feat: Allow limit=0 in query params to return an empty array

src/PostgREST/RangeQuery.hs

@@ -10,6 +10,8 @@ module PostgREST.RangeQuery (
 , restrictRange
 , rangeGeq
 , allRange
+, limitZeroRange
+, hasLimitZero
 , NonnegRange
 , rangeStatusHeader
 , contentRangeH
@@ -74,6 +76,15 @@ rangeLeq :: Integer -> NonnegRange
 rangeLeq n =
   Range BoundaryBelowAll (BoundaryAbove n)
 
+-- Special case to allow limit 0 queries
+-- https://github.com/PostgREST/postgrest/issues/1121
+-- 0 <= x <= -1
+limitZeroRange :: Range Integer
+limitZeroRange = Range (BoundaryBelow 0) (BoundaryAbove (-1))
+
+hasLimitZero :: Range Integer -> Bool
+hasLimitZero r = rangeUpper r == rangeUpper limitZeroRange
+
 rangeStatusHeader :: NonnegRange -> Int64 -> Maybe Int64 -> (Status, Header)
 rangeStatusHeader topLevelRange queryTotal tableTotal =
   let lower = rangeOffset topLevelRange

src/PostgREST/Request/ApiRequest.hs

@@ -50,6 +50,8 @@ import PostgREST.DbStructure.Identifiers (FieldName,
 import PostgREST.DbStructure.Proc        (ProcDescription (..),
                                           ProcParam (..), ProcsMap)
 import PostgREST.RangeQuery              (NonnegRange, allRange,
+                                          hasLimitZero,
+                                          limitZeroRange,
                                           rangeRequested)
 import PostgREST.Request.Preferences     (PreferCount (..),
                                           PreferParameters (..),
@@ -180,7 +182,7 @@ apiRequest :: AppConfig -> DbStructure -> Request -> RequestBody -> QueryParams.
 apiRequest conf@AppConfig{..} dbStructure req reqBody queryparams@QueryParams{..}
   | isJust profile && fromJust profile `notElem` configDbSchemas = Left $ UnacceptableSchema $ toList configDbSchemas
   | isTargetingProc && method `notElem` ["HEAD", "GET", "POST"] = Left ActionInappropriate
-  | topLevelRange == emptyRange = Left InvalidRange
+  | isInvalidRange = Left InvalidRange
   | shouldParsePayload && isLeft payload = either (Left . InvalidBody) witness payload
   | not expectParams && not (L.null qsParams) = Left $ ParseRequestError "Unexpected param or filter missing operator" ("Failed to parse " <> show qsParams)
   | otherwise = do
@@ -330,8 +332,14 @@ apiRequest conf@AppConfig{..} dbStructure req reqBody queryparams@QueryParams{..
   lookupHeader    = flip lookup hdrs
   Preferences.Preferences{..} = Preferences.fromHeaders hdrs
   headerRange = rangeRequested hdrs
+  limitRange = fromMaybe allRange (M.lookup "limit" qsRanges)
+  headerAndLimitRange = rangeIntersection headerRange limitRange
 
-  ranges = M.insert "limit" (rangeIntersection headerRange (fromMaybe allRange (M.lookup "limit" qsRanges))) qsRanges
+  -- Bypass all the ranges and send only the limit zero range (0 <= x <= -1) if
+  -- limit=0 is present in the query params (not allowed for the Range header)
+  ranges = M.insert "limit" (if hasLimitZero limitRange then limitZeroRange else headerAndLimitRange) qsRanges
+  -- The only emptyRange allowed is the limit zero range
+  isInvalidRange = topLevelRange == emptyRange && not (hasLimitZero limitRange)
 
 {-|
   Find the best match from a list of content types accepted by the