cmd/go: improve 'go mod download' and 'go list -m' error messages

modload.ListModules now wraps errors as module.ModuleError as
appropriate. The resulting errors always include the module path and
will include the version, if known.

'go mod download' no longer ignores errors reported by ListModules.
Previously, it started requesting module info, go.mod, and zip. Those
requests would fail, overwriting the original failure. They were
usually less descriptive.

'go mod download' with a module not in the build list (and no version
query) is now an error. Previously, this was silently ignored.

Fixes #30743

Change-Id: Icee8c1c6c5240de135a8b6ba42d6bbcdb757cdac
Reviewed-on: https://go-review.googlesource.com/c/go/+/189323
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -390,7 +390,7 @@ func runList(cmd *base.Command, args []string) {
 		if !*listE {
 			for _, m := range mods {
 				if m.Error != nil {
-					base.Errorf("go list -m %s: %v", m.Path, m.Error.Err)
+					base.Errorf("go list -m: %v", m.Error.Err)
 				}
 			}
 			base.ExitIfErrors()

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

@@ -89,7 +89,8 @@ func runDownload(cmd *base.Command, args []string) {
 		if info.Replace != nil {
 			info = info.Replace
 		}
-		if info.Version == "" {
+		if info.Version == "" && info.Error == nil {
+			// main module
 			continue
 		}
 		m := &moduleJSON{
@@ -97,6 +98,10 @@ func runDownload(cmd *base.Command, args []string) {
 			Version: info.Version,
 		}
 		mods = append(mods, m)
+		if info.Error != nil {
+			m.Error = info.Error.Err
+			continue
+		}
 		work.Add(m)
 	}
 
@@ -110,15 +115,20 @@ func runDownload(cmd *base.Command, args []string) {
 		// downloading the modules.
 		var latestArgs []string
 		for _, m := range mods {
+			if m.Error != "" {
+				continue
+			}
 			latestArgs = append(latestArgs, m.Path+"@latest")
 		}
 
+		if len(latestArgs) > 0 {
 			for _, info := range modload.ListModules(latestArgs, listU, listVersions) {
 				if info.Version != "" {
 					latest[info.Path] = info.Version
 				}
 			}
 		}
+	}
 
 	work.Do(10, func(item interface{}) {
 		m := item.(*moduleJSON)
@@ -169,7 +179,7 @@ func runDownload(cmd *base.Command, args []string) {
 	} else {
 		for _, m := range mods {
 			if m.Error != "" {
-				base.Errorf("%s@%s: %s\n", m.Path, m.Version, m.Error)
+				base.Errorf("%s", m.Error)
 			}
 		}
 		base.ExitIfErrors()

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

@@ -5,6 +5,7 @@
 package modload
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -70,9 +71,7 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 				mods = append(mods, &modinfo.ModulePublic{
 					Path:    path,
 					Version: vers,
-					Error: &modinfo.ModuleError{
-						Err: err.Error(),
-					},
+					Error:   modinfoError(path, vers, err),
 				})
 				continue
 			}
@@ -117,18 +116,14 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 					} else {
 						mods = append(mods, &modinfo.ModulePublic{
 							Path:  arg,
-							Error: &modinfo.ModuleError{
-								Err: err.Error(),
-							},
+							Error: modinfoError(arg, "", err),
 						})
 					}
 					continue
 				}
 				mods = append(mods, &modinfo.ModulePublic{
 					Path:  arg,
-					Error: &modinfo.ModuleError{
-						Err: fmt.Sprintf("module %q is not a known dependency", arg),
-					},
+					Error: modinfoError(arg, "", errors.New("not a known dependency")),
 				})
 			} else {
 				fmt.Fprintf(os.Stderr, "warning: pattern %q matched no module dependencies\n", arg)
@@ -138,3 +133,21 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 
 	return mods
 }
+
+// modinfoError wraps an error to create an error message in
+// modinfo.ModuleError with minimal redundancy.
+func modinfoError(path, vers string, err error) *modinfo.ModuleError {
+	var nerr *NoMatchingVersionError
+	var merr *module.ModuleError
+	if errors.As(err, &nerr) {
+		// NoMatchingVersionError contains the query, so we don't mention the
+		// query again in ModuleError.
+		err = &module.ModuleError{Path: path, Err: err}
+	} else if !errors.As(err, &merr) {
+		// If the error does not contain path and version, wrap it in a
+		// module.ModuleError.
+		err = &module.ModuleError{Path: path, Version: vers, Err: err}
+	}
+
+	return &modinfo.ModuleError{Err: err.Error()}
+}

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

@@ -85,6 +85,16 @@ exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
 go mod download -json rsc.io/quote@v1.5.1
 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$'
+! go mod download bad/path@latest
+stderr '^bad/path@latest: malformed module path "bad/path": missing dot in first path element$'
+! go mod download rsc.io/quote@v1.999.999
+stderr '^rsc.io/quote@v1.999.999: reading .*/v1.999.999.info: 404 Not Found$'
+! go mod download -json bad/path
+stdout '^\t"Error": "module bad/path: not a known dependency"'
+
 # allow go mod download without go.mod
 env GO111MODULE=auto
 rm go.mod

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

@@ -34,12 +34,12 @@ go list rsc.io/quote/buggy
 
 # rsc.io/quote/buggy should not be listable as a module
 go list -m -e -f '{{.Error.Err}}' nonexist rsc.io/quote/buggy
-stdout '^module "nonexist" is not a known dependency'
-stdout '^module "rsc.io/quote/buggy" is not a known dependency'
+stdout '^module nonexist: not a known dependency$'
+stdout '^module rsc.io/quote/buggy: not a known dependency$'
 
 ! go list -m nonexist rsc.io/quote/buggy
-stderr '^go list -m nonexist: module "nonexist" is not a known dependency'
-stderr '^go list -m rsc.io/quote/buggy: module "rsc.io/quote/buggy" is not a known dependency'
+stderr '^go list -m: module nonexist: not a known dependency'
+stderr '^go list -m: module rsc.io/quote/buggy: not a known dependency'
 
 # Module loader does not interfere with list -e (golang.org/issue/24149).
 go list -e -f '{{.Error.Err}}' database

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

@@ -22,7 +22,7 @@ go list -m rsc.io/quote@<v1.5.4
 stdout 'rsc.io/quote v1.5.2$'
 
 ! go list -m rsc.io/quote@>v1.5.3
-stderr 'go list -m rsc.io/quote: no matching versions for query ">v1.5.3"'
+stderr 'go list -m: module rsc.io/quote: no matching versions for query ">v1.5.3"'
 
 go list -m -e -f '{{.Error.Err}}' rsc.io/quote@>v1.5.3
 stdout 'no matching versions for query ">v1.5.3"'