cmd/go: check for source files in relative paths before attempting to determine the package path

This is a more minimial fix for the immediate symptom of 32917 and
30590, but does not improve 'list -e' behavior or error
messages resulting from other package loading issues.

Fixes #32917
Fixes #30590

Change-Id: I6088d14d864410159ebf228d9392d186322fd2a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/185417
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/load.go

@@ -100,11 +100,31 @@ func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 						dir = filepath.Clean(dir)
 					}
 
+					// golang.org/issue/32917: We should resolve a relative path to a
+					// package path only if the relative path actually contains the code
+					// for that package.
+					if !dirContainsPackage(dir) {
+						// If we're outside of a module, ensure that the failure mode
+						// indicates that.
+						ModRoot()
+
+						// If the directory is local but does not exist, don't return it
+						// while loader is iterating, since this might trigger a fetch.
+						// After loader is done iterating, we still need to return the
+						// path, so that "go list -e" produces valid output.
+						if !iterating {
+							// We don't have a valid path to resolve to, so report the
+							// unresolved path.
+							m.Pkgs = append(m.Pkgs, pkg)
+						}
+						continue
+					}
+
 					// Note: The checks for @ here are just to avoid misinterpreting
 					// the module cache directories (formerly GOPATH/src/mod/foo@v1.5.2/bar).
 					// It's not strictly necessary but helpful to keep the checks.
 					if modRoot != "" && dir == modRoot {
-						pkg = Target.Path
+						pkg = targetPrefix
 					} else if modRoot != "" && strings.HasPrefix(dir, modRoot+string(filepath.Separator)) && !strings.Contains(dir[len(modRoot):], "@") {
 						suffix := filepath.ToSlash(dir[len(modRoot):])
 						if strings.HasPrefix(suffix, "/vendor/") {
@@ -121,7 +141,13 @@ func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 								continue
 							}
 						} else {
-							pkg = Target.Path + suffix
+							modPkg := targetPrefix + suffix
+							if _, ok := dirInModule(modPkg, targetPrefix, modRoot, true); ok {
+								pkg = modPkg
+							} else if !iterating {
+								ModRoot()
+								base.Errorf("go: directory %s is outside main module", base.ShortPath(dir))
+							}
 						}
 					} else if sub := search.InDir(dir, cfg.GOROOTsrc); sub != "" && sub != "." && !strings.Contains(sub, "@") {
 						pkg = filepath.ToSlash(sub)
@@ -134,16 +160,6 @@ func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 							base.Errorf("go: directory %s outside available modules", base.ShortPath(dir))
 						}
 					}
-					info, err := os.Stat(dir)
-					if err != nil || !info.IsDir() {
-						// If the directory is local but does not exist, don't return it
-						// while loader is iterating, since this would trigger a fetch.
-						// After loader is done iterating, we still need to return the
-						// path, so that "go list -e" produces valid output.
-						if iterating {
-							continue
-						}
-					}
 					m.Pkgs = append(m.Pkgs, pkg)
 				}
 
@@ -247,6 +263,32 @@ func pathInModuleCache(dir string) string {
 	return ""
 }
 
+var dirContainsPackageCache sync.Map // absolute dir → bool
+
+func dirContainsPackage(dir string) bool {
+	isPkg, ok := dirContainsPackageCache.Load(dir)
+	if !ok {
+		_, err := cfg.BuildContext.ImportDir(dir, 0)
+		if err == nil {
+			isPkg = true
+		} else {
+			if fi, statErr := os.Stat(dir); statErr != nil || !fi.IsDir() {
+				// A non-directory or inaccessible directory is not a Go package.
+				isPkg = false
+			} else if _, noGo := err.(*build.NoGoError); noGo {
+				// A directory containing no Go source files is not a Go package.
+				isPkg = false
+			} else {
+				// An error other than *build.NoGoError indicates that the package exists
+				// but has some other problem (such as a syntax error).
+				isPkg = true
+			}
+		}
+		isPkg, _ = dirContainsPackageCache.LoadOrStore(dir, isPkg)
+	}
+	return isPkg.(bool)
+}
+
 // ImportFromFiles adds modules to the build list as needed
 // to satisfy the imports in the named Go source files.
 func ImportFromFiles(gofiles []string) {

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

+env GO111MODULE=on
+
+# golang.org/issue/32917 and golang.org/issue/28459: 'go build' and 'go test'
+# in an empty directory should refer to the path '.' and should not attempt
+# to resolve an external module.
+cd dir
+! go get .
+stderr 'go get \.: path .* is not a package in module rooted at .*[/\\]dir$'
+! go list
+! stderr 'cannot find module providing package'
+stderr '^can.t load package: package \.: no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir$'
+
+cd subdir
+! go list
+! stderr 'cannot find module providing package'
+stderr '^can.t load package: package \.: no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]subdir$'
+cd ..
+
+# golang.org/issue/30590: if a package is found in the filesystem
+# but is not in the main module, the error message should not say
+# "cannot find module providing package", and we shouldn't try
+# to find a module providing the package.
+! go list ./othermodule
+! stderr 'cannot find module providing package'
+stderr 'go: directory othermodule is outside main module'
+
+-- dir/go.mod --
+module example.com
+go 1.13
+-- dir/subdir/README --
+There are no Go source files in this directory.
+-- dir/othermodule/go.mod --
+module example.com/othermodule
+go 1.13
+-- dir/othermodule/om.go --
+package othermodule

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

@@ -34,11 +34,11 @@ stderr 'import lookup disabled'
 
 ! go build -mod=readonly ./nonexist
 ! stderr 'import lookup disabled'
-stderr 'unknown import path "m/nonexist": cannot find package'
+stderr '^can.t load package: package ./nonexist: cannot find package "." in:\n\t'$WORK'[/\\]gopath[/\\]src[/\\]x[/\\]nonexist$'
 
 ! go build -mod=readonly ./go.mod
 ! stderr 'import lookup disabled'
-stderr 'unknown import path "m/go.mod": cannot find package'
+stderr 'can.t load package: package ./go.mod: cannot find package'
 
 -- x/go.mod --
 module m

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

@@ -12,9 +12,10 @@ stdout ^math$
 go list -f '{{.ImportPath}}' .
 stdout ^x$
 ! go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
-stderr 'unknown import path "rsc.io/quote": cannot find package'
+stderr '^can.t load package: package '$WORK'[/\\]gopath/pkg/mod/rsc.io/quote@v1.5.2: can only use path@version syntax with .go get.'
+
 go list -e -f '{{with .Error}}{{.}}{{end}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
-stdout 'unknown import path "rsc.io/quote": cannot find package'
+stdout '^package '$WORK'[/\\]gopath/pkg/mod/rsc.io/quote@v1.5.2: can only use path@version syntax with .go get.'
 go mod download rsc.io/quote@v1.5.2
 go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
 stdout '^rsc.io/quote$'

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

@@ -6,7 +6,7 @@ env GO111MODULE=on
 go mod download
 
 ! go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
-stderr 'outside available modules'
+stderr 'can only use path@version syntax with .go get.'
 
 go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.1
 stdout 'rsc.io/quote'