fix(context): fallback method to get current cid

The hostname method can fail if the host name name has been customised.
The fallback method match the first 64 characters hex string found.

Author: buchdag

context.go

@@ -2,11 +2,14 @@ package dockergen
 
 import (
 	"bufio"
+	"bytes"
+	"fmt"
+	"io"
 	"os"
 	"regexp"
 	"sync"
-	"fmt"
-	"github.com/fsouza/go-dockerclient"
+
+	docker "github.com/fsouza/go-dockerclient"
 )
 
 var (
@@ -158,57 +161,85 @@ type Docker struct {
 	CurrentContainerID string
 }
 
-func GetCurrentContainerID() string {
-	filepaths := []string{"/proc/self/cgroup", "/proc/self/mountinfo"}
+// GetCurrentContainerID attempts to extract the current container ID from the provided file paths.
+// If no files paths are provided, it will default to /proc/1/cpuset, /proc/self/cgroup and /proc/self/mountinfo.
+// It attempts to match the HOSTNAME first then use the fallback method, and returns with the first valid match.
+func GetCurrentContainerID(filepaths ...string) (id string) {
+	if len(filepaths) == 0 {
+		filepaths = []string{"/proc/1/cpuset", "/proc/self/cgroup", "/proc/self/mountinfo"}
+	}
+
+	var files []io.Reader
 
 	for _, filepath := range filepaths {
 		file, err := os.Open(filepath)
 		if err != nil {
 			continue
 		}
-		reader := bufio.NewReader(file)
-		scanner := bufio.NewScanner(reader)
-		scanner.Split(bufio.ScanLines)
-		for scanner.Scan() {
-			_, lines, err := bufio.ScanLines([]byte(scanner.Text()), true)
-			if err == nil {
-				strLines := string(lines)
-				if id := matchDockerCurrentContainerID(strLines); id != "" {
-					return id
-				} else if id := matchECSCurrentContainerID(strLines); id != "" {
-					return id
-				}
+		defer file.Close()
+		files = append(files, file)
+	}
+
+	reader := io.MultiReader(files...)
+	var buffer bytes.Buffer
+	tee := io.TeeReader(reader, &buffer)
+
+	// We try to match a 64 character hex string starting with the hostname first
+	scanner := bufio.NewScanner(tee)
+	scanner.Split(bufio.ScanLines)
+	for scanner.Scan() {
+		_, lines, err := bufio.ScanLines([]byte(scanner.Text()), true)
+		if err == nil {
+			strLines := string(lines)
+			if id = matchContainerIDWithHostname(strLines); len(id) == 64 {
+				return
 			}
 		}
 	}
 
-	return ""
+	// If we didn't get any ID that matches the hostname, fall back to matching the first 64 character hex string
+	scanner = bufio.NewScanner(&buffer)
+	scanner.Split(bufio.ScanLines)
+	for scanner.Scan() {
+		_, lines, err := bufio.ScanLines([]byte(scanner.Text()), true)
+		if err == nil {
+			strLines := string(lines)
+			if id = matchContainerID(strLines); len(id) == 64 {
+				return
+			}
+		}
+	}
+
+	return
 }
 
-func matchDockerCurrentContainerID(lines string) string {
+func matchContainerIDWithHostname(lines string) string {
 	hostname := os.Getenv("HOSTNAME")
-	regex := fmt.Sprintf("(%s[[:alnum:]]{52})", hostname)
-	re := regexp.MustCompilePOSIX(regex)
+	re := regexp.MustCompilePOSIX("^[[:alnum:]]{12}$")
 
-	if re.MatchString(lines) {
-		submatches := re.FindStringSubmatch(string(lines))
-		containerID := submatches[1]
+	if re.MatchString(hostname) {
+		regex := fmt.Sprintf("(%s[[:alnum:]]{52})", hostname)
+		re := regexp.MustCompilePOSIX(regex)
 
-		return containerID
+		if re.MatchString(lines) {
+			submatches := re.FindStringSubmatch(string(lines))
+			containerID := submatches[1]
+
+			return containerID
+		}
 	}
 	return ""
 }
 
-func matchECSCurrentContainerID(lines string) string {
-	regex := "/ecs\\/[^\\/]+\\/(.+)$"
+func matchContainerID(lines string) string {
+	regex := "([[:alnum:]]{64})"
 	re := regexp.MustCompilePOSIX(regex)
 
-	if re.MatchString(string(lines)) {
+	if re.MatchString(lines) {
 		submatches := re.FindStringSubmatch(string(lines))
 		containerID := submatches[1]
 
 		return containerID
 	}
-
 	return ""
 }