Fix patching from dir with gomod generator

This is just a difference in how the states are stored when there is a
subpath in a directory that is the patch source (as compared to a patch
file).

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

source.go

@@ -527,8 +527,14 @@ func patchSource(worker, sourceState llb.State, sourceToState map[string]llb.Sta
 		patchState := sourceToState[p.Source]
 		// on each iteration, mount source state to /src to run `patch`, and
 		// set the state under /src to be the source state for the next iteration
+
+		subPath := p.Source
+		if p.Path != "" {
+			subPath = p.Path
+		}
+
 		sourceState = worker.Run(
-			llb.AddMount("/patch", patchState, llb.Readonly, llb.SourcePath(filepath.Join(p.Source, p.Path))),
+			llb.AddMount("/patch", patchState, llb.Readonly, llb.SourcePath(subPath)),
 			llb.Dir("src"),
 			ShArgs(fmt.Sprintf("patch -p%d < /patch", *p.Strip)),
 			WithConstraints(opts...),

test/source_test.go

@@ -260,6 +260,7 @@ index ea874f5..ba38f84 100644
 
 	// Note: module here should be moduyle+version because this is checking the go module path on disk
 	checkModule := func(ctx context.Context, gwc gwclient.Client, module string, spec *dalec.Spec) {
+		t.Helper()
 		res, err := gwc.Solve(ctx, newSolveRequest(withBuildTarget("debug/gomods"), withSpec(ctx, t, spec)))
 		if err != nil {
 			t.Fatal(err)
@@ -314,24 +315,49 @@ index ea874f5..ba38f84 100644
 	})
 
 	t.Run("with patch", func(t *testing.T) {
-		t.Parallel()
-		testEnv.RunTest(baseCtx, t, func(ctx context.Context, gwc gwclient.Client) {
-			spec := baseSpec()
+		t.Run("file", func(t *testing.T) {
+			t.Parallel()
+			testEnv.RunTest(baseCtx, t, func(ctx context.Context, gwc gwclient.Client) {
+				spec := baseSpec()
 
-			patchName := "patch"
-			spec.Sources[patchName] = dalec.Source{
-				Inline: &dalec.SourceInline{
-					File: &dalec.SourceInlineFile{
-						Contents: downgradePatch,
+				patchName := "patch"
+				spec.Sources[patchName] = dalec.Source{
+					Inline: &dalec.SourceInline{
+						File: &dalec.SourceInlineFile{
+							Contents: downgradePatch,
+						},
 					},
-				},
-			}
+				}
 
-			spec.Patches = map[string][]dalec.PatchSpec{
-				srcName: {{Source: patchName}},
-			}
+				spec.Patches = map[string][]dalec.PatchSpec{
+					srcName: {{Source: patchName}},
+				}
 
-			checkModule(ctx, gwc, "github.com/cpuguy83/tar2go@v0.3.0", spec)
+				checkModule(ctx, gwc, "github.com/cpuguy83/tar2go@v0.3.0", spec)
+			})
+		})
+		t.Run("dir", func(t *testing.T) {
+			t.Parallel()
+			testEnv.RunTest(baseCtx, t, func(ctx context.Context, gwc gwclient.Client) {
+				spec := baseSpec()
+
+				patchName := "patch"
+				spec.Sources[patchName] = dalec.Source{
+					Inline: &dalec.SourceInline{
+						Dir: &dalec.SourceInlineDir{
+							Files: map[string]*dalec.SourceInlineFile{
+								"patch-file": {Contents: downgradePatch},
+							},
+						},
+					},
+				}
+
+				spec.Patches = map[string][]dalec.PatchSpec{
+					srcName: {{Source: patchName, Path: "patch-file"}},
+				}
+
+				checkModule(ctx, gwc, "github.com/cpuguy83/tar2go@v0.3.0", spec)
+			})
 		})
 	})
 }