Add response.headers on GET/POST/PATCH/PUT/DELETE

CHANGELOG.md

@@ -13,8 +13,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - #1378, Add support for `Prefer: count=planned` and `Prefer: count=estimated` on GET /table - @steve-chavez
 - #1327, Add support for optional query parameter `on_conflict` to upsert with specified keys for POST - @ykst
 - #1430, Allow specifying the foreign key constraint name(`/source?select=fk_constraint(*)`) to disambiguate an embedding - @steve-chavez
-- #1168, Allow access to the Authorization header through the request.header.authorization GUC - @steve-chavez
+- #1168, Allow access to the `Authorization` header through the `request.header.authorization` GUC - @steve-chavez
-- #1435, Add request.method and request.path GUCs - @steve-chavez
+- #1435, Add `request.method` and `request.path` GUCs - @steve-chavez
+- #1088, Allow adding headers to GET/POST/PATCH/PUT/DELETE responses through the `response.headers` GUC - @steve-chavez
 
 ### Fixed
 

src/PostgREST/App.hs

@@ -131,10 +131,13 @@ app dbStructure proc cols conf apiRequest =
                              then limitedQuery cq ((+ 1) <$> maxRows) -- LIMIT maxRows + 1 so we can determine below that maxRows was surpassed
                              else cq
                   stm = createReadStatement q cQuery (contentType == CTSingularJSON) shouldCount
-                        (contentType == CTTextCSV) bField
+                        (contentType == CTTextCSV) bField pgVer
                   explStm = createExplainStatement cq
               row <- H.statement () stm
-              let (tableTotal, queryTotal, _ , body) = row
+              let (tableTotal, queryTotal, _ , body, gucHeaders) = row
+              case gucHeaders of
+                Left _ -> return . errorResponseFor $ GucHeadersError
+                Right hs -> do
                   total <- if | plannedCount   -> H.statement () explStm
                               | estimatedCount -> if tableTotal > (fromIntegral <$> maxRows)
                                                     then do estTotal <- H.statement () explStm
@@ -146,8 +149,7 @@ app dbStructure proc cols conf apiRequest =
                     if contentType == CTSingularJSON && queryTotal /= 1
                       then errorResponseFor . singularityError $ queryTotal
                       else responseLBS status
-                    [toHeader contentType, contentRange,
+                        ([toHeader contentType, contentRange, contentLocationH tName (iCanonicalQS apiRequest)] ++ (gucHToHeader <$> hs))
-                     contentLocationH tName (iCanonicalQS apiRequest)]
                         (if headersOnly then mempty else toS body)
 
         (ActionCreate, TargetIdent (QualifiedIdentifier tSchema tName), Just pJson) ->
@@ -157,10 +159,13 @@ app dbStructure proc cols conf apiRequest =
               let pkCols = tablePKCols dbStructure tSchema tName
                   stm = createWriteStatement sq mq
                     (contentType == CTSingularJSON) True
-                    (contentType == CTTextCSV) (iPreferRepresentation apiRequest) pkCols
+                    (contentType == CTTextCSV) (iPreferRepresentation apiRequest) pkCols pgVer
               row <- H.statement (toS $ pjRaw pJson) stm
-              let (_, queryTotal, fs, body) = row
+              let (_, queryTotal, fs, body, gucHeaders) = row
-                  headers = catMaybes [
+              case gucHeaders of
+                Left _ -> return . errorResponseFor $ GucHeadersError
+                Right hdrs -> do
+                  let headers = catMaybes [
                           if null fs
                             then Nothing
                             else Just $ locationH tName fs
@@ -172,16 +177,13 @@ app dbStructure proc cols conf apiRequest =
                         , if null pkCols && isNothing (iOnConflict apiRequest)
                             then Nothing
                             else (\x -> ("Preference-Applied", show x)) <$> iPreferResolution apiRequest
-                    ]
+                        ] ++ (gucHToHeader <$> hdrs)
-              if contentType == CTSingularJSON
+                  if contentType == CTSingularJSON && queryTotal /= 1
-                 && queryTotal /= 1
                     then do
                       HT.condemn
                       return . errorResponseFor . singularityError $ queryTotal
                     else
-                return . responseLBS status201 headers $
+                      return . responseLBS status201 headers $ if iPreferRepresentation apiRequest == Full then toS body else mempty
-                  if iPreferRepresentation apiRequest == Full
-                    then toS body else ""
 
