fix: same TCP connection appears twice (prometheus#631)

```
TCP, TCP6, UDP, and UDP6 are dynamically changing,
and when we read these files, we should read them all at once.
there will be data consistency issues if using line by lin reading

fix: prometheus#576
```

Signed-off-by: weidongkl <weidong@uniontech.com>

net_ip_socket.go

@@ -14,26 +14,14 @@
 package procfs
 
 import (
-	"bufio"
 	"encoding/hex"
 	"fmt"
-	"io"
 	"net"
 	"os"
 	"strconv"
 	"strings"
 )
 
-const (
-	// readLimit is used by io.LimitReader while reading the content of the
-	// /proc/net/udp{,6} files. The number of lines inside such a file is dynamic
-	// as each line represents a single used socket.
-	// In theory, the number of available sockets is 65535 (2^16 - 1) per IP.
-	// With e.g. 150 Byte per line and the maximum number of 65535,
-	// the reader needs to handle 150 Byte * 65535 =~ 10 MB for a single IP.
-	readLimit = 4294967296 // Byte -> 4 GiB
-)
-
 // This contains generic data structures for both udp and tcp sockets.
 type (
 	// NetIPSocket represents the contents of /proc/net/{t,u}dp{,6} file without the header.
@@ -74,49 +62,50 @@ type (
 )
 
 func newNetIPSocket(file string) (NetIPSocket, error) {
-	f, err := os.Open(file)
+	var netIPSocket NetIPSocket
+	isUDP := strings.Contains(file, "udp")
+	content, err := os.ReadFile(file)
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
-
-	var netIPSocket NetIPSocket
-	isUDP := strings.Contains(file, "udp")
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 1 {
+		return nil, ErrFileParse
+	}
 
-	lr := io.LimitReader(f, readLimit)
-	s := bufio.NewScanner(lr)
-	s.Scan() // skip first line with headers
-	for s.Scan() {
-		fields := strings.Fields(s.Text())
+	for _, line := range lines[1:] {
+		fields := strings.Fields(line)
+		if len(fields) == 0 {
+			continue
+		}
 		line, err := parseNetIPSocketLine(fields, isUDP)
 		if err != nil {
 			return nil, err
 		}
 		netIPSocket = append(netIPSocket, line)
 	}
-	if err := s.Err(); err != nil {
-		return nil, err
-	}
 	return netIPSocket, nil
 }
 
 // newNetIPSocketSummary creates a new NetIPSocket{,6} from the contents of the given file.
 func newNetIPSocketSummary(file string) (*NetIPSocketSummary, error) {
-	f, err := os.Open(file)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-
 	var netIPSocketSummary NetIPSocketSummary
 	var udpPacketDrops uint64
 	isUDP := strings.Contains(file, "udp")
+	content, err := os.ReadFile(file)
+	if err != nil {
+		return nil, err
+	}
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 1 {
+		return nil, ErrFileParse
+	}
 
-	lr := io.LimitReader(f, readLimit)
-	s := bufio.NewScanner(lr)
-	s.Scan() // skip first line with headers
-	for s.Scan() {
-		fields := strings.Fields(s.Text())
+	for _, line := range lines[1:] {
+		fields := strings.Fields(line)
+		if len(fields) == 0 {
+			continue
+		}
 		line, err := parseNetIPSocketLine(fields, isUDP)
 		if err != nil {
 			return nil, err
@@ -129,9 +118,6 @@ func newNetIPSocketSummary(file string) (*NetIPSocketSummary, error) {
 			netIPSocketSummary.Drops = &udpPacketDrops
 		}
 	}
-	if err := s.Err(); err != nil {
-		return nil, err
-	}
 	return &netIPSocketSummary, nil
 }