telegraf/metric/reader.go

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3.5 KiB
Go
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package metric
import (
"io"
"github.com/influxdata/telegraf"
)
type state int
const (
_ state = iota
// normal state copies whole metrics into the given buffer until we can't
// fit the next metric.
normal
// split state means that we have a metric that we were able to split, so
// that we can fit it into multiple metrics (and calls to Read)
split
// overflow state means that we have a metric that didn't fit into a single
// buffer, and needs to be split across multiple calls to Read.
overflow
// splitOverflow state means that a split metric didn't fit into a single
// buffer, and needs to be split across multiple calls to Read.
splitOverflow
// done means we're done reading metrics, and now always return (0, io.EOF)
done
)
type reader struct {
metrics []telegraf.Metric
splitMetrics []telegraf.Metric
buf []byte
state state
// metric index
iM int
// split metric index
iSM int
// buffer index
iB int
}
func NewReader(metrics []telegraf.Metric) io.Reader {
return &reader{
metrics: metrics,
state: normal,
}
}
func (r *reader) Read(p []byte) (n int, err error) {
var i int
switch r.state {
case done:
return 0, io.EOF
case normal:
for {
// this for-loop is the sunny-day scenario, where we are given a
// buffer that is large enough to hold at least a single metric.
// all of the cases below it are edge-cases.
if r.metrics[r.iM].Len() <= len(p[i:]) {
i += r.metrics[r.iM].SerializeTo(p[i:])
} else {
break
}
r.iM++
if r.iM == len(r.metrics) {
r.state = done
return i, io.EOF
}
}
// if we haven't written any bytes, check if we can split the current
// metric into multiple full metrics at a smaller size.
if i == 0 {
tmp := r.metrics[r.iM].Split(len(p))
if len(tmp) > 1 {
r.splitMetrics = tmp
r.state = split
if r.splitMetrics[0].Len() <= len(p) {
i += r.splitMetrics[0].SerializeTo(p)
r.iSM = 1
} else {
// splitting didn't quite work, so we'll drop down and
// overflow the metric.
r.state = normal
r.iSM = 0
}
}
}
// if we haven't written any bytes and we're not at the end of the metrics
// slice, then it means we have a single metric that is larger than the
// provided buffer.
if i == 0 {
r.buf = r.metrics[r.iM].Serialize()
i += copy(p, r.buf[r.iB:])
r.iB += i
r.state = overflow
}
case split:
if r.splitMetrics[r.iSM].Len() <= len(p) {
// write the current split metric
i += r.splitMetrics[r.iSM].SerializeTo(p)
r.iSM++
if r.iSM >= len(r.splitMetrics) {
// done writing the current split metrics
r.iSM = 0
r.iM++
if r.iM == len(r.metrics) {
r.state = done
return i, io.EOF
}
r.state = normal
}
} else {
// This would only happen if we split the metric, and then a
// subsequent buffer was smaller than the initial one given,
// so that our split metric no longer fits.
r.buf = r.splitMetrics[r.iSM].Serialize()
i += copy(p, r.buf[r.iB:])
r.iB += i
r.state = splitOverflow
}
case splitOverflow:
i = copy(p, r.buf[r.iB:])
r.iB += i
if r.iB >= len(r.buf) {
r.iB = 0
r.iSM++
if r.iSM == len(r.splitMetrics) {
r.iM++
if r.iM == len(r.metrics) {
r.state = done
return i, io.EOF
}
r.state = normal
} else {
r.state = split
}
}
case overflow:
i = copy(p, r.buf[r.iB:])
r.iB += i
if r.iB >= len(r.buf) {
r.iB = 0
r.iM++
if r.iM == len(r.metrics) {
r.state = done
return i, io.EOF
}
r.state = normal
}
}
return i, nil
}