cmd/go: make 'go get' preserve sums for content of new requirements

This preserves zip sums when 'go get' is run on a module that does not
have a package in the root directory. The zip must be fetched to
determine whether the package should be loaded, so we already load and
verify the sum.

Note that 'go mod tidy' may still remove these sums, since they
aren't needed to load packages.

Fixes #41103

Change-Id: I78f10a25f0392461fdc98518a7c92a38ee3233c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/251880
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

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

@@ -862,14 +862,11 @@ func WriteGoMod() {
 		}
 	}
 
-	// Always update go.sum, even if we didn't change go.mod: we may have
-	// downloaded modules that we didn't have before.
-	modfetch.WriteGoSum(keepSums())
-
 	if !dirty && cfg.CmdName != "mod tidy" {
 		// The go.mod file has the same semantic content that it had before
 		// (but not necessarily the same exact bytes).
-		// Ignore any intervening edits.
+		// Don't write go.mod, but write go.sum in case we added or trimmed sums.
+		modfetch.WriteGoSum(keepSums(true))
 		return
 	}
 
@@ -880,6 +877,9 @@ func WriteGoMod() {
 	defer func() {
 		// At this point we have determined to make the go.mod file on disk equal to new.
 		index = indexModFile(new, modFile, false)
+
+		// Update go.sum after releasing the side lock and refreshing the index.
+		modfetch.WriteGoSum(keepSums(true))
 	}()
 
 	// Make a best-effort attempt to acquire the side lock, only to exclude
@@ -920,7 +920,10 @@ func WriteGoMod() {
 // the last load function like ImportPaths, LoadALL, etc.). It also contains
 // entries for go.mod files needed for MVS (the version of these entries
 // ends with "/go.mod").
-func keepSums() map[module.Version]bool {
+//
+// If addDirect is true, the set also includes sums for modules directly
+// required by go.mod, as represented by the index, with replacements applied.
+func keepSums(addDirect bool) map[module.Version]bool {
 	// Walk the module graph and keep sums needed by MVS.
 	modkey := func(m module.Version) module.Version {
 		return module.Version{Path: m.Path, Version: m.Version + "/go.mod"}
@@ -932,9 +935,6 @@ func keepSums() map[module.Version]bool {
 	walk = func(m module.Version) {
 		// If we build using a replacement module, keep the sum for the replacement,
 		// since that's the code we'll actually use during a build.
-		//
-		// TODO(golang.org/issue/29182): Perhaps we should keep both sums, and the
-		// sums for both sets of transitive requirements.
 		r := Replacement(m)
 		if r.Path == "" {
 			keep[modkey(m)] = true
@@ -964,9 +964,27 @@ func keepSums() map[module.Version]bool {
 		}
 	}
 
+	// Add entries for modules directly required by go.mod.
+	if addDirect {
+		for m := range index.require {
+			var kept module.Version
+			if r := Replacement(m); r.Path != "" {
+				kept = r
+			} else {
+				kept = m
+			}
+			keep[kept] = true
+			keep[module.Version{Path: kept.Path, Version: kept.Version + "/go.mod"}] = true
+		}
+	}
+
 	return keep
 }
 
 func TrimGoSum() {
-	modfetch.TrimGoSum(keepSums())
+	// Don't retain sums for direct requirements in go.mod. When TrimGoSum is
+	// called, go.mod has not been updated, and it may contain requirements on
+	// modules deleted from the build list.
+	addDirect := false
+	modfetch.TrimGoSum(keepSums(addDirect))
 }

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

@@ -0,0 +1,11 @@
+# When 'go get' is invoked on a module without a package in the root directory,
+# it should add sums for the module's go.mod file and its content to go.sum.
+# Verifies golang.org/issue/41103.
+go mod init m
+go get rsc.io/QUOTE
+grep '^rsc.io/QUOTE v1.5.2/go.mod ' go.sum
+grep '^rsc.io/QUOTE v1.5.2 ' go.sum
+
+# Double-check rsc.io/QUOTE does not have a root package.
+! go list -mod=readonly rsc.io/QUOTE
+stderr '^cannot find module providing package rsc.io/QUOTE: import lookup disabled by -mod=readonly$'