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When serving data from remote location, one might need to pass the request context to backend storage of entries, for example for distributed tracing to work.
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| Original file line number | Diff line number | Diff line change |
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| @@ -1,5 +1,3 @@ | ||
| module github.com/martin-sucha/zipserve | ||
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| go 1.12 | ||
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| require go4.org v0.0.0-20180417224846-9599cf28b011 |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,2 +0,0 @@ | ||
| go4.org v0.0.0-20180417224846-9599cf28b011 h1:i0QTVNl3j6yciHiQIHxz+mnsSQqo/xi78EGN7yNpMVw= | ||
| go4.org v0.0.0-20180417224846-9599cf28b011/go.mod h1:MkTOUMDaeVYJUOUsaDXIhWPZYa1yOyC1qaOBpL57BhE= | ||
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,123 @@ | ||
| package zipserve | ||
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| import ( | ||
| "context" | ||
| "fmt" | ||
| "io" | ||
| "sort" | ||
| ) | ||
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| // ReaderAt is like io.ReaderAt, but also takes context. | ||
| type ReaderAt interface { | ||
| // ReadAtContext has same semantics as ReadAt from io.ReaderAt, but takes context. | ||
| ReadAtContext(ctx context.Context, p []byte, off int64) (n int, err error) | ||
| } | ||
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| type sizeReaderAt interface { | ||
| io.ReaderAt | ||
| Size() int64 | ||
| } | ||
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| type offsetAndData struct { | ||
| offset int64 | ||
| data ReaderAt | ||
| } | ||
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| // multiReaderAt is a ReaderAt that joins multiple ReaderAt sequentially together. | ||
| type multiReaderAt struct { | ||
| parts []offsetAndData | ||
| size int64 | ||
| } | ||
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| // add a part to the multiContextReader. | ||
| // add can be used only before the reader is read from. | ||
| func (mcr *multiReaderAt) add(data ReaderAt, size int64) { | ||
| switch { | ||
| case size < 0: | ||
| panic(fmt.Sprintf("size cannot be negative: %v", size)) | ||
| case size == 0: | ||
| return | ||
| } | ||
| mcr.parts = append(mcr.parts, offsetAndData{ | ||
| offset: mcr.size, | ||
| data: data, | ||
| }) | ||
| mcr.size += size | ||
| } | ||
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| // addSizeReaderAt is like add, but takes sizeReaderAt | ||
| func (mcr *multiReaderAt) addSizeReaderAt(r sizeReaderAt) { | ||
| mcr.add(ignoreContext{r: r}, r.Size()) | ||
| } | ||
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| // endOffset is offset where the given part ends. | ||
| func (mcr *multiReaderAt) endOffset(partIndex int) int64 { | ||
| if partIndex == len(mcr.parts)-1 { | ||
| return mcr.size | ||
| } | ||
| return mcr.parts[partIndex+1].offset | ||
| } | ||
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| func (mcr *multiReaderAt) ReadAtContext(ctx context.Context, p []byte, off int64) (n int, err error) { | ||
| if len(p) == 0 { | ||
| return 0, nil | ||
| } | ||
| if off >= mcr.size { | ||
| return 0, io.EOF | ||
| } | ||
| // find first part that has data for p | ||
| firstPartIndex := sort.Search(len(mcr.parts), func(i int) bool { | ||
| return mcr.endOffset(i) > off | ||
| }) | ||
| for partIndex := firstPartIndex; partIndex < len(mcr.parts) && len(p) > 0; partIndex++ { | ||
| if partIndex > firstPartIndex { | ||
| off = mcr.parts[partIndex].offset | ||
| } | ||
| partRemainingBytes := mcr.endOffset(partIndex) - off | ||
| sizeToRead := int64(len(p)) | ||
| if sizeToRead > partRemainingBytes { | ||
| sizeToRead = partRemainingBytes | ||
| } | ||
| n2, err2 := mcr.parts[partIndex].data.ReadAtContext(ctx, p[0:sizeToRead], off - mcr.parts[partIndex].offset) | ||
| n += n2 | ||
| if err2 != nil { | ||
| return n, err2 | ||
| } | ||
| p = p[n2:] | ||
| } | ||
| if len(p) > 0 { | ||
| // tried reading beyond size | ||
| return n, io.EOF | ||
| } | ||
| return n, nil | ||
| } | ||
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| func (mcr *multiReaderAt) ReadAt(p []byte, off int64) (n int, err error) { | ||
| return mcr.ReadAtContext(context.TODO(), p, off) | ||
| } | ||
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| func (mcr *multiReaderAt) Size() int64 { | ||
| return mcr.size | ||
| } | ||
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| // ignoreContext converts io.ReaderAt to ReaderAt | ||
| type ignoreContext struct { | ||
| r io.ReaderAt | ||
| } | ||
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| func (a ignoreContext) ReadAtContext(_ context.Context, p []byte, off int64) (n int, err error) { | ||
| return a.r.ReadAt(p, off) | ||
| } | ||
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| // withContext converts ReaderAt to io.ReaderAt. | ||
| // | ||
| // While usually we shouldn't store context in a structure, we ensure that withContext lives only within single | ||
| // request. | ||
| type withContext struct { | ||
| ctx context.Context | ||
| r ReaderAt | ||
| } | ||
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| func (w withContext) ReadAt(p []byte, off int64) (n int, err error) { | ||
| return w.r.ReadAtContext(w.ctx, p, off) | ||
| } |
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