Add CanonPath wrapper to represent canonicalized paths

src/libutil/canon-path.cc

@@ -0,0 +1,103 @@
+#include "canon-path.hh"
+#include "util.hh"
+
+namespace nix {
+
+CanonPath CanonPath::root = CanonPath("/");
+
+CanonPath::CanonPath(std::string_view raw)
+    : path(absPath((Path) raw, "/"))
+{ }
+
+CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
+    : path(absPath((Path) raw, root.abs()))
+{ }
+
+std::optional<CanonPath> CanonPath::parent() const
+{
+    if (isRoot()) return std::nullopt;
+    return CanonPath(unchecked_t(), path.substr(0, std::max((size_t) 1, path.rfind('/'))));
+}
+
+void CanonPath::pop()
+{
+    assert(!isRoot());
+    path.resize(std::max((size_t) 1, path.rfind('/')));
+}
+
+bool CanonPath::isWithin(const CanonPath & parent) const
+{
+    return !(
+        path.size() < parent.path.size()
+        || path.substr(0, parent.path.size()) != parent.path
+        || (parent.path.size() > 1 && path.size() > parent.path.size()
+            && path[parent.path.size()] != '/'));
+}
+
+CanonPath CanonPath::removePrefix(const CanonPath & prefix) const
+{
+    assert(isWithin(prefix));
+    if (prefix.isRoot()) return *this;
+    if (path.size() == prefix.path.size()) return root;
+    return CanonPath(unchecked_t(), path.substr(prefix.path.size()));
+}
+
+void CanonPath::extend(const CanonPath & x)
+{
+    if (x.isRoot()) return;
+    if (isRoot())
+        path += x.rel();
+    else
+        path += x.abs();
+}
+
+CanonPath CanonPath::operator + (const CanonPath & x) const
+{
+    auto res = *this;
+    res.extend(x);
+    return res;
+}
+
+void CanonPath::push(std::string_view c)
+{
+    assert(c.find('/') == c.npos);
+    assert(c != "." && c != "..");
+    if (!isRoot()) path += '/';
+    path += c;
+}
+
+CanonPath CanonPath::operator + (std::string_view c) const
+{
+    auto res = *this;
+    res.push(c);
+    return res;
+}
+
+bool CanonPath::isAllowed(const std::set<CanonPath> & allowed) const
+{
+    /* Check if `this` is an exact match or the parent of an
+       allowed path. */
+    auto lb = allowed.lower_bound(*this);
+    if (lb != allowed.end()) {
+        if (lb->isWithin(*this))
+            return true;
+    }
+
+    /* Check if a parent of `this` is allowed. */
+    auto path = *this;
+    while (!path.isRoot()) {
+        path.pop();
+        if (allowed.count(path))
+            return true;
+    }
+
+    return false;
+}
+
+std::ostream & operator << (std::ostream & stream, const CanonPath & path)
+{
+    stream << path.abs();
+    return stream;
+}
+
+}

