Add libPQVersion function

Author: steve-chavez

postgresql-libpq.cabal

@@ -36,6 +36,13 @@ Library
   Build-depends:       base >= 4 && < 5
                      , bytestring
 
+  if !os(windows)
+    Build-depends:     unix
+
+  if os(windows)
+    Build-depends:     Win32
+
+
   GHC-Options:       -Wall
   if flag(use-pkg-config)
     Pkgconfig-depends:   libpq

src/Database/PostgreSQL/LibPQ.hsc

@@ -78,6 +78,7 @@ module Database.PostgreSQL.LibPQ
     , parameterStatus
     , protocolVersion
     , serverVersion
+    , libPQVersion
     , errorMessage
     , socket
     , backendPID
@@ -254,6 +255,14 @@ import qualified Control.Exception
 mask_ = Control.Exception.block
 #endif
 
+import Control.Exception (try, IOException)
+
+#ifndef mingw32_HOST_OS
+import System.Posix.DynamicLinker
+#else
+import System.Win32.DLL
+#endif
+
 -- $dbconn
 -- The following functions deal with making a connection to a
 -- PostgreSQL backend server. An application program can have several
@@ -632,6 +641,30 @@ serverVersion :: Connection
 serverVersion connection =
     fmap fromIntegral $ withConn connection c_PQserverVersion
 
+-- | Return the version of libpq that is being used.
+--
+-- The result of this function can be used to determine, at
+-- run time, if specific functionality is available in the currently
+-- loaded version of libpq. The function can be used, for example,
+-- to determine which connection options are available for
+-- PQconnectdb or if the hex bytea output added in PostgreSQL 9.0 is supported.
+--
+-- This function appeared in PostgreSQL version 9.1, so
+-- it cannot be used to detect required functionality in earlier
+-- versions, since linking to it will create a link dependency
+-- on version 9.1.
+libPQVersion :: IO Int
+libPQVersion = do
+#ifndef mingw32_HOST_OS
+  dl <- dlopen "libpq.so" [RTLD_LAZY]
+  res <- try (dlsym dl "PQlibVersion") :: IO (Either IOException (FunPtr Int))
+#else
+  dl <- loadLibrary "LIBPQ.DLL"
+  res <- try (castPtrToFunPtr `fmap` getProcAddress dl "PQlibVersion") :: IO (Either IOException (FunPtr Int))
+#endif
+  case res of
+    Left _       -> error "libPQVersion is not supported for libpq < 9.1"
+    Right funPtr -> pure $ mkLibPQVersion funPtr
 
 -- | Returns the error message most recently generated by an operation
 -- on the connection.
@@ -2392,6 +2425,9 @@ foreign import ccall unsafe "libpq-fe.h PQprotocolVersion"
 foreign import ccall unsafe "libpq-fe.h PQserverVersion"
     c_PQserverVersion :: Ptr PGconn -> IO CInt
 
+foreign import ccall "dynamic"
+    mkLibPQVersion :: FunPtr Int -> Int
+
 foreign import ccall unsafe "libpq-fe.h PQsocket"
     c_PQsocket :: Ptr PGconn -> IO CInt