         (ActionUpdate, TargetIdent (QualifiedIdentifier tSchema tName), Just pJson) ->
           case mutateSqlParts tSchema tName of
@@ -189,26 +191,27 @@ app dbStructure proc cols conf apiRequest =
             Right (sq, mq) -> do
               let stm = createWriteStatement sq mq
                     (contentType == CTSingularJSON) False (contentType == CTTextCSV)
-                    (iPreferRepresentation apiRequest) []
+                    (iPreferRepresentation apiRequest) [] pgVer
               row <- H.statement (toS $ pjRaw pJson) stm
-              let (_, queryTotal, _, body) = row
+              let (_, queryTotal, _, body, gucHeaders) = row
+              case gucHeaders of
+                Left _ -> return . errorResponseFor $ GucHeadersError
+                Right hdrs -> do
+                  let
                     updateIsNoOp = S.null cols
-                  contentRangeHeader = contentRangeH 0 (queryTotal - 1) $
-                        if shouldCount then Just queryTotal else Nothing
-                  headers | iPreferRepresentation apiRequest == Full = [toHeader contentType, contentRangeHeader]
-                          | otherwise                                = [contentRangeHeader]
                     status | queryTotal == 0 && not updateIsNoOp      = status404
                            | iPreferRepresentation apiRequest == Full = status200
                            | otherwise                                = status204
-              if contentType == CTSingularJSON
+                    contentRangeHeader = contentRangeH 0 (queryTotal - 1) $ if shouldCount then Just queryTotal else Nothing
-                 && queryTotal /= 1
+                    headers = [contentRangeHeader] ++
+                              [if iPreferRepresentation apiRequest == Full then toHeader contentType else mempty] ++
+                              (gucHToHeader <$> hdrs)
+                  if contentType == CTSingularJSON && queryTotal /= 1
                     then do
                       HT.condemn
                       return . errorResponseFor . singularityError $ queryTotal
                     else
-                  return $ if iPreferRepresentation apiRequest == Full
+                      return . responseLBS status headers $ if iPreferRepresentation apiRequest == Full then toS body else mempty
-                    then responseLBS status headers (toS body)
-                    else responseLBS status headers mempty
 
         (ActionSingleUpsert, TargetIdent (QualifiedIdentifier tSchema tName), Just ProcessedJSON{pjRaw, pjType, pjKeys}) ->
           case mutateSqlParts tSchema tName of
@@ -227,8 +230,13 @@ app dbStructure proc cols conf apiRequest =
               else do
                 row <- H.statement (toS pjRaw) $
                        createWriteStatement sq mq (contentType == CTSingularJSON) False
-                                            (contentType == CTTextCSV) (iPreferRepresentation apiRequest) []
+                                            (contentType == CTTextCSV) (iPreferRepresentation apiRequest) [] pgVer
-                let (_, queryTotal, _, body) = row
+                let (_, queryTotal, _, body, gucHeaders) = row
+                case gucHeaders of
+                  Left _ -> return . errorResponseFor $ GucHeadersError
+                  Right hdrs -> do
+                    let headers = toHeader contentType : (gucHToHeader <$> hdrs)
+                        status = if iPreferRepresentation apiRequest == Full then status200 else status204
                     -- Makes sure the querystring pk matches the payload pk
                     -- e.g. PUT /items?id=eq.1 { "id" : 1, .. } is accepted, PUT /items?id=eq.14 { "id" : 2, .. } is rejected
                     -- If this condition is not satisfied then nothing is inserted, check the WHERE for INSERT in QueryBuilder.hs to see how it's done
@@ -237,9 +245,7 @@ app dbStructure proc cols conf apiRequest =
                         HT.condemn
                         return . errorResponseFor $ PutMatchingPkError
                       else
-                    return $ if iPreferRepresentation apiRequest == Full
+                        return . responseLBS status headers $ if iPreferRepresentation apiRequest == Full then toS body else mempty
-                      then responseLBS status200 [toHeader contentType] (toS body)
-                      else responseLBS status204 [] ""
 
