packer/rpc: don't panic on cache errors [GH-1328]

CHANGELOG.md

@@ -22,6 +22,7 @@ BUG FIXES:
 
   * core: nicer error message if an encrypted private key is used for
       SSH. [GH-1445]
+  * core: Fix crash that could happen with a well timed double Ctrl-C. [GH-1328]
   * builder/amazon-chroot: Can properly build HVM images now. [GH-1360]
   * builder/amazon-chroot: Fix crash in root device check. [GH-1360]
   * builder/amazon-instance: Fix deprecation warning for `ec2-bundle-vol`

packer/rpc/cache.go

@@ -2,6 +2,7 @@ package rpc
 
 import (
 	"github.com/mitchellh/packer/packer"
+	"log"
 	"net/rpc"
 )
 
@@ -24,7 +25,8 @@ type CacheRLockResponse struct {
 
 func (c *cache) Lock(key string) (result string) {
 	if err := c.client.Call("Cache.Lock", key, &result); err != nil {
-		panic(err)
+		log.Printf("[ERR] Cache.Lock error: %s", err)
+		return
 	}
 
 	return
@@ -33,7 +35,8 @@ func (c *cache) Lock(key string) (result string) {
 func (c *cache) RLock(key string) (string, bool) {
 	var result CacheRLockResponse
 	if err := c.client.Call("Cache.RLock", key, &result); err != nil {
-		panic(err)
+		log.Printf("[ERR] Cache.RLock error: %s", err)
+		return "", false
 	}
 
 	return result.Path, result.Exists
@@ -41,13 +44,15 @@ func (c *cache) RLock(key string) (string, bool) {
 
 func (c *cache) Unlock(key string) {
 	if err := c.client.Call("Cache.Unlock", key, new(interface{})); err != nil {
-		panic(err)
+		log.Printf("[ERR] Cache.Unlock error: %s", err)
+		return
 	}
 }
 
 func (c *cache) RUnlock(key string) {
 	if err := c.client.Call("Cache.RUnlock", key, new(interface{})); err != nil {
-		panic(err)
+		log.Printf("[ERR] Cache.RUnlock error: %s", err)
+		return
 	}
 }