cmd/go/internal/modload: rework import resolution

modload.Import previously performed two otherwise-separable tasks:

1. Identify which module in the build list contains the requested
   package.

2. If no such module exists, search available modules to try to find
   the missing package.

This change splits those two tasks into two separate unexported
functions, and reports import-resolution errors by attaching them to
the package rather than emitting them directly to stderr. That allows
'list' to report the errors, but 'list -e' to ignore them.

With the two tasks now separate, it will be easier to avoid the
overhead of resolving missing packages during lazy loading if we
discover that some existing dependency needs to be promoted to the top
level (potentially altering the main module's selected versions, and
thus suppling packages that were previously missing).

For #36460
Updates #26909

Change-Id: I32bd853b266d7cd231d1f45f92b0650d95c4bcbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/251445
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index 6d81c1cad1..65003dc883 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -545,7 +545,7 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		// Note that -deps is applied after -test,
 		// so that you only get descriptions of tests for the things named
 		// explicitly on the command line, not for all dependencies.
-		pkgs = load.PackageList(pkgs)
+		pkgs = loadPackageList(pkgs)
 	}
 
 	// Do we need to run a build to gather information?
@@ -580,7 +580,7 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 	if *listTest {
 		all := pkgs
 		if !*listDeps {
-			all = load.PackageList(pkgs)
+			all = loadPackageList(pkgs)
 		}
 		// Update import paths to distinguish the real package p
 		// from p recompiled for q.test.
@@ -697,6 +697,23 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 	}
 }
 
+// loadPackageList is like load.PackageList, but prints error messages and exits
+// with nonzero status if listE is not set and any package in the expanded list
+// has errors.
+func loadPackageList(roots []*load.Package) []*load.Package {
+	pkgs := load.PackageList(roots)
+
+	if !*listE {
+		for _, pkg := range pkgs {
+			if pkg.Error != nil {
+				base.Errorf("%v", pkg.Error)
+			}
+		}
+	}
+
+	return pkgs
+}
+
 // TrackingWriter tracks the last byte written on every write so
 // we can avoid printing a newline if one was already written or
 // if there is no output at all.