Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/canon-path.hh

@@ -0,0 +1,239 @@
+#pragma once
+///@file
+
+#include <string>
+#include <optional>
+#include <cassert>
+#include <iostream>
+#include <set>
+#include <vector>
+
+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 '..'.
+ *
+ * `CanonPath` are "virtual" Nix paths for abstract file system objects;
+ * they are always Unix-style paths, regardless of what OS Nix is
+ * running on. The `/` root doesn't denote the ambient host file system
+ * root, but some virtual FS root.
+ *
+ * @note It might be useful to compare `openat(some_fd, "foo/bar")` on
+ * Unix. `"foo/bar"` is a relative path because an absolute path would
+ * "override" the `some_fd` directory file descriptor and escape to the
+ * "system root". Conversely, Nix's abstract file operations *never* escape the
+ * designated virtual file system (i.e. `SourceAccessor` or
+ * `ParseSink`), so `CanonPath` does not need an absolute/relative
+ * distinction.
+ *
+ * @note The path does not need to correspond to an actually existing
+ * path, and the path may or may not have unresolved symlinks.
+ */
+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))
+    { }
+
+    /**
+     * Construct a canon path from a vector of elements.
+     */
+    CanonPath(const std::vector<std::string> & elems);
+
+    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); }
+
+    const char * rel_c_str() const
+    {
+        auto cs = path.c_str();
+        assert(cs[0]); // for safety if invariant is broken
+        return &cs[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(); }
+
+        bool operator == (const Iterator & x) const
+        { return !(*this != x); }
+
+        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 ((std::string_view) 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!'.
+     */
+    auto 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 (auto cmp = c_i <=> c_j; cmp != 0) return cmp;
+        }
+        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;
+
+    /**
+     * Return a representation `x` of `path` relative to `this`, i.e.
+     * `CanonPath(this.makeRelative(x), this) == path`.
+     */
+    std::string makeRelative(const CanonPath & path) const;
+
+    friend class std::hash<CanonPath>;
+};
+
+std::ostream & operator << (std::ostream & stream, const CanonPath & path);
+
+}
+
+template<>
+struct std::hash<nix::CanonPath>
+{
+    std::size_t operator ()(const nix::CanonPath & s) const noexcept
+    {
+        return std::hash<std::string>{}(s.path);
+    }
+};