lib/protoparser: handle unexpected EOF error when parsing lines in prometheus exposition format (#4851)

Previously only io.EOF was handled, and io.ErrUnexpectedEOF was ignored, but it may happen if the client interrupts the connection.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4817

docs/CHANGELOG.md

@@ -25,6 +25,7 @@ The following `tip` changes can be tested by building VictoriaMetrics components
 ## tip
 
 * FEATURE: [vmbackup](https://docs.victoriametrics.com/vmbackup.html): add support for server-side copy of existing backups. See [these docs](https://docs.victoriametrics.com/vmbackup.html#server-side-copy-of-the-existing-backup) for details.
+* FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly handle `unexpected EOF` error when parsing metrics in Prometheus exposition format. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4817). 
 
 * BUGFIX: do not allow starting VictoriaMetrics components with improperly set boolean command-line flags in the form `-boolFlagName value`, since this leads to silent incomplete flags' parsing. This form should be replaced with `-boolFlagName=value`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4845).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly set labels from `-remoteWrite.label` command-line flag just before sending samples to the configured `-remoteWrite.url` according to [these docs](https://docs.victoriametrics.com/vmagent.html#adding-labels-to-metrics). Previously these labels were incorrectly set before [the relabeling](https://docs.victoriametrics.com/vmagent.html#relabeling) configured via `-remoteWrite.urlRelabelConfigs` and [the stream aggregation](https://docs.victoriametrics.com/stream-aggregation.html) configured via `-remoteWrite.streamAggr.config`, so these labels could be lost or incorrectly transformed before sending the samples to remote storage. The fix allows using `-remoteWrite.label` for identifying `vmagent` instances in [cluster mode](https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4247) and [these docs](https://docs.victoriametrics.com/stream-aggregation.html#cluster-mode) for more details.

lib/protoparser/common/lines_reader.go

@@ -93,7 +93,7 @@ again:
 }
 
 func isEOFLikeError(err error) bool {
-	if errors.Is(err, io.EOF) {
+	if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
 		return true
 	}
 	s := err.Error()

lib/protoparser/common/lines_reader_test.go

@@ -2,6 +2,7 @@ package common
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"reflect"
@@ -25,6 +26,20 @@ func TestReadLinesBlockFailure(t *testing.T) {
 		if _, _, err := ReadLinesBlock(fr, nil, nil); err == nil {
 			t.Fatalf("expecting non-nil error")
 		}
+
+		un := &unexpectedEOF{}
+		if _, _, err := ReadLinesBlock(un, nil, nil); err != nil {
+			if !errors.Is(err, io.EOF) {
+				t.Fatalf("get unexpected error, expecting io.EOF")
+			}
+		}
+
+		ef := eofErr{}
+		if _, _, err := ReadLinesBlock(ef, nil, nil); err != nil {
+			if !errors.Is(err, io.EOF) {
+				t.Fatalf("get unexpected error, expecting io.EOF")
+			}
+		}
 	}
 
 	// empty string
@@ -41,6 +56,18 @@ func (fr *failureReader) Read(p []byte) (int, error) {
 	return 0, fmt.Errorf("some error")
 }
 
+type unexpectedEOF struct{}
+
+func (un unexpectedEOF) Read(p []byte) (int, error) {
+	return 0, io.ErrUnexpectedEOF
+}
+
+type eofErr struct{}
+
+func (eo eofErr) Read(p []byte) (int, error) {
+	return 0, io.EOF
+}
+
 func TestReadLinesBlockMultiLinesSingleByteReader(t *testing.T) {
 	f := func(s string, linesExpected []string) {
 		t.Helper()