[release-branch.go1.16] cmd/go: in 'go mod download' without args, don't save module zip sums

'go mod download' without arguments is frequently used to populate the
module cache. It tends to fetch a lot of extra files (for modules in
the build list that aren't needed to build packages in the main
module). It's annoying when sums are written for these extra files.

'go mod download mod@version' will still write sums for specific
modules in the build list. 'go mod download all' still has the
previous behavior.

For now, all invocations of 'go mod download' still update go.mod and
go.sum with changes needed to load the build list (1.15 behavior).

Fixes #46214

Change-Id: I9e17d18a7466ac7271a0e1a2b663f6b3cb168c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/318629
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 4fb10b2118cb16445f2d089f79beb3d32db3db12)
Reviewed-on: https://go-review.googlesource.com/c/go/+/321892
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

6 files changed, 75 insertions, 18 deletions

diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index e7d3d869cb..a602bb9bbe 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -86,9 +86,11 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	if !modload.HasModRoot() && len(args) == 0 {
 		base.Fatalf("go mod download: no modules specified (see 'go help mod download')")
 	}
-	if len(args) == 0 {
+	haveExplicitArgs := len(args) > 0
+	if !haveExplicitArgs {
 		args = []string{"all"}
-	} else if modload.HasModRoot() {
+	}
+	if modload.HasModRoot() {
 		modload.LoadModFile(ctx) // to fill Target
 		targetAtUpgrade := modload.Target.Path + "@upgrade"
 		targetAtPatch := modload.Target.Path + "@patch"
@@ -137,7 +139,20 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	listRetractions := false
 	type token struct{}
 	sem := make(chan token, runtime.GOMAXPROCS(0))
-	for _, info := range modload.ListModules(ctx, args, listU, listVersions, listRetractions) {
+	infos := modload.ListModules(ctx, args, listU, listVersions, listRetractions)
+	if !haveExplicitArgs {
+		// 'go mod download' is sometimes run without arguments to pre-populate
+		// the module cache. It may fetch modules that aren't needed to build
+		// packages in the main mdoule. This is usually not intended, so don't save
+		// sums for downloaded modules (golang.org/issue/45332).
+		// TODO(golang.org/issue/45551): For now, save sums needed to load the
+		// build list (same as 1.15 behavior). In the future, report an error if
+		// go.mod or go.sum need to be updated after loading the build list.
+		modload.WriteGoMod()
+		modload.DisallowWriteGoMod()
+	}
+
+	for _, info := range infos {
 		if info.Replace != nil {
 			info = info.Replace
 		}
@@ -187,6 +202,13 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 		base.ExitIfErrors()
 	}
 
-	// Update go.mod and especially go.sum if needed.
-	modload.WriteGoMod()
+	// If there were explicit arguments, update go.mod and especially go.sum.
+	// 'go mod download mod@version' is a useful way to add a sum without using
+	// 'go get mod@version', which may have other side effects. We print this in
+	// some error message hints.
+	//
+	// Don't save sums for 'go mod download' without arguments; see comment above.
+	if haveExplicitArgs {
+		modload.WriteGoMod()
+	}
 }
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
index 00b71bf0d5..7982cccea1 100644
--- a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
@@ -5,7 +5,7 @@ module "rsc.io/sampler"
 
 require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
 -- .info --
-{"Version":"v1.2.1","Name":"cac3af4f8a0ab40054fa6f8d423108a63a1255bb","Short":"cac3af4f8a0a","Time":"2018-02-13T18:16:22Z"}EOF
+{"Version":"v1.2.1","Name":"cac3af4f8a0ab40054fa6f8d423108a63a1255bb","Short":"cac3af4f8a0a","Time":"2018-02-13T18:16:22Z"}
 -- hello.go --
 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
diff --git a/src/cmd/go/testdata/script/mod_download.txt b/src/cmd/go/testdata/script/mod_download.txt
index 8a9faffe4e..c2b72b2a02 100644
--- a/src/cmd/go/testdata/script/mod_download.txt
+++ b/src/cmd/go/testdata/script/mod_download.txt
@@ -107,13 +107,28 @@ stderr '^go mod download: skipping argument m that resolves to the main module\n
 ! go mod download m@latest
 stderr '^go mod download: m@latest: malformed module path "m": missing dot in first path element$'
 
-# download updates go.mod and populates go.sum
+# download without arguments updates go.mod and go.sum after loading the
+# build list, but does not save sums for downloaded zips.
 cd update
+cp go.mod.orig go.mod
 ! exists go.sum
 go mod download
+cmp go.mod.update go.mod
+cmp go.sum.update go.sum
+cp go.mod.orig go.mod
+rm go.sum
+
+# download with arguments (even "all") does update go.mod and go.sum.
+go mod download rsc.io/sampler
+cmp go.mod.update go.mod
 grep '^rsc.io/sampler v1.3.0 ' go.sum
-go list -m rsc.io/sampler
-stdout '^rsc.io/sampler v1.3.0$'
+cp go.mod.orig go.mod
+rm go.sum
+
+go mod download all
+cmp go.mod.update go.mod
+grep '^rsc.io/sampler v1.3.0 ' go.sum
+cd ..
 
