prometheus/tsdb/wlog/live_reader.go
beorn7 5b53aa1108 style: Replace else if cascades with switch
Wiser coders than myself have come to the conclusion that a `switch`
statement is almost always superior to a statement that includes any
`else if`.

The exceptions that I have found in our codebase are just these two:

* The `if else` is followed by an additional statement before the next
  condition (separated by a `;`).
* The whole thing is within a `for` loop and `break` statements are
  used. In this case, using `switch` would require tagging the `for`
  loop, which probably tips the balance.

Why are `switch` statements more readable?

For one, fewer curly braces. But more importantly, the conditions all
have the same alignment, so the whole thing follows the natural flow
of going down a list of conditions. With `else if`, in contrast, all
conditions but the first are "hidden" behind `} else if `, harder to
spot and (for no good reason) presented differently from the first
condition.

I'm sure the aforemention wise coders can list even more reasons.

In any case, I like it so much that I have found myself recommending
it in code reviews. I would like to make it a habit in our code base,
without making it a hard requirement that we would test on the CI. But
for that, there has to be a role model, so this commit eliminates all
`if else` occurrences, unless it is autogenerated code or fits one of
the exceptions above.

Signed-off-by: beorn7 <beorn@grafana.com>
2023-04-19 17:22:31 +02:00

324 lines
10 KiB
Go

// Copyright 2019 The Prometheus Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package wlog
import (
"encoding/binary"
"fmt"
"hash/crc32"
"io"
"github.com/go-kit/log"
"github.com/go-kit/log/level"
"github.com/golang/snappy"
"github.com/pkg/errors"
"github.com/prometheus/client_golang/prometheus"
)
// LiveReaderMetrics holds all metrics exposed by the LiveReader.
type LiveReaderMetrics struct {
readerCorruptionErrors *prometheus.CounterVec
}
// NewLiveReaderMetrics instantiates, registers and returns metrics to be injected
// at LiveReader instantiation.
func NewLiveReaderMetrics(reg prometheus.Registerer) *LiveReaderMetrics {
m := &LiveReaderMetrics{
readerCorruptionErrors: prometheus.NewCounterVec(prometheus.CounterOpts{
Name: "prometheus_tsdb_wal_reader_corruption_errors_total",
Help: "Errors encountered when reading the WAL.",
}, []string{"error"}),
}
if reg != nil {
reg.MustRegister(m.readerCorruptionErrors)
}
return m
}
// NewLiveReader returns a new live reader.
func NewLiveReader(logger log.Logger, metrics *LiveReaderMetrics, r io.Reader) *LiveReader {
lr := &LiveReader{
logger: logger,
rdr: r,
metrics: metrics,
// Until we understand how they come about, make readers permissive
// to records spanning pages.
permissive: true,
}
return lr
}
// LiveReader reads WAL records from an io.Reader. It allows reading of WALs
// that are still in the process of being written, and returns records as soon
// as they can be read.
type LiveReader struct {
logger log.Logger
rdr io.Reader
err error
rec []byte
snappyBuf []byte
hdr [recordHeaderSize]byte
buf [pageSize]byte
readIndex int // Index in buf to start at for next read.
writeIndex int // Index in buf to start at for next write.
total int64 // Total bytes processed during reading in calls to Next().
index int // Used to track partial records, should be 0 at the start of every new record.
// For testing, we can treat EOF as a non-error.
eofNonErr bool
// We sometime see records span page boundaries. Should never happen, but it
// does. Until we track down why, set permissive to true to tolerate it.
// NB the non-ive Reader implementation allows for this.
permissive bool
metrics *LiveReaderMetrics
}
// Err returns any errors encountered reading the WAL. io.EOFs are not terminal
// and Next can be tried again. Non-EOFs are terminal, and the reader should
// not be used again. It is up to the user to decide when to stop trying should
// io.EOF be returned.
func (r *LiveReader) Err() error {
if r.eofNonErr && errors.Is(r.err, io.EOF) {
return nil
}
return r.err
}
// Offset returns the number of bytes consumed from this segment.
func (r *LiveReader) Offset() int64 {
return r.total
}
func (r *LiveReader) fillBuffer() (int, error) {
n, err := r.rdr.Read(r.buf[r.writeIndex:len(r.buf)])
r.writeIndex += n
return n, err
}
// Next returns true if Record() will contain a full record.
// If Next returns false, you should always checked the contents of Error().
// Return false guarantees there are no more records if the segment is closed
// and not corrupt, otherwise if Err() == io.EOF you should try again when more
// data has been written.
func (r *LiveReader) Next() bool {
for {
// If buildRecord returns a non-EOF error, its game up - the segment is
// corrupt. If buildRecord returns an EOF, we try and read more in
// fillBuffer later on. If that fails to read anything (n=0 && err=EOF),
// we return EOF and the user can try again later. If we have a full
// page, buildRecord is guaranteed to return a record or a non-EOF; it
// has checks the records fit in pages.
switch ok, err := r.buildRecord(); {
case ok:
return true
case err != nil && err != io.EOF:
r.err = err
return false
}
// If we've filled the page and not found a record, this
// means records have started to span pages. Shouldn't happen
