Re-land "Optimize path::remove_dots"

This reverts commit fb5fd74685e728b1d5e68d33e9842bcd734b98e6.
Re-instates commit 53913a65b408ade2956061b4c0aaed6bba907403

The fix is to trim off trailing separators, as in `/foo/bar/` and
produce `/foo/bar`. VFS tests rely on this. I added unit tests for
remove_dots.

lib/Support/Path.cpp

@@ -684,43 +684,69 @@ StringRef remove_leading_dotslash(StringRef Path, Style style) {
   return Path;
 }
 
-static SmallString<256> remove_dots(StringRef path, bool remove_dot_dot,
-                                    Style style) {
+// Remove path traversal components ("." and "..") when possible, and
+// canonicalize slashes.
+bool remove_dots(SmallVectorImpl<char> &the_path, bool remove_dot_dot,
+                 Style style) {
+  style = real_style(style);
+  StringRef remaining(the_path.data(), the_path.size());
+  bool needs_change = false;
   SmallVector<StringRef, 16> components;
 
-  // Skip the root path, then look for traversal in the components.
-  StringRef rel = path::relative_path(path, style);
-  for (StringRef C :
-       llvm::make_range(path::begin(rel, style), path::end(rel))) {
-    if (C == ".")
-      continue;
-    // Leading ".." will remain in the path unless it's at the root.
-    if (remove_dot_dot && C == "..") {
+  // Consume the root path, if present.
+  StringRef root = path::root_path(remaining, style);
+  bool absolute = !root.empty();
+  if (absolute)
+    remaining = remaining.drop_front(root.size());
+
+  // Loop over path components manually. This makes it easier to detect
+  // non-preferred slashes and double separators that must be canonicalized.
+  while (!remaining.empty()) {
+    size_t next_slash = remaining.find_first_of(separators(style));
+    if (next_slash == StringRef::npos)
+      next_slash = remaining.size();
+    StringRef component = remaining.take_front(next_slash);
+    remaining = remaining.drop_front(next_slash);
+
+    // Eat the slash, and check if it is the preferred separator.
+    if (!remaining.empty()) {
+      needs_change |= remaining.front() != preferred_separator(style);
+      remaining = remaining.drop_front();
+      // The path needs to be rewritten if it has a trailing slash.
+      // FIXME: This is emergent behavior that could be removed.
+      needs_change |= remaining.empty();
+    }
+
+    // Check for path traversal components or double separators.
+    if (component.empty() || component == ".") {
+      needs_change = true;
+    } else if (remove_dot_dot && component == "..") {
+      needs_change = true;
+      // Do not allow ".." to remove the root component. If this is the
+      // beginning of a relative path, keep the ".." component.
       if (!components.empty() && components.back() != "..") {
         components.pop_back();
-        continue;
+      } else if (!absolute) {
+        components.push_back(component);
       }
-      if (path::is_absolute(path, style))
-        continue;
+    } else {
+      components.push_back(component);
     }
-    components.push_back(C);
   }
 
-  SmallString<256> buffer = path::root_path(path, style);
-  for (StringRef C : components)
-    path::append(buffer, style, C);
-  return buffer;
-}
-
-bool remove_dots(SmallVectorImpl<char> &path, bool remove_dot_dot,
-                 Style style) {
-  StringRef p(path.data(), path.size());
-
-  SmallString<256> result = remove_dots(p, remove_dot_dot, style);
-  if (result == path)
+  // Avoid rewriting the path unless we have to.
+  if (!needs_change)
     return false;
 
-  path.swap(result);
+  SmallString<256> buffer = root;
+  if (!components.empty()) {
+    buffer += components[0];
+    for (StringRef C : makeArrayRef(components).drop_front()) {
+      buffer += preferred_separator(style);
+      buffer += C;
+    }
+  }
+  the_path.swap(buffer);
   return true;
 }
 

lib/Support/Windows/Path.inc

@@ -101,18 +101,13 @@ std::error_code widenPath(const Twine &Path8, SmallVectorImpl<wchar_t> &Path16,
   }
 
   // Remove '.' and '..' because long paths treat these as real path components.
+  llvm::sys::path::native(Path8Str, path::Style::windows);
   llvm::sys::path::remove_dots(Path8Str, true);
 
   const StringRef RootName = llvm::sys::path::root_name(Path8Str);
   assert(!RootName.empty() &&
          "Root name cannot be empty for an absolute path!");
 
-  // llvm::sys::path::remove_dots, used above, can leave a '/' after the root
-  // name and long paths must use '\' as the separator.
-  const size_t RootNameSize = RootName.size();
-  if (RootNameSize < Path8Str.size() && Path8Str[RootNameSize] == '/')
-    Path8Str[RootNameSize] = '\\';
-
   SmallString<2 * MAX_PATH> FullPath(LongPathPrefix);
   if (RootName[1] != ':') { // Check if UNC.
     FullPath.append("UNC\\");

unittests/Support/Path.cpp

@@ -1253,6 +1253,30 @@ TEST(Support, RemoveDots) {
             remove_dots("..\\a\\b\\..\\c", true, path::Style::windows));
   EXPECT_EQ("..\\..\\a\\c",
             remove_dots("..\\..\\a\\b\\..\\c", true, path::Style::windows));
+  EXPECT_EQ("C:\\a\\c", remove_dots("C:\\foo\\bar//..\\..\\a\\c", true,
+                                    path::Style::windows));
+
+  // FIXME: These leading forward slashes are emergent behavior. VFS depends on
+  // this behavior now.
+  EXPECT_EQ("C:/bar",
+            remove_dots("C:/foo/../bar", true, path::Style::windows));
+  EXPECT_EQ("C:/foo\\bar",
+            remove_dots("C:/foo/bar", true, path::Style::windows));
+  EXPECT_EQ("C:/foo\\bar",
+            remove_dots("C:/foo\\bar", true, path::Style::windows));
+  EXPECT_EQ("/", remove_dots("/", true, path::Style::windows));
+  EXPECT_EQ("C:/", remove_dots("C:/", true, path::Style::windows));
+
+  // Some clients of remove_dots expect it to remove trailing slashes. Again,
+  // this is emergent behavior that VFS relies on, and not inherently part of
+  // the specification.
+  EXPECT_EQ("C:\\foo\\bar",
+            remove_dots("C:\\foo\\bar\\", true, path::Style::windows));
+  EXPECT_EQ("/foo/bar",
+            remove_dots("/foo/bar/", true, path::Style::posix));
+
+  // A double separator is rewritten.
+  EXPECT_EQ("C:/foo\\bar", remove_dots("C:/foo//bar", true, path::Style::windows));
 
   SmallString<64> Path1(".\\.\\c");
   EXPECT_TRUE(path::remove_dots(Path1, true, path::Style::windows));
@@ -1271,6 +1295,11 @@ TEST(Support, RemoveDots) {
   EXPECT_EQ("/a/c", remove_dots("/../../a/c", true, path::Style::posix));
   EXPECT_EQ("/a/c",
             remove_dots("/../a/b//../././/c", true, path::Style::posix));
+  EXPECT_EQ("/", remove_dots("/", true, path::Style::posix));
+
+  // FIXME: Leaving behind this double leading slash seems like a bug.
+  EXPECT_EQ("//foo/bar",
+            remove_dots("//foo/bar/", true, path::Style::posix));
 
   SmallString<64> Path2("././c");
   EXPECT_TRUE(path::remove_dots(Path2, true, path::Style::posix));