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>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index 1137982e47..60d0d5b6e2 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/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)
 	}
 
@@ -110,12 +115,17 @@ 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")
 		}
 
-		for _, info := range modload.ListModules(latestArgs, listU, listVersions) {
-			if info.Version != "" {
-				latest[info.Path] = info.Version
+		if len(latestArgs) > 0 {
+			for _, info := range modload.ListModules(latestArgs, listU, listVersions) {
+				if info.Version != "" {
+					latest[info.Path] = info.Version
+				}
 			}
 		}
 	}
@@ -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()