cmd/go: scrub go.sum during go mod -sync

go.sum accumulates cruft as modules are added and removed as
direct and indirect dependencies. Instead of exposing all that cruft,
let "go mod -sync" clean it out.

Fixes #26381.

Change-Id: I7c9534cf7cc4579f7f82646d00ff691c87a13c4a
Reviewed-on: https://go-review.googlesource.com/124713
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 25 insertions, 1 deletions

diff --git a/src/cmd/go/internal/modcmd/mod.go b/src/cmd/go/internal/modcmd/mod.go
index b2769fd5d6..d01f699d88 100644
--- a/src/cmd/go/internal/modcmd/mod.go
+++ b/src/cmd/go/internal/modcmd/mod.go
@@ -14,6 +14,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modfile"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
@@ -154,7 +155,8 @@ effectively imply 'go mod -fix'.
 The -sync flag synchronizes go.mod with the source code in the module.
 It adds any missing modules necessary to build the current module's
 packages and dependencies, and it removes unused modules that
-don't provide any relevant packages.
+don't provide any relevant packages. It also adds any missing entries
+to go.sum and removes any unnecessary ones.
 
 The -vendor flag resets the module's vendor directory to include all
 packages needed to build and test all the module's packages.
@@ -291,6 +293,7 @@ func runMod(cmd *base.Command, args []string) {
 				}
 			}
 			modload.SetBuildList(keep)
+			modSyncGoSum() // updates memory copy; WriteGoMod on next line flushes it out
 		}
 		modload.WriteGoMod()
 		if *modVendor {
@@ -530,3 +533,24 @@ func modPrintGraph() {
 	}
 	w.Flush()
 }
+
+// modSyncGoSum resets the go.sum file content
+// to be exactly what's needed for the current go.mod.
+func modSyncGoSum() {
+	// Assuming go.sum already has at least enough from the successful load,
+	// we only have to tell modfetch what needs keeping.
+	reqs := modload.Reqs()
+	keep := make(map[module.Version]bool)
+	var walk func(module.Version)
+	walk = func(m module.Version) {
+		keep[m] = true
+		list, _ := reqs.Required(m)
+		for _, r := range list {
+			if !keep[r] {
+				walk(r)
+			}
+		}
+	}
+	walk(modload.Target)
+	modfetch.TrimGoSum(keep)
+}