cmd/go: avoid accidental downgrades in 'go get' with latest and patch

Currently, 'go get -u' and 'go get -u=patch' avoid accidentally
downgrading modules by preventing upgrades in two cases:

1) If the current version is a prerelease that is semantically later
   than the "latest" or "patch" version.
2) If the current version is a pseudoversion that is chronologically
   newer than the "latest" or "patch" version.

With this change, 'go get m@latest' and 'go get m@patch' prevent
downgrades using the same checks.

Also: 'go get m@patch' now works if m is a module path but not a
package path (i.e., there is no package in the module root directory).

Fixes #30634
Fixes #32537

Change-Id: I916630c385b5f3ba7c13e0d65ba08f73a1a67829
Reviewed-on: https://go-review.googlesource.com/c/go/+/180337
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
index 5d5c3288b8..c571ddc5f5 100644
--- a/src/cmd/go/internal/modload/list.go
+++ b/src/cmd/go/internal/modload/list.go
@@ -55,18 +55,28 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 			base.Fatalf("go: cannot use relative path %s to specify module", arg)
 		}
 		if i := strings.Index(arg, "@"); i >= 0 {
-			info, err := Query(arg[:i], arg[i+1:], nil)
+			path := arg[:i]
+			vers := arg[i+1:]
+			var current string
+			for _, m := range buildList {
+				if m.Path == path {
+					current = m.Version
+					break
+				}
+			}
+
+			info, err := Query(path, vers, current, nil)
 			if err != nil {
 				mods = append(mods, &modinfo.ModulePublic{
-					Path:    arg[:i],
-					Version: arg[i+1:],
+					Path:    path,
+					Version: vers,
 					Error: &modinfo.ModuleError{
 						Err: err.Error(),
 					},
 				})
 				continue
 			}
-			mods = append(mods, moduleInfo(module.Version{Path: arg[:i], Version: info.Version}, false))
+			mods = append(mods, moduleInfo(module.Version{Path: path, Version: info.Version}, false))
 			continue
 		}
 
@@ -101,7 +111,7 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 					// Don't make the user provide an explicit '@latest' when they're
 					// explicitly asking what the available versions are.
 					// Instead, resolve the module, even if it isn't an existing dependency.
-					info, err := Query(arg, "latest", nil)
+					info, err := Query(arg, "latest", "", nil)
 					if err == nil {
 						mods = append(mods, moduleInfo(module.Version{Path: arg, Version: info.Version}, false))
 					} else {