cmd/go: reset modfetch state between modules in go work sync

go work sync resets the state in the modload package before each
iteration where it updates the workspace modules' go.mod files. But
before this change it wasn't resetting the global state in the modfetch
package. This is necessary because the modfetch package keeps track of
the sums that will be written to go.sum. Further, the fetch caches
will update information about which modules are used when fetching
packages, and so those caches need to be cleared between each workspace
module.

Thanks bcmills for helping me debug!

Fixes #50038

Change-Id: I5679c18a80feb7c5194c4a5f7e7129c7d198ef7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/376655
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modfetch/fetch.go

@@ -405,6 +405,28 @@ type modSumStatus struct {
 	used, dirty bool
 }
 
+// Reset resets globals in the modfetch package, so previous loads don't affect
+// contents of go.sum files
+func Reset() {
+	GoSumFile = ""
+	WorkspaceGoSumFiles = nil
+
+	// Uses of lookupCache and downloadCache both can call checkModSum,
+	// which in turn sets the used bit on goSum.status for modules.
+	// Reset them so used can be computed properly.
+	lookupCache = par.Cache{}
+	downloadCache = par.Cache{}
+
+	// Clear all fields on goSum. It will be initialized later
+	goSum.mu.Lock()
+	goSum.m = nil
+	goSum.w = nil
+	goSum.status = nil
+	goSum.overwrite = false
+	goSum.enabled = false
+	goSum.mu.Unlock()
+}
+
 // initGoSum initializes the go.sum data.
 // The boolean it returns reports whether the
 // use of go.sum is now enabled.

src/cmd/go/internal/modload/init.go

@@ -75,6 +75,7 @@ func EnterModule(ctx context.Context, enterModroot string) {
 	MainModules = nil // reset MainModules
 	requirements = nil
 	workFilePath = "" // Force module mode
+	modfetch.Reset()
 
 	modRoots = []string{enterModroot}
 	LoadModFile(ctx)

src/cmd/go/internal/workcmd/sync.go

@@ -108,13 +108,13 @@ func runSync(ctx context.Context, cmd *base.Command, args []string) {
 
 		modload.LoadPackages(ctx, modload.PackageOpts{
 			Tags:                     imports.AnyTags(),
+			Tidy:                     true,
 			VendorModulesInGOROOTSrc: true,
 			ResolveMissingImports:    false,
 			LoadTests:                true,
 			AllowErrors:              true,
+			SilenceMissingStdImports: true,
 			SilencePackageErrors:     true,
-			Tidy:                     true,
-			SilenceUnmatchedWarnings: true,
 		}, "all")
 		modload.WriteGoMod(ctx)
 	}

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

@@ -0,0 +1,40 @@
+# Test that the sum file data state is properly reset between modules in
+# go work sync so that the sum file that's written is correct.
+# Exercises the fix to #50038.
+
+cp b/go.sum b/go.sum.want
+
+# As a sanity check, verify b/go.sum is tidy.
+cd b
+go mod tidy
+cd ..
+cmp b/go.sum b/go.sum.want
+
+# Run go work sync and verify it doesn't change b/go.sum.
+go work sync
+cmp b/go.sum b/go.sum.want
+
+-- b/go.sum --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+-- go.work --
+go 1.18
+use (
+    ./a
+    ./b
+)
+replace example.com/c => ./c
+-- a/go.mod --
+module example.com/a
+go 1.18
+require rsc.io/fortune v1.0.0
+-- a/a.go --
+package a
+import "rsc.io/fortune"
+-- b/go.mod --
+module example.com/b
+go 1.18
+require rsc.io/quote v1.0.0
+-- b/b.go --
+package b
+import _ "rsc.io/quote"