src/libutil/canon-path.hh

@@ -0,0 +1,173 @@
+#pragma once
+
+#include <string>
+#include <optional>
+#include <cassert>
+#include <iostream>
+#include <set>
+
+namespace nix {
+
+/* A canonical representation of a path. It ensures the following:
+
+   - It always starts with a slash.
+
+   - It never ends with a slash, except if the path is "/".
+
+   - A slash is never followed by a slash (i.e. no empty components).
+
+   - There are no components equal to '.' or '..'.
+
+   Note that the path does not need to correspond to an actually
+   existing path, and there is no guarantee that symlinks are
+   resolved.
+*/
+class CanonPath
+{
+    std::string path;
+
+public:
+
+    /* Construct a canon path from a non-canonical path. Any '.', '..'
+       or empty components are removed. */
+    CanonPath(std::string_view raw);
+
+    explicit CanonPath(const char * raw)
+        : CanonPath(std::string_view(raw))
+    { }
+
+    struct unchecked_t { };
+
+    CanonPath(unchecked_t _, std::string path)
+        : path(std::move(path))
+    { }
+
+    static CanonPath root;
+
+    /* If `raw` starts with a slash, return
+       `CanonPath(raw)`. Otherwise return a `CanonPath` representing
+       `root + "/" + raw`. */
+    CanonPath(std::string_view raw, const CanonPath & root);
+
+    bool isRoot() const
+    { return path.size() <= 1; }
+
+    explicit operator std::string_view() const
+    { return path; }
+
+    const std::string & abs() const
+    { return path; }
+
+    /* Like abs(), but return an empty string if this path is
+       '/'. Thus the returned string never ends in a slash. */
+    const std::string & absOrEmpty() const
+    {
+        const static std::string epsilon;
+        return isRoot() ? epsilon : path;
+    }
+
+    const char * c_str() const
+    { return path.c_str(); }
+
+    std::string_view rel() const
+    { return ((std::string_view) path).substr(1); }
+
+    struct Iterator
+    {
+        std::string_view remaining;
+        size_t slash;
+
+        Iterator(std::string_view remaining)
+            : remaining(remaining)
+            , slash(remaining.find('/'))
+        { }
+
+        bool operator != (const Iterator & x) const
+        { return remaining.data() != x.remaining.data(); }
+
+        const std::string_view operator * () const
+        { return remaining.substr(0, slash); }
+
+        void operator ++ ()
+        {
+            if (slash == remaining.npos)
+                remaining = remaining.substr(remaining.size());
+            else {
+                remaining = remaining.substr(slash + 1);
+                slash = remaining.find('/');
+            }
+        }
+    };
+
+    Iterator begin() const { return Iterator(rel()); }
+    Iterator end() const { return Iterator(rel().substr(path.size() - 1)); }
+
+    std::optional<CanonPath> parent() const;
+
+    /* Remove the last component. Panics if this path is the root.  */
+    void pop();
+
+    std::optional<std::string_view> dirOf() const
+    {
+        if (isRoot()) return std::nullopt;
+        return path.substr(0, path.rfind('/'));
+    }
+
+    std::optional<std::string_view> baseName() const
+    {
+        if (isRoot()) return std::nullopt;
+        return ((std::string_view) path).substr(path.rfind('/') + 1);
+    }
+
+    bool operator == (const CanonPath & x) const
+    { return path == x.path; }
+
+    bool operator != (const CanonPath & x) const
+    { return path != x.path; }
+
+    /* Compare paths lexicographically except that path separators
+       are sorted before any other character. That is, in the sorted order
+       a directory is always followed directly by its children. For
+       instance, 'foo' < 'foo/bar' < 'foo!'. */
+    bool operator < (const CanonPath & x) const
+    {
+        auto i = path.begin();
+        auto j = x.path.begin();
+        for ( ; i != path.end() && j != x.path.end(); ++i, ++j) {
+            auto c_i = *i;
+            if (c_i == '/') c_i = 0;
+            auto c_j = *j;
+            if (c_j == '/') c_j = 0;
+            if (c_i < c_j) return true;
+            if (c_i > c_j) return false;
+        }
+        return i == path.end() && j != x.path.end();
+    }
+
+    /* Return true if `this` is equal to `parent` or a child of
+       `parent`. */
+    bool isWithin(const CanonPath & parent) const;
+
+    CanonPath removePrefix(const CanonPath & prefix) const;
+
+    /* Append another path to this one. */
+    void extend(const CanonPath & x);
+
+    /* Concatenate two paths. */
+    CanonPath operator + (const CanonPath & x) const;
+
+    /* Add a path component to this one. It must not contain any slashes. */
+    void push(std::string_view c);
+
+    CanonPath operator + (std::string_view c) const;
+
+    /* Check whether access to this path is allowed, which is the case
+       if 1) `this` is within any of the `allowed` paths; or 2) any of
+       the `allowed` paths are within `this`. (The latter condition
+       ensures access to the parents of allowed paths.) */
+    bool isAllowed(const std::set<CanonPath> & allowed) const;
+};
+
+std::ostream & operator << (std::ostream & stream, const CanonPath & path);
+
+}

