build: move main solve request into main gateway call

Now, we always perform the full solve request in the main gateway call.
This ensures that progress works properly, and makes the lifetime
semantics much clearer.

NewResultContext abstracts the details of a successful/failed build, to
always return a single ResultContext, even though the details of how a
gateway is created is different:
- For a failed build, we can just keep the gateway open.
- For a successful build, we immediately open another gateway and
  re-evaluate the build definition in that gateway. This should give an
  instant cache hit (since the build was just successful).

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/build.go

@@ -882,10 +882,11 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 
 					ch, done := progress.NewChannel(pw)
 					defer func() { <-done }()
+					_ = done
 
 					cc := c
 					var printRes map[string][]byte
-					rr, err := c.Build(ctx, so, "buildx", func(ctx context.Context, c gateway.Client) (*gateway.Result, error) {
+					buildFunc := func(ctx context.Context, c gateway.Client) (*gateway.Result, error) {
 						if opt.PrintFunc != nil {
 							if _, ok := req.FrontendOpt["frontend.caps"]; !ok {
 								req.FrontendOpt["frontend.caps"] = "moby.buildkit.frontend.subrequests+forward"
@@ -915,7 +916,6 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 							}
 
 							if fallback {
-								fmt.Println("falling back!")
 								req.FrontendOpt["build-arg:BUILDKIT_SYNTAX"] = printFallbackImage
 								res2, err2 := c.Solve(ctx, req)
 								if err2 != nil {
@@ -931,16 +931,16 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 						}
 
 						results.Set(resultKey(dp.driverIndex, k), res)
-						if resultHandleFunc != nil {
-							resultCtx, err := NewResultContext(ctx, cc, so, res)
-							if err == nil {
-								resultHandleFunc(dp.driverIndex, resultCtx)
-							} else {
-								logrus.Warnf("failed to record result: %s", err)
-							}
-						}
 						return res, nil
-					}, ch)
+					}
+					var rr *client.SolveResponse
+					if resultHandleFunc != nil {
+						var resultCtx *ResultContext
+						resultCtx, rr, err = NewResultContext(ctx, cc, so, "buildx", buildFunc, ch)
+						resultHandleFunc(dp.driverIndex, resultCtx)
+					} else {
+						rr, err = c.Build(ctx, so, "buildx", buildFunc, ch)
+					}
 					if err != nil {
 						return err
 					}