Lock row exclusive on update requests

lockrow.diff

diff --git a/src/PostgREST/App.hs b/src/PostgREST/App.hs
index d1d27b4..b701501 100644
--- a/src/PostgREST/App.hs
+++ b/src/PostgREST/App.hs
@@ -53,6 +53,7 @@ import           PostgREST.QueryBuilder ( callProc
                                         , addRelations
                                         , createReadStatement
                                         , createWriteStatement
+                                        , lockRowExclusive
                                         , ResultsWithCount
                                         )
 
@@ -124,6 +125,7 @@ app dbStructure conf reqBody req =
         Left e -> return $ responseLBS status400 [jsonH] $ cs e
         Right (sq,mq) -> do
           let stm = createWriteStatement qi sq mq False (iPreferRepresentation apiRequest) [] (contentType == TextCSV) payload
+          lockRowExclusive qi
           row <- H.query uniform stm
           let (_, queryTotal, _, body) = extractQueryResult row
               r = contentRangeH 0 (toInteger $ queryTotal-1) (toInteger <$> Just queryTotal)
diff --git a/src/PostgREST/QueryBuilder.hs b/src/PostgREST/QueryBuilder.hs
index 0da8118..103999e 100644
--- a/src/PostgREST/QueryBuilder.hs
+++ b/src/PostgREST/QueryBuilder.hs
@@ -19,6 +19,7 @@ module PostgREST.QueryBuilder (
   , createReadStatement
   , createWriteStatement
   , inTransaction
+  , lockRowExclusive
   , operators
   , pgFmtIdent
   , pgFmtLit
@@ -361,6 +362,11 @@ requestToQuery schema (DbMutate (Delete mainTbl conditions)) =
       ("WHERE " <> intercalate " AND " ( map (pgFmtCondition qi ) conditions )) `emptyOnNull` conditions
       ]
 
+lockRowExclusive :: QualifiedIdentifier -> H.Session ()
+lockRowExclusive qi =
+  H.sql [qc| LOCK TABLE {fromQi qi} IN SHARE ROW EXCLUSIVE MODE; |]
+
+
 sourceCTEName :: SqlFragment
 sourceCTEName = "pg_source"