cmd/go: re-enable 'go list -m' with -mod=vendor for limited patterns

I had prohibited 'go list -m' with -mod=vendor because the module
graph is incomplete, but I've realized that many queries do not
actually require the full graph — and may, in fact, be driven using
modules previously reported by 'go list' for specific, vendored
packages. Queries for those modules should succeed.

Updates #33848

Change-Id: I1000b4cf586a830bb78faf620ebf62d73a3cb300
Reviewed-on: https://go-review.googlesource.com/c/go/+/203138
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/list/list.go

@@ -381,17 +381,22 @@ func runList(cmd *base.Command, args []string) {
 			base.Fatalf("go list -test cannot be used with -m")
 		}
 
-		buildModIsDefault := (cfg.BuildMod == "")
 		if modload.Init(); !modload.Enabled() {
 			base.Fatalf("go list -m: not using modules")
 		}
 
 		modload.InitMod() // Parses go.mod and sets cfg.BuildMod.
 		if cfg.BuildMod == "vendor" {
-			if buildModIsDefault {
-				base.Fatalf("go list -m: can't list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.")
-			} else {
-				base.Fatalf("go list -m: can't list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.")
+			for _, arg := range args {
+				// In vendor mode, the module graph is incomplete: it contains only the
+				// explicit module dependencies and the modules that supply packages in
+				// the import graph. Reject queries that imply more information than that.
+				if arg == "all" {
+					base.Fatalf("go list -m: can't compute 'all' using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")
+				}
+				if strings.Contains(arg, "...") {
+					base.Fatalf("go list -m: can't match module patterns using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")
+				}
 			}
 		}
 

src/cmd/go/internal/modload/list.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/modinfo"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
@@ -124,10 +125,20 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 					}
 					continue
 				}
-				mods = append(mods, &modinfo.ModulePublic{
-					Path:  arg,
-					Error: modinfoError(arg, "", errors.New("not a known dependency")),
-				})
+				if cfg.BuildMod == "vendor" {
+					// In vendor mode, we can't determine whether a missing module is “a
+					// known dependency” because the module graph is incomplete.
+					// Give a more explicit error message.
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("can't resolve module using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")),
+					})
+				} else {
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("not a known dependency")),
+					})
+				}
 			} else {
 				fmt.Fprintf(os.Stderr, "warning: pattern %q matched no module dependencies\n", arg)
 			}

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

@@ -10,7 +10,7 @@ stderr '\s*cd \.\. && go mod init'
 # The command we suggested should succeed.
 cd ..
 go mod init
-go list -mod=mod -m all
+go list -m
 stdout '^m$'
 
 -- $WORK/test/vendor/vendor.json --

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

@@ -10,7 +10,7 @@ stderr '\s*cd \.\. && go mod init'
 # The command we suggested should succeed.
 cd ..
 go mod init
-go list -mod=mod -m all
+go list -m
 stdout '^m$'
 
 -- $WORK/test/vendor/manifest --

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

@@ -11,10 +11,17 @@ stdout '^rsc.io/quote v1.5.1 .*vendor[\\/]rsc.io[\\/]quote$'
 stdout '^golang.org/x/text v0.0.0.* .*vendor[\\/]golang.org[\\/]x[\\/]text[\\/]language$'
 
 ! go list -mod=vendor -m rsc.io/quote@latest
-stderr 'go list -m: can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
+stderr 'go list -m: rsc.io/quote@latest: module lookup disabled by -mod=vendor'
 ! go get -mod=vendor -u
 stderr 'flag provided but not defined: -mod'
 
+# Since we don't have a complete module graph, 'go list -m' queries
+# that require the complete graph should fail with a useful error.
+! go list -mod=vendor -m all
+stderr 'go list -m: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+! go list -mod=vendor -m ...
+stderr 'go list -m: can''t match module patterns using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
 -- go.mod --
 module x
 

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

@@ -32,19 +32,28 @@ stdout 'src[\\/]x'
 go list -mod=vendor -f {{.Dir}} x
 stdout 'src[\\/]vendor[\\/]x'
 
-# 'go list -mod=vendor -m' does not have enough information to list modules
-# accurately, and should fail.
-! go list -mod=vendor -f {{.Dir}} -m x
-stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
+# 'go list -mod=vendor -m' should successfully list vendored modules,
+# but should not provide a module directory because no directory contains
+# the complete module.
+go list -mod=vendor -f '{{.Version}} {{.Dir}}' -m x
+stdout '^v1.0.0 $'
+
+# 'go list -mod=vendor -m' on a transitive dependency that does not
+# provide vendored packages should give a helpful error rather than
+# 'not a known dependency'.
+! go list -mod=vendor -f '{{.Version}} {{.Dir}}' -m diamondright
+stderr 'go list -m: module diamondright: can''t resolve module using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
 
 # 'go list -mod=mod' should report packages outside the import graph,
 # but 'go list -mod=vendor' should error out for them.
 go list -mod=mod -f {{.Dir}} w
 stdout 'src[\\/]w'
-
 ! go list -mod=vendor -f {{.Dir}} w
 stderr 'src[\\/]vendor[\\/]w'
 
+go list -mod=mod -f {{.Dir}} diamondright
+stdout 'src[\\/]diamondright'
+
 # Test dependencies should not be copied.
 ! exists vendor/x/testdata
 ! exists vendor/a/foo/bar/b/ignored.go
@@ -79,6 +88,8 @@ go fmt -mod=vendor ./...
 -- go.mod --
 module m
 
+go 1.13
+
 require (
 	a v1.0.0
 	diamondroot v0.0.0
@@ -264,10 +275,11 @@ require (
 -- diamondroot/x.go --
 package diamondroot
 
-import (
-	_ "diamondleft"
-	_ "diamondright"
-)
+import _ "diamondleft"
+-- diamondroot/unused/unused.go --
+package unused
+
+import _ "diamondright"
 -- diamondleft/go.mod --
 module diamondleft
 

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

@@ -17,10 +17,10 @@ stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
 
 ! go list -m all
-stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
+stderr 'go list -m: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
 
 ! go list -m -f '{{.Dir}}' all
-stderr 'can''t list modules with -mod=vendor\n\tUse -mod=mod or -mod=readonly to ignore the vendor directory.'
+stderr 'go list -m: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
 
 # An explicit -mod=mod should force the vendor directory to be ignored.
 env GOFLAGS=-mod=mod
@@ -106,10 +106,10 @@ stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
 # ...but 'go list -m' should continue to fail, this time without
 # referring to a -mod default that the user didn't set.
 ! go list -m all
-stderr 'can''t list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.'
+stderr 'go list -m: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
 
 ! go list -m -f '{{.Dir}}' all
-stderr 'can''t list modules using the vendor directory\n\tUse -mod=mod or -mod=readonly to ignore it.'
+stderr 'go list -m: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
 
 
 # 'go mod init' should work if there is already a GOPATH-mode vendor directory