fix some tests and some config behavior to prevent null dereference errors and incorrect precedence between iso checksum and iso checksum url

common/iso_config.go

@@ -58,13 +58,19 @@ func (c *ISOConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs [
 	}
 
 	if c.ISOChecksumURL != "" {
-		if strings.HasSuffix(strings.ToLower(c.ISOChecksumURL), ".iso") {
-			errs = append(errs, fmt.Errorf("Error parsing checksum:"+
-				" .iso is not a valid checksum extension"))
+		if c.ISOChecksum != "" {
+			warnings = append(warnings, "You have provided both an "+
+				"iso_checksum and an iso_checksum_url. Discarding the "+
+				"iso_checksum_url and using the checksum.")
+		} else {
+			if strings.HasSuffix(strings.ToLower(c.ISOChecksumURL), ".iso") {
+				errs = append(errs, fmt.Errorf("Error parsing checksum:"+
+					" .iso is not a valid checksum extension"))
+			}
+			// go-getter auto-parses checksum files
+			c.ISOChecksumType = "file"
+			c.ISOChecksum = c.ISOChecksumURL
 		}
-		// go-getter auto-parses checksum files
-		c.ISOChecksumType = "file"
-		c.ISOChecksum = c.ISOChecksumURL
 	}
 
 	if c.ISOChecksum == "" {
@@ -86,11 +92,12 @@ func (c *ISOConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs [
 			Getters: getter.Getters,
 		}
 		cksum, err := gc.ChecksumFromFile(c.ISOChecksumURL, u)
-		if err != nil {
+		if cksum == nil || err != nil {
 			errs = append(errs, fmt.Errorf("Couldn't extract checksum from checksum file"))
+		} else {
+			c.ISOChecksumType = cksum.Type
+			c.ISOChecksum = hex.EncodeToString(cksum.Value)
 		}
-		c.ISOChecksumType = cksum.Type
-		c.ISOChecksum = hex.EncodeToString(cksum.Value)
 	}
 
 	return warnings, errs

common/iso_config_test.go

@@ -75,11 +75,16 @@ func TestISOConfigPrepare_ISOChecksum(t *testing.T) {
 func TestISOConfigPrepare_ISOChecksumURLBad(t *testing.T) {
 	i := testISOConfig()
 	i.ISOChecksumURL = "file:///not_read"
+	i.ISOChecksum = "shouldoverride"
 
 	// Test ISOChecksum overrides url
 	warns, err := i.Prepare(nil)
-	if len(warns) > 0 && len(err) > 0 {
-		t.Fatalf("bad: %#v, %#v", warns, err)
+	if len(warns) != 1 {
+		t.Fatalf("Bad: should have warned because both checksum and " +
+			"checksumURL are set.")
+	}
+	if len(err) > 0 {
+		t.Fatalf("Bad; should have warned but not errored.")
 	}
 
 	// Test that we won't try to read an iso into memory because of a user