cmd/go: allow querying other versions of the main module

'go mod download' and a few other commands can now query specific
versions of the main module.

'go get' still reports an error when attempting to update the main
module.

Fixes #42524

Change-Id: Ia93ef8f5f34443e938667c48a0db432200108c63
Reviewed-on: https://go-review.googlesource.com/c/go/+/270520
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

src/cmd/go/internal/modcmd/download.go

@@ -88,12 +88,11 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 		args = []string{"all"}
 	} else if modload.HasModRoot() {
 		modload.LoadModFile(ctx) // to fill Target
-		targetAtLatest := modload.Target.Path + "@latest"
 		targetAtUpgrade := modload.Target.Path + "@upgrade"
 		targetAtPatch := modload.Target.Path + "@patch"
 		for _, arg := range args {
 			switch arg {
-			case modload.Target.Path, targetAtLatest, targetAtUpgrade, targetAtPatch:
+			case modload.Target.Path, targetAtUpgrade, targetAtPatch:
 				os.Stderr.WriteString("go mod download: skipping argument " + arg + " that resolves to the main module\n")
 			}
 		}
@@ -170,7 +169,7 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 		for _, m := range mods {
 			b, err := json.MarshalIndent(m, "", "\t")
 			if err != nil {
-				base.Fatalf("%v", err)
+				base.Fatalf("go mod download: %v", err)
 			}
 			os.Stdout.Write(append(b, '\n'))
 			if m.Error != "" {
@@ -180,7 +179,7 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	} else {
 		for _, m := range mods {
 			if m.Error != "" {
-				base.Errorf("%s", m.Error)
+				base.Errorf("go mod download: %v", m.Error)
 			}
 		}
 		base.ExitIfErrors()

src/cmd/go/internal/modload/query.go

@@ -109,10 +109,7 @@ func queryProxy(ctx context.Context, proxy, path, query, current string, allowed
 		allowed = func(context.Context, module.Version) error { return nil }
 	}
 
-	if path == Target.Path {
-		if query != "upgrade" && query != "patch" {
-			return nil, &QueryMatchesMainModuleError{Pattern: path, Query: query}
-		}
+	if path == Target.Path && (query == "upgrade" || query == "patch") {
 		if err := allowed(ctx, Target); err != nil {
 			return nil, fmt.Errorf("internal error: main module version is not allowed: %w", err)
 		}
@@ -582,6 +579,7 @@ func QueryPattern(ctx context.Context, pattern, query string, current func(strin
 		}
 	}
 
+	var queryMatchesMainModule bool
 	if HasModRoot() {
 		m := match(Target, modRoot, true)
 		if len(m.Pkgs) > 0 {
@@ -605,7 +603,11 @@ func QueryPattern(ctx context.Context, pattern, query string, current func(strin
 			return nil, nil, err
 		}
 
-		if query != "upgrade" && query != "patch" && matchPattern(Target.Path) {
+		if matchPattern(Target.Path) {
+			queryMatchesMainModule = true
+		}
+
+		if (query == "upgrade" || query == "patch") && queryMatchesMainModule {
 			if err := allowed(ctx, Target); err == nil {
 				modOnly = &QueryResult{
 					Mod: Target,
@@ -620,14 +622,20 @@ func QueryPattern(ctx context.Context, pattern, query string, current func(strin
 		candidateModules = modulePrefixesExcludingTarget(base)
 	)
 	if len(candidateModules) == 0 {
-		if modOnly == nil {
+		if modOnly != nil {
+			return nil, modOnly, nil
+		} else if queryMatchesMainModule {
+			return nil, nil, &QueryMatchesMainModuleError{
+				Pattern: pattern,
+				Query:   query,
+			}
+		} else {
 			return nil, nil, &PackageNotInModuleError{
 				Mod:     Target,
 				Query:   query,
 				Pattern: pattern,
 			}
 		}
-		return nil, modOnly, nil
 	}
 
 	err = modfetch.TryProxies(func(proxy string) error {
@@ -675,6 +683,12 @@ func QueryPattern(ctx context.Context, pattern, query string, current func(strin
 		return err
 	})
 
