cmd/go: describe dependencies in build list error messages

mvs.BuildList reports errors with a chain of modules to make it
clear why the module where the error occurred was part of the
build. This is a little confusing with "go get -u" since there are
edges in the module graph for requirements and for updates.

With this change, we now print "requires" or "updates to" between
each module version in the chain.

Updates #30661

Change-Id: Ie689500ea86857e715b250b9e0cae0bc6686dc32
Reviewed-on: https://go-review.googlesource.com/c/go/+/171150
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/mvs/mvs.go

@@ -65,33 +65,41 @@ 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
-	Stack []module.Version
+	err   error
+	stack []buildListErrorElem
+}
+
+type buildListErrorElem struct {
+	m module.Version
+
+	// nextReason is the reason this module depends on the next module in the
+	// stack. Typically either "requires", or "upgraded to".
+	nextReason string
 }
 
 func (e *BuildListError) Error() string {
 	b := &strings.Builder{}
-	errMsg := e.Err.Error()
-	stack := e.Stack
+	errMsg := e.err.Error()
+	stack := e.stack
 
 	// Don't print modules at the beginning of the chain without a
 	// version. These always seem to be the main module or a
 	// synthetic module ("target@").
-	for len(stack) > 0 && stack[len(stack)-1].Version == "" {
+	for len(stack) > 0 && stack[len(stack)-1].m.Version == "" {
 		stack = stack[:len(stack)-1]
 	}
 
 	// Don't print the last module if the error message already
 	// starts with module path and version.
-	if len(stack) > 0 && strings.HasPrefix(errMsg, fmt.Sprintf("%s@%s: ", stack[0].Path, stack[0].Version)) {
-		// error already mentions module
-		stack = stack[1:]
-	}
-
-	for i := len(stack) - 1; i >= 0; i-- {
-		fmt.Fprintf(b, "%s@%s ->\n\t", stack[i].Path, stack[i].Version)
+	errMentionsLast := len(stack) > 0 && strings.HasPrefix(errMsg, fmt.Sprintf("%s@%s: ", stack[0].m.Path, stack[0].m.Version))
+	for i := len(stack) - 1; i >= 1; i-- {
+		fmt.Fprintf(b, "%s@%s %s\n\t", stack[i].m.Path, stack[i].m.Version, stack[i].nextReason)
 	}
+	if errMentionsLast || len(stack) == 0 {
 		b.WriteString(errMsg)
+	} else {
+		fmt.Fprintf(b, "%s@%s: %s", stack[0].m.Path, stack[0].m.Version, errMsg)
+	}
 	return b.String()
 }
 
@@ -168,9 +176,16 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 			q = q[1:]
 
 			if node.err != nil {
-				err := &BuildListError{Err: node.err}
-				for n := node; n != nil; n = neededBy[n] {
-					err.Stack = append(err.Stack, n.m)
+				err := &BuildListError{
+					err:   node.err,
+					stack: []buildListErrorElem{{m: node.m}},
+				}
+				for n, prev := neededBy[node], node; n != nil; n, prev = neededBy[n], n {
+					reason := "requires"
+					if n.upgrade == prev.m {
+						reason = "updating to"
+					}
+					err.stack = append(err.stack, buildListErrorElem{m: n.m, nextReason: reason})
 				}
 				return nil, err
 			}

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

@@ -8,18 +8,18 @@ go mod download example.com/badchain/a@v1.1.0
 go mod download example.com/badchain/b@v1.1.0
 go mod download example.com/badchain/c@v1.1.0
 
-# Try to upgrade example.com/badchain/a (and its dependencies).
+# Try to update example.com/badchain/a (and its dependencies).
 ! go get -u example.com/badchain/a
-cmp stderr upgrade-a-expected
+cmp stderr update-a-expected
 cmp go.mod go.mod.orig
 
-# Try to upgrade the main module. This upgrades everything, including
+# Try to update the main module. This updates everything, including
 # modules that aren't direct requirements, so the error stack is shorter.
 ! go get -u
-cmp stderr upgrade-main-expected
+cmp stderr update-main-expected
 cmp go.mod go.mod.orig
 
-# Upgrade 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
@@ -28,14 +28,14 @@ cmp stderr list-expected
 module m
 
 require example.com/badchain/a v1.0.0
--- upgrade-main-expected --
-go get: example.com/badchain/c@v1.0.0 ->
+-- 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"
--- upgrade-a-expected --
-go get: example.com/badchain/a@v1.1.0 ->
-	example.com/badchain/b@v1.1.0 ->
+-- update-a-expected --
+go get: 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-expected --
-go: example.com/badchain/a@v1.1.0 ->
-	example.com/badchain/b@v1.1.0 ->
+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"