src/libutil/tests/canon-path.cc

@@ -0,0 +1,155 @@
+#include "canon-path.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    TEST(CanonPath, basic) {
+        {
+            CanonPath p("/");
+            ASSERT_EQ(p.abs(), "/");
+            ASSERT_EQ(p.rel(), "");
+            ASSERT_EQ(p.baseName(), std::nullopt);
+            ASSERT_EQ(p.dirOf(), std::nullopt);
+            ASSERT_FALSE(p.parent());
+        }
+
+        {
+            CanonPath p("/foo//");
+            ASSERT_EQ(p.abs(), "/foo");
+            ASSERT_EQ(p.rel(), "foo");
+            ASSERT_EQ(*p.baseName(), "foo");
+            ASSERT_EQ(*p.dirOf(), ""); // FIXME: do we want this?
+            ASSERT_EQ(p.parent()->abs(), "/");
+        }
+
+        {
+            CanonPath p("foo/bar");
+            ASSERT_EQ(p.abs(), "/foo/bar");
+            ASSERT_EQ(p.rel(), "foo/bar");
+            ASSERT_EQ(*p.baseName(), "bar");
+            ASSERT_EQ(*p.dirOf(), "/foo");
+            ASSERT_EQ(p.parent()->abs(), "/foo");
+        }
+
+        {
+            CanonPath p("foo//bar/");
+            ASSERT_EQ(p.abs(), "/foo/bar");
+            ASSERT_EQ(p.rel(), "foo/bar");
+            ASSERT_EQ(*p.baseName(), "bar");
+            ASSERT_EQ(*p.dirOf(), "/foo");
+        }
+    }
+
+    TEST(CanonPath, pop) {
+        CanonPath p("foo/bar/x");
+        ASSERT_EQ(p.abs(), "/foo/bar/x");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/foo/bar");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/foo");
+        p.pop();
+        ASSERT_EQ(p.abs(), "/");
+    }
+
+    TEST(CanonPath, removePrefix) {
+        CanonPath p1("foo/bar");
+        CanonPath p2("foo/bar/a/b/c");
+        ASSERT_EQ(p2.removePrefix(p1).abs(), "/a/b/c");
+        ASSERT_EQ(p1.removePrefix(p1).abs(), "/");
+        ASSERT_EQ(p1.removePrefix(CanonPath("/")).abs(), "/foo/bar");
+    }
+
+    TEST(CanonPath, iter) {
+        {
+            CanonPath p("a//foo/bar//");
+            std::vector<std::string_view> ss;
+            for (auto & c : p) ss.push_back(c);
+            ASSERT_EQ(ss, std::vector<std::string_view>({"a", "foo", "bar"}));
+        }
+
+        {
+            CanonPath p("/");
+            std::vector<std::string_view> ss;
+            for (auto & c : p) ss.push_back(c);
+            ASSERT_EQ(ss, std::vector<std::string_view>());
+        }
+    }
+
+    TEST(CanonPath, concat) {
+        {
+            CanonPath p1("a//foo/bar//");
+            CanonPath p2("xyzzy/bla");
+            ASSERT_EQ((p1 + p2).abs(), "/a/foo/bar/xyzzy/bla");
+        }
+
+        {
+            CanonPath p1("/");
+            CanonPath p2("/a/b");
+            ASSERT_EQ((p1 + p2).abs(), "/a/b");
+        }
+
+        {
+            CanonPath p1("/a/b");
+            CanonPath p2("/");
+            ASSERT_EQ((p1 + p2).abs(), "/a/b");
+        }
+
+        {
+            CanonPath p("/foo/bar");
+            ASSERT_EQ((p + "x").abs(), "/foo/bar/x");
+        }
+
+        {
+            CanonPath p("/");
+            ASSERT_EQ((p + "foo" + "bar").abs(), "/foo/bar");
+        }
+    }
+
+    TEST(CanonPath, within) {
+        {
+            ASSERT_TRUE(CanonPath("foo").isWithin(CanonPath("foo")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("bar")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("fo")));
+            ASSERT_TRUE(CanonPath("foo/bar").isWithin(CanonPath("foo")));
+            ASSERT_FALSE(CanonPath("foo").isWithin(CanonPath("foo/bar")));
+            ASSERT_TRUE(CanonPath("/foo/bar/default.nix").isWithin(CanonPath("/")));
+            ASSERT_TRUE(CanonPath("/").isWithin(CanonPath("/")));
+        }
+    }
+
+    TEST(CanonPath, sort) {
+        ASSERT_FALSE(CanonPath("foo") < CanonPath("foo"));
+        ASSERT_TRUE (CanonPath("foo") < CanonPath("foo/bar"));
+        ASSERT_TRUE (CanonPath("foo/bar") < CanonPath("foo!"));
+        ASSERT_FALSE(CanonPath("foo!") < CanonPath("foo"));
+        ASSERT_TRUE (CanonPath("foo") < CanonPath("foo!"));
+    }
+
+    TEST(CanonPath, allowed) {
+        {
+            std::set<CanonPath> allowed {
+                CanonPath("foo/bar"),
+                CanonPath("foo!"),
+                CanonPath("xyzzy"),
+                CanonPath("a/b/c"),
+            };
+
+            ASSERT_TRUE (CanonPath("foo/bar").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("foo/bar/bla").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("foo").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("bar").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("bar/a").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c/d").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("a/b/c/d/e").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("a/b/a").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("a/b/d").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("aaa").isAllowed(allowed));
+            ASSERT_FALSE(CanonPath("zzz").isAllowed(allowed));
+            ASSERT_TRUE (CanonPath("/").isAllowed(allowed));
+        }
+    }
+}