Merge InputAccessor into SourceAccessor

After the removal of the InputAccessor::fetchToStore() method, the
only remaining functionality in InputAccessor was `fingerprint` and
`getLastModified()`, and there is no reason to keep those in a
separate class.

src/libutil/input-accessor.hh

@@ -1,27 +0,0 @@
-#pragma once
-///@file
-
-#include "source-accessor.hh"
-#include "ref.hh"
-#include "repair-flag.hh"
-
-namespace nix {
-
-MakeError(RestrictedPathError, Error);
-
-struct InputAccessor : virtual SourceAccessor, std::enable_shared_from_this<InputAccessor>
-{
-    std::optional<std::string> fingerprint;
-
-    /**
-     * Return the maximum last-modified time of the files in this
-     * tree, if available.
-     */
-    virtual std::optional<time_t> getLastModified()
-    {
-        return std::nullopt;
-    }
-
-};
-
-}

src/libutil/memory-source-accessor.cc

@@ -108,7 +108,7 @@ std::string MemorySourceAccessor::readLink(const CanonPath & path)
         throw Error("file '%s' is not a symbolic link", path);
 }
 
-CanonPath MemorySourceAccessor::addFile(CanonPath path, std::string && contents)
+SourcePath MemorySourceAccessor::addFile(CanonPath path, std::string && contents)
 {
     auto * f = open(path, File { File::Regular {} });
     if (!f)
@@ -118,7 +118,7 @@ CanonPath MemorySourceAccessor::addFile(CanonPath path, std::string && contents)
     else
         throw Error("file '%s' is not a regular file", path);
 
-    return path;
+    return SourcePath{ref(shared_from_this()), path};
 }
 
 
@@ -184,4 +184,10 @@ void MemorySink::createSymlink(const Path & path, const std::string & target)
         throw Error("file '%s' is not a symbolic link", path);
 }
 
+ref<SourceAccessor> makeEmptySourceAccessor()
+{
+    static auto empty = make_ref<MemorySourceAccessor>().cast<SourceAccessor>();
+    return empty;
+}
+
 }

src/libutil/memory-source-accessor.hh

@@ -1,4 +1,4 @@
-#include "source-accessor.hh"
+#include "source-path.hh"
 #include "fs-sink.hh"
 #include "variant-wrapper.hh"
 
@@ -69,7 +69,7 @@ struct MemorySourceAccessor : virtual SourceAccessor
      */
     File * open(const CanonPath & path, std::optional<File> create);
 
-    CanonPath addFile(CanonPath path, std::string && contents);
+    SourcePath addFile(CanonPath path, std::string && contents);
 };
 
 /**

src/libutil/source-accessor.hh

@@ -35,7 +35,7 @@ enum class SymlinkResolution {
  * filesystem-like entities (such as the real filesystem, tarballs or
  * Git repositories).
  */
-struct SourceAccessor
+struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
 {
     const size_t number;
 
@@ -168,6 +168,30 @@ struct SourceAccessor
     CanonPath resolveSymlinks(
         const CanonPath & path,
         SymlinkResolution mode = SymlinkResolution::Full);
+
+    /**
+     * A string that uniquely represents the contents of this
+     * accessor. This is used for caching lookups (see `fetchToStore()`).
+     */
+    std::optional<std::string> fingerprint;
+
+    /**
+     * Return the maximum last-modified time of the files in this
+     * tree, if available.
+     */
+    virtual std::optional<time_t> getLastModified()
+    { return std::nullopt; }
 };
 
+/**
+ * Return a source accessor that contains only an empty root directory.
+ */
+ref<SourceAccessor> makeEmptySourceAccessor();
+
+/**
+ * Exception thrown when accessing a filtered path (see
+ * `FilteringInputAccessor`).
+ */
+MakeError(RestrictedPathError, Error);
+
 }

src/libutil/source-path.cc

@@ -18,13 +18,13 @@ std::string SourcePath::readFile() const
 bool SourcePath::pathExists() const
 { return accessor->pathExists(path); }
 
-InputAccessor::Stat SourcePath::lstat() const
+SourceAccessor::Stat SourcePath::lstat() const
 { return accessor->lstat(path); }
 
-std::optional<InputAccessor::Stat> SourcePath::maybeLstat() const
+std::optional<SourceAccessor::Stat> SourcePath::maybeLstat() const
 { return accessor->maybeLstat(path); }
 
-InputAccessor::DirEntries SourcePath::readDirectory() const
+SourceAccessor::DirEntries SourcePath::readDirectory() const
 { return accessor->readDirectory(path); }
 
 std::string SourcePath::readLink() const

src/libutil/source-path.hh

@@ -7,7 +7,7 @@
 
 #include "ref.hh"
 #include "canon-path.hh"
-#include "input-accessor.hh"
+#include "source-accessor.hh"
 
 namespace nix {
 
@@ -19,10 +19,10 @@ namespace nix {
  */
 struct SourcePath
 {
-    ref<InputAccessor> accessor;
+    ref<SourceAccessor> accessor;
     CanonPath path;
 
-    SourcePath(ref<InputAccessor> accessor, CanonPath path = CanonPath::root)
+    SourcePath(ref<SourceAccessor> accessor, CanonPath path = CanonPath::root)
         : accessor(std::move(accessor))
         , path(std::move(path))
     { }
@@ -51,19 +51,19 @@ struct SourcePath
      * Return stats about this `SourcePath`, or throw an exception if
      * it doesn't exist.
      */
-    InputAccessor::Stat lstat() const;
+    SourceAccessor::Stat lstat() const;
 
     /**
      * Return stats about this `SourcePath`, or std::nullopt if it
      * doesn't exist.
      */
-    std::optional<InputAccessor::Stat> maybeLstat() const;
+    std::optional<SourceAccessor::Stat> maybeLstat() const;
 
     /**
      * If this `SourcePath` denotes a directory (not a symlink),
      * return its directory entries; otherwise throw an error.
      */
-    InputAccessor::DirEntries readDirectory() const;
+    SourceAccessor::DirEntries readDirectory() const;
 
     /**
      * If this `SourcePath` denotes a symlink, return its target;