cmd/go/internal/get: fix "mod over non-mod" preference for meta tags

If there was a mod and non-mod meta tag for a given prefix,
the meta tag extractor was already dropping the non-mod meta tag.
But we might have mod and non-mod meta tags with different
prefixes, in which case the mod tag should prevail when both match.

Fixes #26200.

Change-Id: I17ab361338e270b9fa03999ad1954f2bbe0f5017
Reviewed-on: https://go-review.googlesource.com/124714Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/get/pkg_test.go

@@ -98,6 +98,23 @@ var parseMetaGoImportsTests = []struct {
 		IgnoreMod,
 		[]metaImport{{"chitin.io/chitin", "git", "https://github.com/chitin-io/chitin"}},
 	},
+	{
+		`<meta name="go-import" content="myitcv.io git https://github.com/myitcv/x">
+	        <meta name="go-import" content="myitcv.io/blah2 mod https://raw.githubusercontent.com/myitcv/pubx/master">
+	        `,
+		IgnoreMod,
+		[]metaImport{{"myitcv.io", "git", "https://github.com/myitcv/x"}},
+	},
+	{
+		`<meta name="go-import" content="myitcv.io git https://github.com/myitcv/x">
+	        <meta name="go-import" content="myitcv.io/blah2 mod https://raw.githubusercontent.com/myitcv/pubx/master">
+	        `,
+		PreferMod,
+		[]metaImport{
+			{"myitcv.io/blah2", "mod", "https://raw.githubusercontent.com/myitcv/pubx/master"},
+			{"myitcv.io", "git", "https://github.com/myitcv/x"},
+		},
+	},
 }
 
 func TestParseMetaGoImports(t *testing.T) {

src/cmd/go/internal/get/vcs.go

@@ -948,7 +948,13 @@ func matchGoImport(imports []metaImport, importPath string) (metaImport, error)
 			continue
 		}
 
-		if match != -1 {
+		if match >= 0 {
+			if imports[match].VCS == "mod" && im.VCS != "mod" {
+				// All the mod entries precede all the non-mod entries.
+				// We have a mod entry and don't care about the rest,
+				// matching or not.
+				break
+			}
 			return metaImport{}, fmt.Errorf("multiple meta tags match import path %q", importPath)
 		}
 		match = i

src/cmd/go/internal/get/vcs_test.go

@@ -401,6 +401,22 @@ func TestMatchGoImport(t *testing.T) {
 			path: "different.example.com/user/foo",
 			err:  errors.New("meta tags do not match import path"),
 		},
+		{
+			imports: []metaImport{
+				{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+				{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+			},
+			path: "myitcv.io/blah2/foo",
+			mi:   metaImport{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+				{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+			},
+			path: "myitcv.io/other",
+			mi:   metaImport{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+		},
 	}
 
 	for _, test := range tests {