Allow hail to read data from stdin

This patch makes hail treat '-' as denoting stdin, per the usual Unix
convention. This will help with testing.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

htools/Ganeti/HTools/IAlloc.hs

@@ -347,7 +347,9 @@ processRequest request =
 -- | Reads the request from the data file(s).
 readRequest :: FilePath -> IO Request
 readRequest fp = do
-  input_data <- readFile fp
+  input_data <- case fp of
+                  "-" -> getContents
+                  _   -> readFile fp
   case parseData input_data of
     Bad err -> do
       hPutStrLn stderr $ "Error: " ++ err

man/hail.rst

@@ -24,6 +24,9 @@ state and the request details, and output (on stdout) a JSON-formatted
 response. In case of critical failures, the error message is printed
 on stderr and the exit code is changed to show failure.
 
+If the input file name is ``-`` (a single minus sign), then the request
+data will be read from *stdin*.
+
 ALGORITHM
 ~~~~~~~~~