cmd/go: print package import chains for some build list errors

When we construct the build list by loading packages (e.g., in
"go build", "go list", or "go test"), we may load additional modules
not mentioned in the original build list. If we encounter an error
loading one of these modules, mvs.BuildList currently returns a
BuildListError with a chain of requirments. Unfortunately, this is not
helpful, since the graph is structured such that these missing modules
are direct requirements of the main module.

With this change, loader.load keeps track of the package that caused
each "missing" module to be added. If an error occurs in a missing
module, the chain of package imports is printed instead of the module
requirements.

Fixes #31475

Change-Id: Ie484814af42ceea3e85fedc38e705ba3a38cd495
Reviewed-on: https://go-review.googlesource.com/c/go/+/171859
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/modload/load.go

@@ -546,6 +546,7 @@ func (ld *loader) load(roots func() []string) {
 		for _, m := range buildList {
 			haveMod[m] = true
 		}
+		modAddedBy := make(map[module.Version]*loadPkg)
 		for _, pkg := range ld.pkgs {
 			if err, ok := pkg.err.(*ImportMissingError); ok && err.Module.Path != "" {
 				if err.newMissingVersion != "" {
@@ -558,6 +559,7 @@ func (ld *loader) load(roots func() []string) {
 				numAdded++
 				if !haveMod[err.Module] {
 					haveMod[err.Module] = true
+					modAddedBy[err.Module] = pkg
 					buildList = append(buildList, err.Module)
 				}
 				continue
@@ -573,6 +575,14 @@ func (ld *loader) load(roots func() []string) {
 		reqs = Reqs()
 		buildList, err = mvs.BuildList(Target, reqs)
 		if err != nil {
+			// If an error was found in a newly added module, report the package
+			// import stack instead of the module requirement stack. Packages
+			// are more descriptive.
+			if err, ok := err.(*mvs.BuildListError); ok {
+				if pkg := modAddedBy[err.Module()]; pkg != nil {
+					base.Fatalf("go: %s: %v", pkg.stackText(), err.Err)
+				}
+			}
 			base.Fatalf("go: %v", err)
 		}
 	}
@@ -804,27 +814,33 @@ func (ld *loader) buildStacks() {
 // stackText builds the import stack text to use when
 // reporting an error in pkg. It has the general form
 //
-//	import root ->
-//		import other ->
-//		import other2 ->
-//		import pkg
+//	root imports
+//		other imports
+//		other2 tested by
+//		other2.test imports
+//		pkg
 //
 func (pkg *loadPkg) stackText() string {
 	var stack []*loadPkg
-	for p := pkg.stack; p != nil; p = p.stack {
+	for p := pkg; p != nil; p = p.stack {
 		stack = append(stack, p)
 	}
 
 	var buf bytes.Buffer
 	for i := len(stack) - 1; i >= 0; i-- {
 		p := stack[i]
+		fmt.Fprint(&buf, p.path)
 		if p.testOf != nil {
-			fmt.Fprintf(&buf, "test ->\n\t")
+			fmt.Fprint(&buf, ".test")
+		}
+		if i > 0 {
+			if stack[i-1].testOf == p {
+				fmt.Fprint(&buf, " tested by\n\t")
 			} else {
-			fmt.Fprintf(&buf, "import %q ->\n\t", p.path)
+				fmt.Fprint(&buf, " imports\n\t")
+			}
 		}
 	}
-	fmt.Fprintf(&buf, "import %q", pkg.path)
 	return buf.String()
 }
 

src/cmd/go/internal/mvs/mvs.go

@@ -65,7 +65,7 @@ type Reqs interface {
 // while constructing a build list. BuildListError prints the chain
 // of requirements to the module where the error occurred.
 type BuildListError struct {
-	err   error
+	Err   error
 	stack []buildListErrorElem
 }
 
@@ -77,9 +77,18 @@ type buildListErrorElem struct {
 	nextReason string
 }
 
+// Module returns the module where the error occurred. If the module stack
+// is empty, this returns a zero value.
+func (e *BuildListError) Module() module.Version {
+	if len(e.stack) == 0 {
+		return module.Version{}
+	}
+	return e.stack[0].m
+}
+
 func (e *BuildListError) Error() string {
 	b := &strings.Builder{}
-	errMsg := e.err.Error()
+	errMsg := e.Err.Error()
 	stack := e.stack
 
 	// Don't print modules at the beginning of the chain without a
@@ -177,7 +186,7 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 
 			if node.err != nil {
 				err := &BuildListError{
-					err:   node.err,
+					Err:   node.err,
 					stack: []buildListErrorElem{{m: node.m}},
 				}
 				for n, prev := neededBy[node], node; n != nil; n, prev = neededBy[n], n {

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

@@ -19,15 +19,39 @@ cmp go.mod go.mod.orig
 cmp stderr update-main-expected
 cmp go.mod go.mod.orig
 
-# update manually. Listing modules should produce an error.
+# Update manually. Listing modules should produce an error.
 go mod edit -require=example.com/badchain/a@v1.1.0
 ! go list -m
 cmp stderr list-expected
 
+# Try listing a package that imports a package
+# in a module without a requirement.
+go mod edit -droprequire example.com/badchain/a
+! go list m/use
+cmp stderr list-missing-expected
+
+! go list -test m/testuse
+cmp stderr list-missing-test-expected
+
 -- go.mod.orig --
 module m
 
 require example.com/badchain/a v1.0.0
+-- use/use.go --
+package use
+
+import _ "example.com/badchain/c"
+-- testuse/testuse.go --
+package testuse
+-- testuse/testuse_test.go --
+package testuse
+
+import (
+	"testing"
+	_ "example.com/badchain/c"
+)
+
+func Test(t *testing.T) {}
 -- update-main-expected --
 go get: example.com/badchain/c@v1.0.0 updating to
 	example.com/badchain/c@v1.1.0: parsing go.mod: unexpected module path "example.com/badchain/wrong"
@@ -39,3 +63,10 @@ go get: example.com/badchain/a@v1.1.0 requires
 go: example.com/badchain/a@v1.1.0 requires
 	example.com/badchain/b@v1.1.0 requires
 	example.com/badchain/c@v1.1.0: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+-- list-missing-expected --
+go: m/use imports
+	example.com/badchain/c: example.com/badchain/c@v1.1.0: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+-- list-missing-test-expected --
+go: m/testuse tested by
+	m/testuse.test imports
+	example.com/badchain/c: example.com/badchain/c@v1.1.0: parsing go.mod: unexpected module path "example.com/badchain/wrong"

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

@@ -1,7 +1,7 @@
 env GO111MODULE=on
 
 ! go list use.go
-stderr 'import "example.com/missingpkg/deprecated": package provided by example.com/missingpkg at latest version v1.0.0 but not at required version v1.0.1-beta'
+stderr 'example.com/missingpkg/deprecated: package provided by example.com/missingpkg at latest version v1.0.0 but not at required version v1.0.1-beta'
 
 -- use.go --
 package use