build: small readability improvements by reversal of conditions

This adds some ideomatic earlier returns and loop continuations
to avoid deeply nested coditional blocks that are several lines
long.

Signed-off-by: Ilya Dmitrichenko <errordeveloper@gmail.com>

build/build.go

@@ -691,7 +691,10 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 					return nil
 				}
 
-				if pushNames != "" {
+				if pushNames == "" {
+					return nil
+				}
+
 				progress.Write(pw, fmt.Sprintf("merging manifest list %s", pushNames), func() error {
 					descs := make([]specs.Descriptor, 0, len(res))
 
@@ -705,7 +708,11 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 							})
 						}
 					}
-						if len(descs) > 0 {
+
+					if len(descs) == 0 {
+						return nil
+					}
+
 					var imageopt imagetools.Opt
 					for _, dp := range dps {
 						imageopt = nodes[dp.driverIndex].ImageOpt
@@ -773,10 +780,10 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 						},
 					}
 					respMu.Unlock()
-						}
+
 					return nil
 				})
-				}
+
 				return nil
 			})
 
@@ -866,7 +873,9 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 										return nil, err
 									}
 									var reqErr *errdefs.UnsupportedSubrequestError
-									if !isFallback {
+									if isFallback {
+										return nil, err
+									}
 									if errors.As(err, &reqErr) {
 										switch reqErr.Name {
 										case "frontend.outline", "frontend.targets":
@@ -882,7 +891,6 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 										origErr = err
 										continue
 									}
-									}
 									return nil, err
 								}
 								if opt.PrintFunc != nil {
@@ -908,10 +916,15 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 						}
 
 						node := nodes[dp.driverIndex].Driver
-						if node.IsMobyDriver() {
+						if !node.IsMobyDriver() {
+							return nil
+						}
+
 						for _, e := range so.Exports {
 							if e.Type == "moby" && e.Attrs["push"] != "" {
-									if ok, _ := strconv.ParseBool(e.Attrs["push"]); ok {
+								if ok, _ := strconv.ParseBool(e.Attrs["push"]); !ok {
+									continue
+								}
 								pushNames = e.Attrs["name"]
 								if pushNames == "" {
 									return errors.Errorf("tag is needed when pushing to registry")
@@ -940,8 +953,7 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 								}
 							}
 						}
-							}
-						}
+
 						return nil
 					})
 
@@ -1315,7 +1327,11 @@ func waitContextDeps(ctx context.Context, index int, results *waitmap.Map, so *c
 			}
 			delete(so.FrontendAttrs, v)
 		}
-		if rr.Ref != nil {
+
+		if rr.Ref == nil {
+			continue
+		}
+
 		st, err := rr.Ref.ToState()
 		if err != nil {
 			return err
@@ -1337,7 +1353,6 @@ func waitContextDeps(ctx context.Context, index int, results *waitmap.Map, so *c
 			so.FrontendAttrs["input-metadata:"+k] = string(dt)
 		}
 	}
-	}
 	return nil
 }