+	if queryMatchesMainModule && len(results) == 0 && modOnly == nil && errors.Is(err, fs.ErrNotExist) {
+		return nil, nil, &QueryMatchesMainModuleError{
+			Pattern: pattern,
+			Query:   query,
+		}
+	}
 	return results[:len(results):len(results)], modOnly, err
 }
 

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

@@ -93,19 +93,19 @@ exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
 
 # download reports errors encountered when locating modules
 ! go mod download bad/path
-stderr '^module bad/path: not a known dependency$'
+stderr '^go mod download: module bad/path: not a known dependency$'
 ! go mod download bad/path@latest
-stderr '^bad/path@latest: malformed module path "bad/path": missing dot in first path element$'
+stderr '^go mod download: bad/path@latest: malformed module path "bad/path": missing dot in first path element$'
 ! go mod download rsc.io/[email protected]
-stderr '^rsc.io/[email protected]: reading .*/v1.999.999.info: 404 Not Found$'
+stderr '^go mod download: rsc.io/[email protected]: reading .*/v1.999.999.info: 404 Not Found$'
 ! go mod download -json bad/path
 stdout '^\t"Error": "module bad/path: not a known dependency"'
 
 # download main module produces a warning or error
 go mod download m
 stderr '^go mod download: skipping argument m that resolves to the main module\n'
 ! go mod download m@latest
-stderr 'm@latest: can''t request version "latest" of the main module \(m\)'
+stderr '^go mod download: m@latest: malformed module path "m": missing dot in first path element$'
 
 # download updates go.mod and populates go.sum
 cd update

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

@@ -30,10 +30,14 @@ grep 'rsc.io/quote v1.5.1' go.mod
 
 
 # The main module cannot be updated to a specific version.
+! go get -d [email protected]
+stderr '^go get: can''t request version "v0.1.0" of the main module \(rsc.io\)$'
+
+# A package in the main module can't be upgraded either.
 ! go get -d rsc.io/[email protected]
 stderr '^go get: package rsc.io/x is in the main module, so can''t request version v0.1.0$'
 
-# The main module cannot be updated to @latest, which is a specific version.
+# Nor can a pattern matching packages in the main module.
 ! go get -d rsc.io/x/...@latest
 stderr '^go get: pattern rsc.io/x/... matches package rsc.io/x in the main module, so can''t request version latest$'
 

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

@@ -0,0 +1,43 @@
+# 'go mod download' can download specific versions of the main module.
+go mod download rsc.io/quote@5d9f230b
+go mod download rsc.io/[email protected]
+go mod download rsc.io/quote@latest
+
+# 'go mod download' will not download @upgrade or @patch, since they always
+# resolve to the main module.
+go mod download rsc.io/quote@upgrade
+stderr '^go mod download: skipping argument rsc.io/quote@upgrade that resolves to the main module$'
+go mod download rsc.io/quote@patch
+stderr '^go mod download: skipping argument rsc.io/quote@patch that resolves to the main module$'
+
+# 'go list -m' can show a version of the main module.
+go list -m rsc.io/quote@5d9f230b
+stdout '^rsc.io/quote v0.0.0-20180710144737-5d9f230bcfba$'
+go list -m rsc.io/[email protected]
+stdout '^rsc.io/quote v1.5.2$'
+go list -m rsc.io/quote@latest
+stdout '^rsc.io/quote v1.5.2$'
+
+# 'go list -m -versions' shows available versions.
+go list -m -versions rsc.io/quote
+stdout '^rsc.io/quote.*v1.5.2'
+
+# 'go list -m' resolves @upgrade and @patch to the main module.
+go list -m rsc.io/quote@upgrade
+stdout '^rsc.io/quote$'
+go list -m rsc.io/quote@patch
+stdout '^rsc.io/quote$'
+
+# 'go get' will not attempt to upgrade the main module to any specific version.
+# See also: mod_get_main.txt.
+! go get rsc.io/quote@5d9f230b
+stderr '^go get: can''t request version "5d9f230b" of the main module \(rsc.io/quote\)$'
+! go get rsc.io/[email protected]
+stderr '^go get: can''t request version "v1.5.2" of the main module \(rsc.io/quote\)$'
+! go get rsc.io/quote@latest
+stderr '^go get: can''t request version "latest" of the main module \(rsc.io/quote\)$'
+
+-- go.mod --
+module rsc.io/quote
+
+go 1.16