packer: closed pipe is caught [GH-875]

CHANGELOG.md

@@ -20,6 +20,8 @@ BUG FIXES:
 
 * core: Fix crash case if blank parameters are given to Packer. [GH-832]
 * core: Fix crash if big file uploads are done. [GH-897]
+* core: Fix crash if machine-readable output is going to a closed
+  pipe. [GH-875]
 * builder/docker: user variables work properly. [GH-777]
 * builder/qemu: reboots are now possible in provisioners. [GH-864]
 * builder/virtualbox,vmware: iso\_checksum is not required if the

packer/ui.go

@@ -11,6 +11,7 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 	"unicode"
 )
@@ -282,6 +283,11 @@ func (u *MachineReadableUi) Machine(category string, args ...string) {
 
 	_, err := fmt.Fprintf(u.Writer, "%d,%s,%s,%s\n", now.Unix(), target, category, argsString)
 	if err != nil {
-		panic(err)
+		if err == syscall.EPIPE {
+			// Ignore epipe errors because that just means that the file
+			// is probably closed or going to /dev/null or something.
+		} else {
+			panic(err)
+		}
 	}
 }