make sure we don't try to load an iso into memory because of a user mistake

common/iso_config.go

@@ -58,6 +58,11 @@ func (c *ISOConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs [
 
 				// If iso_checksum has no value use iso_checksum_url instead.
 				if c.ISOChecksum == "" {
+					if strings.HasSuffix(strings.ToLower(c.ISOChecksumURL), ".iso") {
+						errs = append(errs, fmt.Errorf("Error parsing checksum:"+
+							" .iso is not a valid checksum extension"))
+						return warnings, errs
+					}
 					u, err := url.Parse(c.ISOChecksumURL)
 					if err != nil {
 						errs = append(errs,

common/iso_config_test.go

@@ -239,6 +239,16 @@ func TestISOConfigPrepare_ISOChecksumURL(t *testing.T) {
 	if i.ISOChecksum != "bar0" {
 		t.Fatalf("should've found \"bar0\" got: %s", i.ISOChecksum)
 	}
+
+	// Test that we won't try to read an iso into memory because of a user
+	// error
+	i = testISOConfig()
+	i.ISOChecksumURL = "file:///not_read.iso"
+	i.ISOChecksum = ""
+	warns, err = i.Prepare(nil)
+	if err == nil {
+		t.Fatalf("should have error because iso is bad filetype: %s", err)
+	}
 }
 
 func TestISOConfigPrepare_ISOChecksumType(t *testing.T) {