cmd/go: detect inconsistent 'go get' version requests

If x v1.0.0 requires y v1.2.0, then
go get x@v1.0.0 y@v1.0.0 needs to fail gracefully.

Fixes #25917.

Change-Id: I9b426af23a30310fcb0c3545a8d97feb58b8ddbe
Reviewed-on: https://go-review.googlesource.com/124800Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modget/get.go

@@ -206,9 +206,15 @@ func runGet(cmd *base.Command, args []string) {
 
 	modload.LoadBuildList()
 
+	// Do not allow any updating of go.mod until we've applied
+	// all the requested changes and checked that the result matches
+	// what was requested.
+	modload.DisallowWriteGoMod()
+
 	// A task holds the state for processing a single get argument (path@vers).
 	type task struct {
-		arg             string           // original argument
+		arg             string // original argument
+		index           int
 		path            string           // package path part of arg
 		forceModulePath bool             // path must be interpreted as a module path
 		vers            string           // version part of arg
@@ -429,6 +435,7 @@ func runGet(cmd *base.Command, args []string) {
 			base.Fatalf("go get: %v", err)
 		}
 		required = upgraded[1:] // slice off upgradeTarget
+		base.ExitIfErrors()
 	}
 
 	// Put together the final build list as described above (1) (2) (3).
@@ -441,8 +448,9 @@ func runGet(cmd *base.Command, args []string) {
 	list = append(list, required...)
 	modload.SetBuildList(list)
 	modload.ReloadBuildList() // note: does not update go.mod
+	base.ExitIfErrors()
 
-	// Apply any needed downgrades.
+	// Scan for and apply any needed downgrades.
 	var down []module.Version
 	for _, m := range modload.BuildList() {
 		t := byPath[m.Path]
@@ -458,8 +466,64 @@ func runGet(cmd *base.Command, args []string) {
 		modload.SetBuildList(list)
 		modload.ReloadBuildList() // note: does not update go.mod
 	}
+	base.ExitIfErrors()
+
+	// Scan for any upgrades lost by the downgrades.
+	lost := make(map[string]string)
+	for _, m := range modload.BuildList() {
+		t := byPath[m.Path]
+		if t != nil && semver.Compare(m.Version, t.m.Version) != 0 {
+			lost[m.Path] = m.Version
+		}
+	}
+	if len(lost) > 0 {
+		desc := func(m module.Version) string {
+			s := m.Path + "@" + m.Version
+			t := byPath[m.Path]
+			if t != nil && t.arg != s {
+				s += " from " + t.arg
+			}
+			return s
+		}
+		downByPath := make(map[string]module.Version)
+		for _, d := range down {
+			downByPath[d.Path] = d
+		}
+		var buf strings.Builder
+		fmt.Fprintf(&buf, "go get: inconsistent versions:")
+		for _, t := range tasks {
+			if lost[t.m.Path] == "" {
+				continue
+			}
+			// We lost t because its build list requires a newer version of something in down.
+			// Figure out exactly what.
+			// Repeatedly constructing the build list is inefficient
+			// if there are MANY command-line arguments,
+			// but at least all the necessary requirement lists are cached at this point.
+			list, err := mvs.BuildList(t.m, reqs)
+			if err != nil {
+				base.Fatalf("go get: %v", err)
+			}
+
+			fmt.Fprintf(&buf, "\n\t%s", desc(t.m))
+			sep := " requires"
+			for _, m := range list {
+				if down, ok := downByPath[m.Path]; ok && semver.Compare(down.Version, m.Version) < 0 {
+					fmt.Fprintf(&buf, "%s %s@%s (not %s)", sep, m.Path, m.Version, desc(down))
+					sep = ","
+				}
+			}
+			if sep != "," {
+				// We have no idea why this happened.
+				// At least report the problem.
+				fmt.Fprintf(&buf, " ended up at %v unexpectedly (please report at golang.org/issue/new)", lost[t.m.Path])
+			}
+		}
+		base.Fatalf("%v", buf.String())
+	}
 
 	// Everything succeeded. Update go.mod.
+	modload.AllowWriteGoMod()
 	modload.WriteGoMod()
 
 	// If -m was specified, we're done after the module work. No download, no build.

src/cmd/go/internal/modload/init.go

@@ -465,8 +465,27 @@ func findImportComment(file string) string {
 	return path
 }
 
+var allowWriteGoMod = true
+
+// DisallowWriteGoMod causes future calls to WriteGoMod to do nothing at all.
+func DisallowWriteGoMod() {
+	allowWriteGoMod = false
+}
+
+// AllowWriteGoMod undoes the effect of DisallowWriteGoMod:
+// future calls to WriteGoMod will update go.mod if needed.
+// Note that any past calls have been discarded, so typically
+// a call to AlowWriteGoMod should be followed by a call to WriteGoMod.
+func AllowWriteGoMod() {
+	allowWriteGoMod = true
+}
+
 // WriteGoMod writes the current build list back to go.mod.
 func WriteGoMod() {
+	if !allowWriteGoMod {
+		return
+	}
+
 	modfetch.WriteGoSum()
 
 	if loaded != nil {

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

@@ -11,6 +11,17 @@ go get rsc.io/sampler@none
 go list -m all
 stdout 'rsc.io/quote v1.3.0'
 
+# downgrade should report inconsistencies and not change go.mod
+go get rsc.io/quote@v1.5.1
+go list -m all
+stdout 'rsc.io/quote v1.5.1'
+stdout 'rsc.io/sampler v1.3.0'
+! go get rsc.io/sampler@v1.0.0 rsc.io/quote@v1.5.2 golang.org/x/text@none
+stderr 'go get: inconsistent versions:\n\trsc.io/quote@v1.5.2 requires golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c \(not golang.org/x/text@none\), rsc.io/sampler@v1.3.0 \(not rsc.io/sampler@v1.0.0\)'
+go list -m all
+stdout 'rsc.io/quote v1.5.1'
+stdout 'rsc.io/sampler v1.3.0'
+
 -- go.mod --
 module x
 require rsc.io/quote v1.5.1