriteCloser, error)
io.Closer
}
type NopWriteCloser struct {
io.Writer
}
func (NopWriteCloser) Close() error { return nil }
type WriteCloser2 struct {
io.WriteCloser
Closer io.Closer
}
func (wc WriteCloser2) Close() error {
err1 := wc.WriteCloser.Close()
err2 := wc.Closer.Close()
if err1 != nil {
return err1
}
return err2
}