cmd/go/internal/modload: add a "v" prefix to the indexed go version

This allows semver-based comparisons of the version without additional allocations.

Also comment on the reason for the loops that iterate over modFile instead.

(I was reading the vendor code in order to add the lazy-loading version check,
and this section was a bit unclear to me.)

For #36460

Change-Id: I11559d81ffb4eba0e4e10e6fa3c01990b11f9180
Reviewed-on: https://go-review.googlesource.com/c/go/+/240622
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

1 files changed, 8 insertions, 6 deletions

diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index 9f4ec5a49f..9a166cae54 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -20,7 +20,7 @@ type modFileIndex struct {
 	data         []byte
 	dataNeedsFix bool // true if fixVersion applied a change while parsing data
 	module       module.Version
-	goVersion    string
+	goVersionV   string // GoVersion with "v" prefix
 	require      map[module.Version]requireMeta
 	replace      map[module.Version]module.Version
 	exclude      map[module.Version]bool
@@ -66,9 +66,11 @@ func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileInd
 		i.module = modFile.Module.Mod
 	}
 
-	i.goVersion = ""
+	i.goVersionV = ""
 	if modFile.Go != nil {
-		i.goVersion = modFile.Go.Version
+		// We're going to use the semver package to compare Go versions, so go ahead
+		// and add the "v" prefix it expects once instead of every time.
+		i.goVersionV = "v" + modFile.Go.Version
 	}
 
 	i.require = make(map[module.Version]requireMeta, len(modFile.Require))
@@ -114,11 +116,11 @@ func (i *modFileIndex) modFileIsDirty(modFile *modfile.File) bool {
 	}
 
 	if modFile.Go == nil {
-		if i.goVersion != "" {
+		if i.goVersionV != "" {
 			return true
 		}
-	} else if modFile.Go.Version != i.goVersion {
-		if i.goVersion == "" && cfg.BuildMod == "readonly" {
+	} else if "v"+modFile.Go.Version != i.goVersionV {
+		if i.goVersionV == "" && cfg.BuildMod == "readonly" {
 			// go.mod files did not always require a 'go' version, so do not error out
 			// if one is missing — we may be inside an older module in the module
 			// cache, and should bias toward providing useful behavior.