Extract function to return all current process ids

This implementation should be more efficient because it only uses the
names under /proc, whereas the previous implementation fetched the
full directory entry for each one, and sorted them too.

common/process.go

@@ -0,0 +1,27 @@
+package common
+
+import (
+	"os"
+	"strconv"
+)
+
+func AllPids(procRoot string) ([]int, error) {
+	fh, err := os.Open(procRoot)
+	if err != nil {
+		return nil, err
+	}
+	defer fh.Close()
+	dirNames, err := fh.Readdirnames(-1)
+	if err != nil {
+		return nil, err
+	}
+	pids := make([]int, len(dirNames))
+	for _, dirName := range dirNames {
+		pid, err := strconv.Atoi(dirName)
+		if err != nil { // Only interested in numeric entries - skip /proc/stat, etc.
+			continue
+		}
+		pids = append(pids, pid)
+	}
+	return pids, err
+}

prog/weaveutil/docker_tls_args.go

@@ -8,6 +8,8 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+
+	"github.com/weaveworks/weave/common"
 )
 
 func dockerTLSArgs(args []string) error {
@@ -15,18 +17,14 @@ func dockerTLSArgs(args []string) error {
 		cmdUsage("docker-tls-args", "")
 	}
 	procRoot := "/proc"
-	dirEntries, err := ioutil.ReadDir(procRoot)
+	pids, err := common.AllPids(procRoot)
 	if err != nil {
 		return err
 	}
 
-	for _, dirEntry := range dirEntries {
-		dirName := dirEntry.Name()
-		if _, err := strconv.Atoi(dirName); err != nil {
-			continue
-		}
-
+	for _, pid := range pids {
 		isDaemon := false
+		dirName := strconv.Itoa(pid)
 		if comm, err := ioutil.ReadFile(filepath.Join(procRoot, dirName, "comm")); err != nil {
 			continue
 		} else if string(comm) == "dockerd\n" {