readFile: Only read the full file if it's a non-regular one

Make `SourceAccessor::readFile` stream when reading a regular file, and
only fallback to a non-streaming version otherwise.

src/libutil/posix-source-accessor.cc

@@ -50,6 +50,13 @@ void PosixSourceAccessor::readFile(
     if (fstat(fd.get(), &st) == -1)
         throw SysError("statting file");
 
+    if (!S_ISREG(st.st_mode)) {
+        std::string fileContent = nix::readFile(path.abs());
+        sizeCallback(fileContent.size());
+        sink(fileContent);
+        return;
+    }
+
     sizeCallback(st.st_size);
 
     off_t left = st.st_size;

tests/functional/local.mk

@@ -60,6 +60,7 @@ nix_tests = \
   fetchGitVerification.sh \
   flakes/search-root.sh \
   readfile-context.sh \
+  readfile-stdin.sh \
   nix-channel.sh \
   recursive.sh \
   dependencies.sh \

tests/functional/readfile-stdin.sh

@@ -0,0 +1,3 @@
+source common.sh
+
+[[ $(echo yay | nix eval --raw --impure --expr 'builtins.readFile "/dev/stdin"') = yay ]]