cmd/go: be less strict about go version syntax in dependency go.mod files

It is unclear what the future holds for the go line in go.mod files.
Perhaps at some point we will switch to semver numbering.
Perhaps at some point we will allow specifying minor versions
or even betas and release candidates.
Those kinds of changes are difficult today because the go line
is parsed in dependency modules, meaning that older
versions of the Go toolchain need to understand newer go lines.

This CL makes that case - parsing a go line in a dependency's
go.mod file - a bit more lax about how to find the version.
It allows a leading v and any trailing non-digit-prefixed string
after the MAJOR.MINOR section.

There are no concrete plans to make use of any of these changes,
but if in the future we want to make them, having a few Go releases
under out belt that will accept the syntax in dependencies will
make any changes significantly easier.

Change-Id: I79bb84bba4b769048ac4b14d5c275eb9a3f270c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/317690
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go.mod

@@ -7,7 +7,7 @@ require (
 	github.com/ianlancetaylor/demangle v0.0.0-20200824232613-28f6c0f3b639 // indirect
 	golang.org/x/arch v0.0.0-20210502124803-cbf565b21d1e
 	golang.org/x/crypto v0.0.0-20210503195802-e9a32991a82e // indirect
-	golang.org/x/mod v0.4.3-0.20210504181020-67f1c1edc27a
+	golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd
 	golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744 // indirect
 	golang.org/x/term v0.0.0-20210503060354-a79de5458b56
 	golang.org/x/tools v0.1.1-0.20210505014545-7cab0ef2e9a5

src/cmd/go.sum

@@ -9,8 +9,8 @@ golang.org/x/arch v0.0.0-20210502124803-cbf565b21d1e h1:pv3V0NlNSh5Q6AX/StwGLBjc
 golang.org/x/arch v0.0.0-20210502124803-cbf565b21d1e/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
 golang.org/x/crypto v0.0.0-20210503195802-e9a32991a82e h1:8foAy0aoO5GkqCvAEJ4VC4P3zksTg4X4aJCDpZzmgQI=
 golang.org/x/crypto v0.0.0-20210503195802-e9a32991a82e/go.mod h1:P+XmwS30IXTQdn5tA2iutPOUgjI07+tq3H3K9MVA1s8=
-golang.org/x/mod v0.4.3-0.20210504181020-67f1c1edc27a h1:wbpC/7Wbo5WFVox32n+KjhRRLmTLq8YW/wRlL2iVAhk=
-golang.org/x/mod v0.4.3-0.20210504181020-67f1c1edc27a/go.mod h1:5OXOZSfqPIIbmVBIIKWRFfZjPR0E5r58TLhUjH0a2Ro=
+golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd h1:CuRnpyMrCCBulv0d/y0CswR4K0vGydgE3DZ2wYPIOo8=
+golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd/go.mod h1:5OXOZSfqPIIbmVBIIKWRFfZjPR0E5r58TLhUjH0a2Ro=
 golang.org/x/sys v0.0.0-20191204072324-ce4227a45e2e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744 h1:yhBbb4IRs2HS9PPlAg6DMC6mUOKexJBNsLf4Z+6En1Q=
 golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

src/cmd/go/testdata/script/badgo.txt

@@ -0,0 +1,50 @@
+go get example.net/badgo@v1.0.0
+go get example.net/badgo@v1.1.0
+go get example.net/badgo@v1.2.0
+go get example.net/badgo@v1.3.0
+go get example.net/badgo@v1.4.0
+go get example.net/badgo@v1.5.0
+! go get example.net/badgo@v1.6.0
+stderr 'invalid go version .X.Y.: must match format 1.23'
+
+-- go.mod --
+module m
+
+replace (
+	example.net/badgo v1.0.0 => ./v1.0.0
+	example.net/badgo v1.1.0 => ./v1.1.0
+	example.net/badgo v1.2.0 => ./v1.2.0
+	example.net/badgo v1.3.0 => ./v1.3.0
+	example.net/badgo v1.4.0 => ./v1.4.0
+	example.net/badgo v1.5.0 => ./v1.5.0
+	example.net/badgo v1.6.0 => ./v1.6.0
+)
+
+-- v1.0.0/go.mod --
+module example.net/badgo
+go 1.17.0
+
+-- v1.1.0/go.mod --
+module example.net/badgo
+go 1.17rc2
+
+-- v1.2.0/go.mod --
+module example.net/badgo
+go 1.17.1
+
+-- v1.3.0/go.mod --
+module example.net/badgo
+go v1.17.0
+
+-- v1.4.0/go.mod --
+module example.net/badgo
+go v1.17.0-rc.2
+
+-- v1.5.0/go.mod --
+module example.net/badgo
+go v1.17.1
+
+-- v1.6.0/go.mod --
+module example.net/badgo
+go X.Y
+

src/cmd/vendor/golang.org/x/mod/modfile/rule.go

@@ -217,6 +217,7 @@ func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (parse
 }
 
 var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
+var laxGoVersionRE = lazyregexp.New(`^v?(([1-9][0-9]*)\.(0|[1-9][0-9]*))([^0-9].*)$`)
 
 func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
 	// If strict is false, this module is a dependency.
@@ -267,8 +268,17 @@ func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, a
 			errorf("go directive expects exactly one argument")
 			return
 		} else if !GoVersionRE.MatchString(args[0]) {
-			errorf("invalid go version '%s': must match format 1.23", args[0])
-			return
+			fixed := false
+			if !strict {
+				if m := laxGoVersionRE.FindStringSubmatch(args[0]); m != nil {
+					args[0] = m[1]
+					fixed = true
+				}
+			}
+			if !fixed {
+				errorf("invalid go version '%s': must match format 1.23", args[0])
+				return
+			}
 		}
 
 		f.Go = &Go{Syntax: line}

src/cmd/vendor/modules.txt

@@ -28,7 +28,7 @@ golang.org/x/arch/x86/x86asm
 ## explicit; go 1.17
 golang.org/x/crypto/ed25519
 golang.org/x/crypto/ed25519/internal/edwards25519
-# golang.org/x/mod v0.4.3-0.20210504181020-67f1c1edc27a
+# golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd
 ## explicit; go 1.17
 golang.org/x/mod/internal/lazyregexp
 golang.org/x/mod/modfile