Procstat: don't cache PIDs (#2206)

* Procstat: don't cache PIDs

Changed the procstat input plugin to not cache PIDs. Solves #1636.
The logic of creating a process by pid was moved from `procstat.go` to
`spec_processor.go`.

* Procstat: go fmt

* procstat: modify changelog for #2206

plugins/inputs/procstat/spec_processor.go

@@ -1,6 +1,7 @@
 package procstat
 
 import (
+	"fmt"
 	"time"
 
 	"github.com/shirou/gopsutil/process"
@@ -9,12 +10,13 @@ import (
 )
 
 type SpecProcessor struct {
-	Prefix string
-	pid    int32
-	tags   map[string]string
-	fields map[string]interface{}
-	acc    telegraf.Accumulator
-	proc   *process.Process
+	ProcessName string
+	Prefix      string
+	pid         int32
+	tags        map[string]string
+	fields      map[string]interface{}
+	acc         telegraf.Accumulator
+	proc        *process.Process
 }
 
 func NewSpecProcessor(
@@ -22,29 +24,35 @@ func NewSpecProcessor(
 	prefix string,
 	pid int32,
 	acc telegraf.Accumulator,
-	p *process.Process,
 	tags map[string]string,
 ) *SpecProcessor {
-	if processName != "" {
-		tags["process_name"] = processName
-	} else {
-		name, err := p.Name()
-		if err == nil {
-			tags["process_name"] = name
-		}
-	}
 	return &SpecProcessor{
-		Prefix: prefix,
-		pid:    pid,
-		tags:   tags,
-		fields: make(map[string]interface{}),
-		acc:    acc,
-		proc:   p,
+		ProcessName: processName,
+		Prefix:      prefix,
+		pid:         pid,
+		tags:        tags,
+		fields:      make(map[string]interface{}),
+		acc:         acc,
 	}
 }
 
-func (p *SpecProcessor) pushMetrics() {
+func (p *SpecProcessor) pushMetrics() error {
 	var prefix string
+	proc, err := process.NewProcess(p.pid)
+	if err != nil {
+		return fmt.Errorf("Failed to open process with pid '%d'. Error: '%s'",
+			p.pid, err)
+	}
+	p.proc = proc
+	if p.ProcessName != "" {
+		p.tags["process_name"] = p.ProcessName
+	} else {
+		name, err := p.proc.Name()
+		if err == nil {
+			p.tags["process_name"] = name
+		}
+	}
+
 	if p.Prefix != "" {
 		prefix = p.Prefix + "_"
 	}
@@ -107,4 +115,5 @@ func (p *SpecProcessor) pushMetrics() {
 	}
 
 	p.acc.AddFields("procstat", fields, p.tags)
+	return nil
 }