daemon: Fix read-file so it fails gracefully for large files (RHBZ#589039).

Pengzhen Cao noticed that read-file would fail for files
larger than the protocol size; this is *not* the bug.  However
it would also lose protocol synchronization after this.

The reason was that functions which return RBufferOut in the
generator must not 'touch' the *size_r parameter along error
return paths.

I fixed read-file and initrd-cat, and I checked that pread was
doing the right thing.

This also adds regression tests for read-file with various categories
of large file.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/generator.ml b/src/generator.ml
index 1e6d3c0d..c01e265b 100755
--- a/src/generator.ml
+++ b/src/generator.ml
@@ -3287,7 +3287,20 @@ for full details.");
 
   ("read_file", (RBufferOut "content", [Pathname "path"]), 150, [ProtocolLimitWarning],
    [InitISOFS, Always, TestOutputBuffer (
-      [["read_file"; "/known-4"]], "abc\ndef\nghi")],
+      [["read_file"; "/known-4"]], "abc\ndef\nghi");
+    (* Test various near large, large and too large files (RHBZ#589039). *)
+    InitBasicFS, Always, TestLastFail (
+      [["touch"; "/a"];
+       ["truncate_size"; "/a"; "4194303"]; (* GUESTFS_MESSAGE_MAX - 1 *)
+       ["read_file"; "/a"]]);
+    InitBasicFS, Always, TestLastFail (
+      [["touch"; "/a"];
+       ["truncate_size"; "/a"; "4194304"]; (* GUESTFS_MESSAGE_MAX *)
+       ["read_file"; "/a"]]);
+    InitBasicFS, Always, TestLastFail (
+      [["touch"; "/a"];
+       ["truncate_size"; "/a"; "41943040"]; (* GUESTFS_MESSAGE_MAX * 10 *)
+       ["read_file"; "/a"]])],
    "read a file",
    "\
 This calls returns the contents of the file C<path> as a