         (ActionDelete, TargetIdent (QualifiedIdentifier tSchema tName), Nothing) ->
           case mutateSqlParts tSchema tName of
@@ -248,20 +254,25 @@ app dbStructure proc cols conf apiRequest =
               let stm = createWriteStatement sq mq
                     (contentType == CTSingularJSON) False
                     (contentType == CTTextCSV)
-                    (iPreferRepresentation apiRequest) []
+                    (iPreferRepresentation apiRequest) [] pgVer
               row <- H.statement mempty stm
-              let (_, queryTotal, _, body) = row
+              let (_, queryTotal, _, body, gucHeaders) = row
-                  contentRangeHeader = contentRangeH 1 0 $
+              case gucHeaders of
-                        if shouldCount then Just queryTotal else Nothing
+                Left _ -> return . errorResponseFor $ GucHeadersError
+                Right hdrs -> do
+                  let
+                    status = if iPreferRepresentation apiRequest == Full then status200 else status204
+                    contentRangeHeader = contentRangeH 1 0 $ if shouldCount then Just queryTotal else Nothing
+                    headers = [contentRangeHeader] ++
+                              [if iPreferRepresentation apiRequest == Full then toHeader contentType else mempty] ++
+                              (gucHToHeader <$> hdrs)
                   if contentType == CTSingularJSON
                      && queryTotal /= 1
                     then do
                       HT.condemn
                       return . errorResponseFor . singularityError $ queryTotal
                     else
-                  return $ if iPreferRepresentation apiRequest == Full
+                      return . responseLBS status headers $ if iPreferRepresentation apiRequest == Full then toS body else mempty
-                    then responseLBS status200 [toHeader contentType, contentRangeHeader] (toS body)
-                    else responseLBS status204 [contentRangeHeader] ""
 
         (ActionInfo, TargetIdent (QualifiedIdentifier tSchema tTable), Nothing) ->
           let mTable = find (\t -> tableName t == tTable && tableSchema t == tSchema) (dbTables dbStructure) in
@@ -282,7 +293,7 @@ app dbStructure proc cols conf apiRequest =
                 pq = requestToCallProcQuery qi (specifiedProcArgs cols proc) returnsScalar preferParams
                 stm = callProcStatement returnsScalar pq q cq shouldCount (contentType == CTSingularJSON)
                         (contentType == CTTextCSV) (contentType `elem` rawContentTypes) (preferParams == Just MultipleObjects)
-                        bField (pgVersion dbStructure)
+                        bField pgVer
               row <- H.statement (toS $ pjRaw pJson) stm
               let (tableTotal, queryTotal, body, gucHeaders) = row
                   (status, contentRange) = rangeStatusHeader topLevelRange queryTotal tableTotal
@@ -294,7 +305,7 @@ app dbStructure proc cols conf apiRequest =
                       HT.condemn
                       return . errorResponseFor . singularityError $ queryTotal
                     else
-                      return $ responseLBS status ([toHeader contentType, contentRange] ++ toHeaders hs)
+                      return $ responseLBS status ([toHeader contentType, contentRange] ++ (gucHToHeader <$> hs))
                       (if invMethod == InvHead then mempty else toS body)
 
         (ActionInspect headersOnly, TargetDefaultSpec tSchema, Nothing) -> do
@@ -325,6 +336,7 @@ app dbStructure proc cols conf apiRequest =
         shouldCount = exactCount || estimatedCount
         topLevelRange = iTopLevelRange apiRequest
         returnsScalar = maybe False procReturnsScalar proc
+        pgVer = pgVersion dbStructure
 
         readSqlParts s t =
           let

src/PostgREST/Private/QueryFragment.hs

@@ -197,3 +197,9 @@ returningF qi returnings =
   if null returnings
     then "RETURNING 1" -- For mutation cases where there's no ?select, we return 1 to know how many rows were modified
     else "RETURNING " <> intercalate ", " (pgFmtColumn qi <$> returnings)
+
+responseHeadersF :: PgVersion -> SqlFragment
+responseHeadersF pgVer =
+  if pgVer >= pgVersion96
+    then "coalesce(nullif(current_setting('response.headers', true), ''), '[]')" :: Text -- nullif is used because of https://gist.github.com/steve-chavez/8d7033ea5655096903f3b52f8ed09a15
+    else "'[]'" :: Text

src/PostgREST/Statements.hs

@@ -38,12 +38,12 @@ import           Text.InterpolatedString.Perl6   (qc)
     is represented as a list of strings containing variable bindings like
     @"k1=eq.42"@, or the empty list if there is no location header.
 -}
-type ResultsWithCount = (Maybe Int64, Int64, [BS.ByteString], BS.ByteString)
+type ResultsWithCount = (Maybe Int64, Int64, [BS.ByteString], BS.ByteString, Either Text [GucHeader])
 
