cmd/go: don't attempt to downgrade to incompatible versions

When we downgrade a module (using 'go get m@none' or similar), we
exclude versions of other modules that depend on it. We'll try
previous versions (in the "versions" list returned by the proxy or in
codeRepo.Versions for vcs) until we find a version that doesn't
require an excluded module version.

If older versions of a module are broken for some reason,
mvs.Downgrade currently panics. With this change, we ignore versions
with errors during downgrade.

A frequent cause of this is incompatible v2+ versions. These are
common if a repository tagged v2.0.0 before migrating to modules, then
tagged v2.0.1 with a go.mod file later. v2.0.0 is incorrectly
considered part of the v2 module.

Fixes #31942

Change-Id: Icaa75c5c93f73f18a400c22f18a8cc603aa4011a
Reviewed-on: https://go-review.googlesource.com/c/go/+/177337
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

@@ -13,7 +13,6 @@ import (
 	"sync"
 	"sync/atomic"
 
-	"cmd/go/internal/base"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
 )
@@ -118,7 +117,7 @@ func BuildList(target module.Version, reqs Reqs) ([]module.Version, error) {
 	return buildList(target, reqs, nil)
 }
 
-func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) module.Version) ([]module.Version, error) {
+func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (module.Version, error)) ([]module.Version, error) {
 	// Explore work graph in parallel in case reqs.Required
 	// does high-latency network operations.
 	type modGraphNode struct {
@@ -133,6 +132,10 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 		min      = map[string]string{} // maps module path to minimum required version
 		haveErr  int32
 	)
+	setErr := func(n *modGraphNode, err error) {
+		n.err = err
+		atomic.StoreInt32(&haveErr, 1)
+	}
 
 	var work par.Work
 	work.Add(target)
@@ -149,8 +152,7 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 
 		required, err := reqs.Required(m)
 		if err != nil {
-			node.err = err
-			atomic.StoreInt32(&haveErr, 1)
+			setErr(node, err)
 			return
 		}
 		node.required = required
@@ -159,9 +161,9 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 		}
 
 		if upgrade != nil {
-			u := upgrade(m)
-			if u.Path == "" {
-				base.Errorf("Upgrade(%v) returned zero module", m)
+			u, err := upgrade(m)
+			if err != nil {
+				setErr(node, err)
 				return
 			}
 			if u != m {
@@ -332,17 +334,12 @@ func Req(target module.Version, list []module.Version, base []string, reqs Reqs)
 // UpgradeAll returns a build list for the target module
 // in which every module is upgraded to its latest version.
 func UpgradeAll(target module.Version, reqs Reqs) ([]module.Version, error) {
-	return buildList(target, reqs, func(m module.Version) module.Version {
+	return buildList(target, reqs, func(m module.Version) (module.Version, error) {
 		if m.Path == target.Path {
-			return target
+			return target, nil
 		}
 
-		latest, err := reqs.Upgrade(m)
-		if err != nil {
-			panic(err) // TODO
-		}
-		m.Version = latest.Version
-		return m
+		return reqs.Upgrade(m)
 	})
 }
 
