libutil: return a source from readFile

don't consume a sink, return a source instead. the only reason to not do
this is a very slight reduction in dynamic allocations, but since we are
going to *at least* do disk io that will not be a lot of overhead anyway

Change-Id: Iae2f879ec64c3c3ac1d5310eeb6a85e696d4614a

src/libutil/file-system.cc

@@ -289,12 +289,17 @@ std::string readFile(const Path & path)
 }
 
 
-void readFile(const Path & path, Sink & sink)
+box_ptr<Source> readFileSource(const Path & path)
 {
     AutoCloseFD fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
     if (!fd)
         throw SysError("opening file '%s'", path);
-    drainFD(fd.get(), sink);
+
+    struct FileSource : FdSource {
+        AutoCloseFD fd;
+        explicit FileSource(AutoCloseFD fd) : FdSource(fd.get()), fd(std::move(fd)) {}
+    };
+    return make_box_ptr<FileSource>(std::move(fd));
 }
 
 

src/libutil/file-system.hh

@@ -5,6 +5,7 @@
  * Utiltities for working with the file sytem and file paths.
  */
 
+#include "box_ptr.hh"
 #include "types.hh"
 #include "file-descriptor.hh"
 
@@ -142,7 +143,7 @@ unsigned char getFileType(const Path & path);
  * Read the contents of a file into a string.
  */
 std::string readFile(const Path & path);
-void readFile(const Path & path, Sink & sink);
+box_ptr<Source> readFileSource(const Path & path);
 
 /**
  * Write a string to a file.

src/libutil/hash.cc

@@ -324,7 +324,7 @@ Hash hashString(HashType ht, std::string_view s)
 Hash hashFile(HashType ht, const Path & path)
 {
     HashSink sink(ht);
-    readFile(path, sink);
+    readFileSource(path)->drainInto(sink);
     return sink.finish().first;
 }