 # allow go mod download without go.mod
 env GO111MODULE=auto
@@ -131,7 +146,7 @@ stderr 'get '$GOPROXY
 -- go.mod --
 module m
 
--- update/go.mod --
+-- update/go.mod.orig --
 module m
 
 go 1.16
@@ -140,3 +155,16 @@ require (
 	rsc.io/quote v1.5.2
 	rsc.io/sampler v1.2.1 // older version than in build list
 )
+-- update/go.mod.update --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // older version than in build list
+)
+-- update/go.sum.update --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_get_trailing_slash.txt b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
index 3b38d8ba7d..c536693537 100644
--- a/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
+++ b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
@@ -1,6 +1,3 @@
-# Populate go.sum
-go mod download
-
 # go list should succeed to load a package ending with ".go" if the path does
 # not correspond to an existing local file. Listing a pattern ending with
 # ".go/" should try to list a package regardless of whether a file exists at the
@@ -31,3 +28,10 @@ module m
 go 1.13
 
 require example.com/dotgo.go v1.0.0
+-- go.sum --
+example.com/dotgo.go v1.0.0 h1:XKJfs0V8x2PvY2tX8bJBCEbCDLnt15ma2onwhVpew/I=
+example.com/dotgo.go v1.0.0/go.mod h1:Qi6z/X3AC5vHiuMt6HF2ICx3KhIBGrMdrA7YoPDKqR0=
+-- use.go --
+package use
+
+import _ "example.com/dotgo.go"
diff --git a/src/cmd/go/testdata/script/mod_query.txt b/src/cmd/go/testdata/script/mod_query.txt
index e10185709d..a75f86ed7c 100644
--- a/src/cmd/go/testdata/script/mod_query.txt
+++ b/src/cmd/go/testdata/script/mod_query.txt
@@ -1,9 +1,7 @@
 env GO111MODULE=on
 
-# Populate go.sum.
 # TODO(golang.org/issue/41297): we shouldn't need go.sum. None of the commands
 # below depend on the build list.
-go mod download
 
 go list -m -versions rsc.io/quote
 stdout '^rsc.io/quote v1.0.0 v1.1.0 v1.2.0 v1.2.1 v1.3.0 v1.4.0 v1.5.0 v1.5.1 v1.5.2 v1.5.3-pre1$'
@@ -36,6 +34,9 @@ stdout 'no matching versions for query ">v1.5.3"'
 module x
 require rsc.io/quote v1.0.0
 
+-- go.sum --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
 -- use.go --
 package use
 
diff --git a/src/cmd/go/testdata/script/mod_retract.txt b/src/cmd/go/testdata/script/mod_retract.txt
index a52e05bc72..4f95ece8d7 100644
--- a/src/cmd/go/testdata/script/mod_retract.txt
+++ b/src/cmd/go/testdata/script/mod_retract.txt
@@ -1,8 +1,5 @@
 cp go.mod go.mod.orig
 
-# Populate go.sum.
-go mod download
-
 # 'go list pkg' does not report an error when a retracted version is used.
 go list -e -f '{{if .Error}}{{.Error}}{{end}}' ./use
 ! stdout .
@@ -32,6 +29,11 @@ go 1.15
 
 require example.com/retract v1.0.0-bad
 
+-- go.sum --
+example.com/retract v1.0.0-bad h1:liAW69rbtjY67x2CcNzat668L/w+YGgNX3lhJsWIJis=
+example.com/retract v1.0.0-bad/go.mod h1:0DvGGofJ9hr1q63cBrOY/jSY52OwhRGA0K47NE80I5Y=
+example.com/retract/self/prev v1.1.0 h1:0/8I/GTG+1eJTFeDQ/fUbgrMsVHHyKhh3Z8DSZp1fuA=
+example.com/retract/self/prev v1.1.0/go.mod h1:xl2EcklWuZZHVtHWcpzfSJQmnzAGpKZYpA/Wto7SZN4=
 -- use/use.go --
 package use