Backport SourcePath from the lazy-trees branch

This introduces the SourcePath type from lazy-trees as an abstraction
for accessing files from inputs that may not be materialized in the
real filesystem (e.g. Git repositories). Currently, however, it's just
a wrapper around CanonPath, so it shouldn't change any behaviour. (On
lazy-trees, SourcePath is a <InputAccessor, CanonPath> tuple.)

src/libfetchers/input-accessor.cc

@@ -0,0 +1,100 @@
+#include "input-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path)
+{
+    str << path.to_string();
+    return str;
+}
+
+std::string_view SourcePath::baseName() const
+{
+    return path.baseName().value_or("source");
+}
+
+SourcePath SourcePath::parent() const
+{
+    auto p = path.parent();
+    assert(p);
+    return std::move(*p);
+}
+
+InputAccessor::Stat SourcePath::lstat() const
+{
+    auto st = nix::lstat(path.abs());
+    return InputAccessor::Stat {
+        .type =
+            S_ISREG(st.st_mode) ? InputAccessor::tRegular :
+            S_ISDIR(st.st_mode) ? InputAccessor::tDirectory :
+            S_ISLNK(st.st_mode) ? InputAccessor::tSymlink :
+            InputAccessor::tMisc,
+        .isExecutable = S_ISREG(st.st_mode) && st.st_mode & S_IXUSR
+    };
+}
+
+std::optional<InputAccessor::Stat> SourcePath::maybeLstat() const
+{
+    // FIXME: merge these into one operation.
+    if (!pathExists())
+        return {};
+    return lstat();
+}
+
+InputAccessor::DirEntries SourcePath::readDirectory() const
+{
+    InputAccessor::DirEntries res;
+    for (auto & entry : nix::readDirectory(path.abs())) {
+        std::optional<InputAccessor::Type> type;
+        switch (entry.type) {
+        case DT_REG: type = InputAccessor::Type::tRegular; break;
+        case DT_LNK: type = InputAccessor::Type::tSymlink; break;
+        case DT_DIR: type = InputAccessor::Type::tDirectory; break;
+        }
+        res.emplace(entry.name, type);
+    }
+    return res;
+}
+
+StorePath SourcePath::fetchToStore(
+    ref<Store> store,
+    std::string_view name,
+    PathFilter * filter,
+    RepairFlag repair) const
+{
+    return
+        settings.readOnlyMode
+        ? store->computeStorePathForPath(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter).first
+        : store->addToStore(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter, repair);
+}
+
+SourcePath SourcePath::resolveSymlinks() const
+{
+    SourcePath res(CanonPath::root);
+
+    int linksAllowed = 1024;
+
+    for (auto & component : path) {
+        res.path.push(component);
+        while (true) {
+            if (auto st = res.maybeLstat()) {
+                if (!linksAllowed--)
+                    throw Error("infinite symlink recursion in path '%s'", path);
+                if (st->type != InputAccessor::tSymlink) break;
+                auto target = res.readLink();
+                if (hasPrefix(target, "/"))
+                    res = CanonPath(target);
+                else {
+                    res.path.pop();
+                    res.path.extend(CanonPath(target));
+                }
+            } else
+                break;
+        }
+    }
+
+    return res;
+}
+
+}