// but does and until we found out why, we need to deal with this.
if r.permissive && r.writeIndex == pageSize && r.readIndex > 0 {
copy(r.buf[:], r.buf[r.readIndex:])
r.writeIndex -= r.readIndex
r.readIndex = 0
continue
}
if r.readIndex == pageSize {
r.writeIndex = 0
r.readIndex = 0
}
if r.writeIndex != pageSize {
n, err := r.fillBuffer()
if n == 0 || (err != nil && err != io.EOF) {
r.err = err
return false
}
}
}
}
// Record returns the current record.
// The returned byte slice is only valid until the next call to Next.
func (r *LiveReader) Record() []byte {
return r.rec
}
// Rebuild a full record from potentially partial records. Returns false
// if there was an error or if we weren't able to read a record for any reason.
// Returns true if we read a full record. Any record data is appended to
// LiveReader.rec
func (r *LiveReader) buildRecord() (bool, error) {
for {
// Check that we have data in the internal buffer to read.
if r.writeIndex <= r.readIndex {
return false, nil
}
// Attempt to read a record, partial or otherwise.
temp, n, err := r.readRecord()
if err != nil {
return false, err
}
r.readIndex += n
r.total += int64(n)
if temp == nil {
return false, nil
}
rt := recTypeFromHeader(r.hdr[0])
if rt == recFirst || rt == recFull {
r.rec = r.rec[:0]
r.snappyBuf = r.snappyBuf[:0]
}
compressed := r.hdr[0]&snappyMask != 0
if compressed {
r.snappyBuf = append(r.snappyBuf, temp...)
} else {
r.rec = append(r.rec, temp...)
}
if err := validateRecord(rt, r.index); err != nil {
r.index = 0
return false, err
}
if rt == recLast || rt == recFull {
r.index = 0
if compressed && len(r.snappyBuf) > 0 {
// The snappy library uses `len` to calculate if we need a new buffer.
// In order to allocate as few buffers as possible make the length
// equal to the capacity.
r.rec = r.rec[:cap(r.rec)]
r.rec, err = snappy.Decode(r.rec, r.snappyBuf)
if err != nil {
return false, err
}
}
return true, nil
}
// Only increment i for non-zero records since we use it
// to determine valid content record sequences.
r.index++
}
}
// Returns an error if the recType and i indicate an invalid record sequence.
// As an example, if i is > 0 because we've read some amount of a partial record
// (recFirst, recMiddle, etc. but not recLast) and then we get another recFirst or recFull
// instead of a recLast or recMiddle we would have an invalid record.
func validateRecord(typ recType, i int) error {
switch typ {
case recFull:
if i != 0 {
return errors.New("unexpected full record")
}
return nil
case recFirst:
if i != 0 {
return errors.New("unexpected first record, dropping buffer")
}
return nil
case recMiddle:
if i == 0 {
return errors.New("unexpected middle record, dropping buffer")
}
return nil
case recLast:
if i == 0 {
return errors.New("unexpected last record, dropping buffer")
}
return nil
default:
return errors.Errorf("unexpected record type %d", typ)
}
}
// Read a sub-record (see recType) from the buffer. It could potentially
// be a full record (recFull) if the record fits within the bounds of a single page.
// Returns a byte slice of the record data read, the number of bytes read, and an error
// if there's a non-zero byte in a page term record or the record checksum fails.
// This is a non-method function to make it clear it does not mutate the reader.
func (r *LiveReader) readRecord() ([]byte, int, error) {
// Special case: for recPageTerm, check that are all zeros to end of page,
// consume them but don't return them.
if r.buf[r.readIndex] == byte(recPageTerm) {
// End of page won't necessarily be end of buffer, as we may have
// got misaligned by records spanning page boundaries.
// r.total % pageSize is the offset into the current page
// that r.readIndex points to in buf. Therefore
// pageSize - (r.total % pageSize) is the amount left to read of
// the current page.
remaining := int(pageSize - (r.total % pageSize))
if r.readIndex+remaining > r.writeIndex {
return nil, 0, io.EOF
}
for i := r.readIndex; i < r.readIndex+remaining; i++ {
if r.buf[i] != 0 {
return nil, 0, errors.New("unexpected non-zero byte in page term bytes")
}
}
return nil, remaining, nil
}
// Not a recPageTerm; read the record and check the checksum.
if r.writeIndex-r.readIndex < recordHeaderSize {
return nil, 0, io.EOF
}
copy(r.hdr[:], r.buf[r.readIndex:r.readIndex+recordHeaderSize])
length := int(binary.BigEndian.Uint16(r.hdr[1:]))
crc := binary.BigEndian.Uint32(r.hdr[3:])
if r.readIndex+recordHeaderSize+length > pageSize {
if !r.permissive {
return nil, 0, fmt.Errorf("record would overflow current page: %d > %d", r.readIndex+recordHeaderSize+length, pageSize)
}
r.metrics.readerCorruptionErrors.WithLabelValues("record_span_page").Inc()
level.Warn(r.logger).Log("msg", "Record spans page boundaries", "start", r.readIndex, "end", recordHeaderSize+length, "pageSize", pageSize)
}
if recordHeaderSize+length > pageSize {
return nil, 0, fmt.Errorf("record length greater than a single page: %d > %d", recordHeaderSize+length, pageSize)
}
if r.readIndex+recordHeaderSize+length > r.writeIndex {
return nil, 0, io.EOF
}
rec := r.buf[r.readIndex+recordHeaderSize : r.readIndex+recordHeaderSize+length]
if c := crc32.Checksum(rec, castagnoliTable); c != crc {
return nil, 0, errors.Errorf("unexpected checksum %x, expected %x", c, crc)
}
return rec, length + recordHeaderSize, nil
}
func min(i, j int) int {
if i < j {
return i
}
return j
}