cmd/go/internal/modfile: report error for extra text around version

Fixes #34697

Change-Id: Iedfa3d46d558510f3bd1fdf9466cd974793d9ecd
Reviewed-on: https://go-review.googlesource.com/c/go/+/199017
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfile/read_test.go

@@ -363,3 +363,26 @@ func TestModulePath(t *testing.T) {
 		})
 	}
 }
+
+func TestGoVersion(t *testing.T) {
+	for _, test := range []struct {
+		desc, input string
+		ok          bool
+	}{
+		{desc: "empty", input: "module m\ngo \n", ok: false},
+		{desc: "one", input: "module m\ngo 1\n", ok: false},
+		{desc: "two", input: "module m\ngo 1.22\n", ok: true},
+		{desc: "three", input: "module m\ngo 1.22.333", ok: false},
+		{desc: "before", input: "module m\ngo v1.2\n", ok: false},
+		{desc: "after", input: "module m\ngo 1.2rc1\n", ok: false},
+		{desc: "space", input: "module m\ngo 1.2 3.4\n", ok: false},
+	} {
+		t.Run(test.desc, func(t *testing.T) {
+			if _, err := Parse("go.mod", []byte(test.input), nil); err == nil && !test.ok {
+				t.Error("unexpected success")
+			} else if err != nil && test.ok {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}

src/cmd/go/internal/modfile/rule.go

@@ -153,7 +153,7 @@ func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (*File
 	return f, nil
 }
 
-var GoVersionRE = lazyregexp.New(`([1-9][0-9]*)\.(0|[1-9][0-9]*)`)
+var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
 
 func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
 	// If strict is false, this module is a dependency.