cmd/go: report possibly-relevant ignored symlinks during pattern match

We can't follow symlinks for fear of directory cycles and other problems,
but we can at least notice potentially-relevant symlinks that are being
ignored and report them.

Fixes #17662.

Change-Id: I1fce00bd5b80ea8df45dac8b61bfa08076ec5f4b
Reviewed-on: https://go-review.googlesource.com/46425
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -2070,6 +2070,31 @@ func TestCaseCollisions(t *testing.T) {
 	tg.grepStderr("case-insensitive import collision", "go build example/a/pkg example/a/Pkg did not report import collision")
 }
 
+// Issue 17451, 17662.
+func TestSymlinkWarning(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.tempDir("src/example/xx")
+	tg.tempDir("yy/zz")
+	tg.tempFile("yy/zz/zz.go", "package zz\n")
+	if err := os.Symlink(tg.path("yy"), tg.path("src/example/xx/yy")); err != nil {
+		t.Skip("symlink failed: %v", err)
+	}
+	tg.run("list", "example/xx/z...")
+	tg.grepStdoutNot(".", "list should not have matched anything")
+	tg.grepStderr("matched no packages", "list should have reported that pattern matched no packages")
+	tg.grepStderrNot("symlink", "list should not have reported symlink")
+
+	tg.run("list", "example/xx/...")
+	tg.grepStdoutNot(".", "list should not have matched anything")
+	tg.grepStderr("matched no packages", "list should have reported that pattern matched no packages")
+	tg.grepStderr("ignoring symlink", "list should have reported symlink")
+}
+
 // Issue 8181.
 func TestGoGetDashTIssue8181(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)

src/cmd/go/internal/load/search.go

@@ -67,25 +67,39 @@ func MatchPackages(pattern string) []string {
 			root += "cmd" + string(filepath.Separator)
 		}
 		filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
-			if err != nil || !fi.IsDir() || path == src {
+			if err != nil || path == src {
 				return nil
 			}
 
+			want := true
 			// Avoid .foo, _foo, and testdata directory trees.
 			_, elem := filepath.Split(path)
 			if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
-				return filepath.SkipDir
+				want = false
 			}
 
 			name := filepath.ToSlash(path[len(src):])
 			if pattern == "std" && (!isStandardImportPath(name) || name == "cmd") {
 				// The name "std" is only the standard library.
 				// If the name is cmd, it's the root of the command tree.
-				return filepath.SkipDir
+				want = false
 			}
 			if !treeCanMatch(name) {
+				want = false
+			}
+
+			if !fi.IsDir() {
+				if fi.Mode()&os.ModeSymlink != 0 && want {
+					if target, err := os.Stat(path); err == nil && target.IsDir() {
+						fmt.Fprintf(os.Stderr, "warning: ignoring symlink %s\n", path)
+					}
+				}
+				return nil
+			}
+			if !want {
 				return filepath.SkipDir
 			}
+
 			if have[name] {
 				return nil
 			}