@@ -351,7 +348,7 @@ func UpgradeAll(target module.Version, reqs Reqs) ([]module.Version, error) {
 func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]module.Version, error) {
 	list, err := reqs.Required(target)
 	if err != nil {
-		panic(err) // TODO
+		return nil, err
 	}
 	// TODO: Maybe if an error is given,
 	// rerun with BuildList(upgrade[0], reqs) etc
@@ -370,7 +367,7 @@ func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]mod
 func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([]module.Version, error) {
 	list, err := reqs.Required(target)
 	if err != nil {
-		panic(err) // TODO
+		return nil, err
 	}
 	max := make(map[string]string)
 	for _, r := range list {
@@ -409,7 +406,17 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 		}
 		list, err := reqs.Required(m)
 		if err != nil {
-			panic(err) // TODO
+			// If we can't load the requirements, we couldn't load the go.mod file.
+			// There are a number of reasons this can happen, but this usually
+			// means an older version of the module had a missing or invalid
+			// go.mod file. For example, if example.com/mod released v2.0.0 before
+			// migrating to modules (v2.0.0+incompatible), then added a valid go.mod
+			// in v2.0.1, downgrading from v2.0.1 would cause this error.
+			//
+			// TODO(golang.org/issue/31730, golang.org/issue/30134): if the error
+			// is transient (we couldn't download go.mod), return the error from
+			// Downgrade. Currently, we can't tell what kind of error it is.
+			exclude(m)
 		}
 		for _, r := range list {
 			add(r)
@@ -429,7 +436,12 @@ List:
 		for excluded[r] {
 			p, err := reqs.Previous(r)
 			if err != nil {
-				return nil, err // TODO
+				// This is likely a transient error reaching the repository,
+				// rather than a permanent error with the retrieved version.
+				//
+				// TODO(golang.org/issue/31730, golang.org/issue/30134):
+				// decode what to do based on the actual error.
+				return nil, err
 			}
 			// If the target version is a pseudo-version, it may not be
 			// included when iterating over prior versions using reqs.Previous.

src/cmd/go/testdata/mod/example.com_downgrade_v2.0.0.txt

@@ -0,0 +1,9 @@
+example.com/downgrade v2.0.0
+written by hand
+
+-- .mod --
+module example.com/downgrade
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v2.0.0"}

src/cmd/go/testdata/mod/example.com_downgrade_v2_v2.0.1.txt

@@ -0,0 +1,13 @@
+example.com/downgrade/v2 v2.0.1
+written by hand
+
+-- .mod --
+module example.com/downgrade/v2
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v2.0.1"}
+-- go.mod --
+module example.com/downgrade/v2
+
+require rsc.io/quote v1.5.2

src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.0.txt

@@ -0,0 +1,14 @@
+example.com/latemigrate/v2 v2.0.0
+written by hand
+
+This repository migrated to modules in v2.0.1 after v2.0.0 was already tagged.
+All versions require rsc.io/quote so we can test downgrades.
+
+v2.0.0 is technically part of example.com/latemigrate as v2.0.0+incompatible.
+Proxies may serve it as part of the version list for example.com/latemigrate/v2.
+'go get' must be able to ignore these versions.
+
+-- .mod --
+module example.com/latemigrate
+-- .info --
+{"Version":"v2.0.0"}

src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.1.txt

@@ -0,0 +1,20 @@
+example.com/latemigrate/v2 v2.0.1
+written by hand
+
+This repository migrated to modules in v2.0.1 after v2.0.0 was already tagged.
+All versions require rsc.io/quote so we can test downgrades.
+
+v2.0.1 belongs to example.com/latemigrate/v2.
+
+-- .mod --
+module example.com/latemigrate/v2
+
+require rsc.io/quote v1.3.0
+-- .info --
+{"Version":"v2.0.1"}
+-- go.mod --
+module example.com/latemigrate/v2
+
+require rsc.io/quote v1.3.0
+-- late.go --
+package late

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

@@ -2,6 +2,7 @@ env GO111MODULE=on
 [short] skip
 
 # downgrade sampler should downgrade quote
+cp go.mod.orig go.mod
 go get rsc.io/sampler@v1.0.0
 go list -m all
 stdout 'rsc.io/quote v1.4.0'
@@ -31,9 +32,21 @@ stdout 'rsc.io/quote v1.4.0'
 stdout 'rsc.io/sampler v1.0.0'
 ! stdout golang.org/x/text
 
--- go.mod --
+# downgrading away quote should also downgrade away latemigrate/v2,
+# since there are no older versions. v2.0.0 is incompatible.
+cp go.mod.orig go.mod
+go list -m -versions example.com/latemigrate/v2
+stdout v2.0.0 # proxy may serve incompatible versions
+go get rsc.io/quote@none
+go list -m all
+! stdout 'example.com/latemigrate/v2'
+
+-- go.mod.orig --
 module x
-require rsc.io/quote v1.5.1
+require (
+	rsc.io/quote v1.5.1
+	example.com/latemigrate/v2 v2.0.1
+)
 -- go.mod.empty --
 module x
 -- x.go --