os: drop unnecessary trailing . from symlink targets

Adjust splitPathInRoot to match its documented behavior
of dropping . path components except at the end of the path.
This function takes a prefix, path, and suffix; previously
it would preserve a trailing . at the end of the path
even when joining to a suffix.

The practical effect of this change is that we we'll skip
a pointless open of . when following a symlink under some
circumstances:

  - open "a/target"
  - "a" is a symlink to "b/."
  - previously: we rewrite our path to "b/./target"
  - now: we rewrite our path to "b/target"

This is a fairly unimportant edge case, and our observable
behavior isn't changing. The main motivation for this change is
that the overall behavior is more comprehensible if splitPathInRoot
follows its documentation.

Change-Id: I96c6a5e3f489cdac991ba1bd702180d69625bc64
Reviewed-on: https://go-review.googlesource.com/c/go/+/630615
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/root.go

@@ -230,6 +230,10 @@ func splitPathInRoot(s string, prefix, suffix []string) (_ []string, err error)
 		}
 		i = j
 	}
+	if len(suffix) > 0 && len(parts) > 0 && parts[len(parts)-1] == "." {
+		// Remove a trailing "." component if we're joining to a suffix.
+		parts = parts[:len(parts)-1]
+	}
 	parts = append(parts, suffix...)
 	return parts, nil
 }

src/os/root_test.go

@@ -242,6 +242,14 @@ var rootTestCases = []rootTest{{
 	},
 	open:   "b/../a/target",
 	target: "b/c/target",
+}, {
+	name: "symlink ends in dot",
+	fs: []string{
+		"a => b/.",
+		"b/",
+	},
+	open:   "a/target",
+	target: "b/target",
 }, {
 	name:        "directory does not exist",
 	fs:          []string{},