Support major module version paths in imports during save (robfig#47)

When an import that contains a (possible) major module version fails,
it is reattempted without the version component.

For example, imports like "sigs.k8s.io/structured-merge-diff/v4/value"
and "k8s.io/klog/v2" will fail because on disk, they reside in
GOPATH/src/sigs.k8s.io/structured-merge-diff/value & GOPATH/src/k8s.io/klog,
respectively.

save.go

@@ -134,6 +134,32 @@ func (p byImportPath) Len() int           { return len(p) }
 func (p byImportPath) Less(i, j int) bool { return p[i].root < p[j].root }
 func (p byImportPath) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
 
+var majorVersionComponent = regexp.MustCompile(`v[\d]+`)
+
+// pathWithoutMajorVersion returns path with the 1st major version /component
+// (if any) stripped out. If one was found, the 2nd return value is true.
+func pathWithoutMajorVersion(path string) (string, bool) {
+	parts := strings.Split(path, "/")
+	for idx, part := range strings.Split(path, "/") {
+		if majorVersionComponent.MatchString(part) {
+			return strings.Join(append(parts[:idx], parts[idx+1:]...), "/"), true
+		}
+	}
+	return path, false
+}
+
+// tryImport attempts to import the path as-is and, if it fails to be found and
+// path contains a major module version, reattempts with the version removed.
+func tryImport(ctx build.Context, path, srcDir string, mode build.ImportMode) (*build.Package, error) {
+	pkg, err := ctx.Import(path, srcDir, mode)
+	if err != nil && strings.HasPrefix(err.Error(), "cannot find package ") {
+		if versionlessPath, ok := pathWithoutMajorVersion(path); ok {
+			return ctx.Import(versionlessPath, srcDir, mode)
+		}
+	}
+	return pkg, err
+}
+
 // getAllDeps returns a slice of package import paths for all dependencies
 // (including test dependencies) of the given import path (and subpackages) and commands.
 func getAllDeps(importPath string, cmds []string) []string {
@@ -169,7 +195,7 @@ func getAllDeps(importPath string, cmds []string) []string {
 
 		// Add the subpackages.
 		for path := range buildutil.ExpandPatterns(&buildContext, []string{subpackagePrefix + "..."}) {
-			_, err := buildContext.Import(path, "", 0)
+			_, err := tryImport(buildContext, path, "", 0)
 			if _, ok := err.(*build.NoGoError); ok {
 				continue
 			}
@@ -179,7 +205,7 @@ func getAllDeps(importPath string, cmds []string) []string {
 
 		var addTransitiveClosure func(string)
 		addTransitiveClosure = func(path string) {
-			pkg, err := buildContext.Import(path, "", 0)
+			pkg, err := tryImport(buildContext, path, "", 0)
 			printLoadingError(path, err)
 
 			importPaths := append([]string(nil), pkg.Imports...)
@@ -194,7 +220,7 @@ func getAllDeps(importPath string, cmds []string) []string {
 				}
 
 				// Resolve the import path relative to the importing package.
-				if bp2, _ := buildContext.Import(path, pkg.Dir, build.FindOnly); bp2 != nil {
+				if bp2, _ := tryImport(buildContext, path, pkg.Dir, build.FindOnly); bp2 != nil {
 					path = bp2.ImportPath
 				}
 

save_test.go

@@ -204,6 +204,39 @@ var saveTests = []saveTest{
 			"github.com/test/p2",
 		},
 	},
+
+	{
+		"module major versions",
+		[]pkg{{
+			"github.com/test/p1",
+			[]file{
+				{"foo.go", false, []string{"github.com/test/p2/v2"}},
+				{"foo_test.go", false, []string{"github.com/test/p3/v2"}},
+			}}, {
+			"github.com/test/p2",
+			[]file{
+				{"foo.go", false, []string{"github.com/test/p4/v2/subpkg"}},
+			}}, {
+			"github.com/test/p3",
+			[]file{
+				{"v2/foo.go", false, []string{"os"}},
+			}}, {
+			"github.com/test/p4",
+			[]file{
+				{"subpkg/foo.go", false, []string{"github.com/test/p5"}},
+			}}, {
+			"github.com/test/p5",
+			[]file{
+				{"foo.go", false, []string{"os"}},
+			}},
+		},
+		[]string{
+			"github.com/test/p2",
+			"github.com/test/p3",
+			"github.com/test/p4",
+			"github.com/test/p5",
+		},
+	},
 }
 
 func TestSave(t *testing.T) {
@@ -315,3 +348,26 @@ import (
 		}
 	}
 }
+
+func TestPathWithoutMajorVersion(t *testing.T) {
+	tests := []struct{
+		path         string
+		expectedPath string
+		expectedBool bool
+	}{
+		{"github.com/p1", "github.com/p1", false},
+		{"github.com/p1/v1", "github.com/p1", true},
+		{"github.com/p2/p3", "github.com/p2/p3", false},
+		{"github.com/p1/v2/p3", "github.com/p1/p3", true},
+	}
+
+	for _, test := range tests {
+		actualPath, actualBool := pathWithoutMajorVersion(test.path)
+		if actualPath != test.expectedPath {
+			t.Errorf("%v: expected: %v got: %v", test.path, test.expectedPath, actualPath)
+		}
+		if actualBool != test.expectedBool {
+			t.Errorf("%v: expected: %v got: %v", test.path, test.expectedBool, actualBool)
+		}
+	}
+}