 createWriteStatement :: SqlQuery -> SqlQuery -> Bool -> Bool -> Bool ->
-                        PreferRepresentation -> [Text] ->
+                        PreferRepresentation -> [Text] -> PgVersion ->
                         H.Statement ByteString ResultsWithCount
-createWriteStatement selectQuery mutateQuery wantSingle isInsert asCsv rep pKeys =
+createWriteStatement selectQuery mutateQuery wantSingle isInsert asCsv rep pKeys pgVer =
   unicodeStatement sql (param HE.unknown) decodeStandard True
  where
   sql = [qc|
@@ -53,7 +53,8 @@ createWriteStatement selectQuery mutateQuery wantSingle isInsert asCsv rep pKeys
         '' AS total_result_set,
         pg_catalog.count(_postgrest_t) AS page_total,
         {locF} AS header,
-        {bodyF} AS body
+        {bodyF} AS body,
+        {responseHeadersF pgVer} AS response_headers
       FROM ({selectQuery}) _postgrest_t |]
 
   locF =
@@ -73,11 +74,11 @@ createWriteStatement selectQuery mutateQuery wantSingle isInsert asCsv rep pKeys
 
   decodeStandard :: HD.Result ResultsWithCount
   decodeStandard =
-   fromMaybe (Nothing, 0, [], "") <$> HD.rowMaybe standardRow
+   fromMaybe (Nothing, 0, [], mempty, Right []) <$> HD.rowMaybe standardRow
 
-createReadStatement :: SqlQuery -> SqlQuery -> Bool -> Bool -> Bool -> Maybe FieldName ->
+createReadStatement :: SqlQuery -> SqlQuery -> Bool -> Bool -> Bool -> Maybe FieldName -> PgVersion ->
                        H.Statement () ResultsWithCount
-createReadStatement selectQuery countQuery isSingle countTotal asCsv binaryField =
+createReadStatement selectQuery countQuery isSingle countTotal asCsv binaryField pgVer =
   unicodeStatement sql HE.noParams decodeStandard False
  where
   sql = [qc|
@@ -88,7 +89,8 @@ createReadStatement selectQuery countQuery isSingle countTotal asCsv binaryField
         {countResultF} AS total_result_set,
         pg_catalog.count(_postgrest_t) AS page_total,
         {noLocationF} AS header,
-        {bodyF} AS body
+        {bodyF} AS body,
+        {responseHeadersF pgVer} AS response_headers
       FROM ( SELECT * FROM {sourceCTEName}) _postgrest_t |]
 
   (countCTEF, countResultF) = countF countQuery countTotal
@@ -108,8 +110,8 @@ createReadStatement selectQuery countQuery isSingle countTotal asCsv binaryField
     for that common type of query.
 -}
 standardRow :: HD.Row ResultsWithCount
-standardRow = (,,,) <$> nullableColumn HD.int8 <*> column HD.int8
+standardRow = (,,,,) <$> nullableColumn HD.int8 <*> column HD.int8
-                    <*> column header <*> column HD.bytea
+                    <*> column header <*> column HD.bytea <*> column decodeGucHeaders
   where
     header = HD.array $ HD.dimension replicateM $ element HD.bytea
 
@@ -128,7 +130,7 @@ callProcStatement returnsScalar callProcQuery selectQuery countQuery countTotal
         {countResultF} AS total_result_set,
         pg_catalog.count(_postgrest_t) AS page_total,
         {bodyF} AS body,
-        {responseHeaders} AS response_headers
+        {responseHeadersF pgVer} AS response_headers
       FROM ({selectQuery}) _postgrest_t;|]
 
     (countCTEF, countResultF) = countF countQuery countTotal
@@ -145,18 +147,12 @@ callProcStatement returnsScalar callProcQuery selectQuery countQuery countTotal
      | multObjects = "json_agg(_postgrest_t.pgrst_scalar)::character varying"
      | otherwise   = "(json_agg(_postgrest_t.pgrst_scalar)->0)::character varying"
 
-    responseHeaders =
-      if pgVer >= pgVersion96
-        then "coalesce(nullif(current_setting('response.headers', true), ''), '[]')" :: Text -- nullif is used because of https://gist.github.com/steve-chavez/8d7033ea5655096903f3b52f8ed09a15
-        else "'[]'" :: Text
-
     decodeProc :: HD.Result ProcResults
     decodeProc =
-      let row = fromMaybe (Just 0, 0, "[]", "[]") <$> HD.rowMaybe procRow in
+      fromMaybe (Just 0, 0, mempty, Right []) <$> HD.rowMaybe procRow
-      (\(a, b, c, d) -> (a, b, c, first toS $ JSON.eitherDecode $ toS d)) <$> row
       where
         procRow = (,,,) <$> nullableColumn HD.int8 <*> column HD.int8
-                        <*> column HD.bytea <*> column HD.bytea
+                        <*> column HD.bytea <*> column decodeGucHeaders
 
 createExplainStatement :: SqlQuery -> H.Statement () (Maybe Int64)
 createExplainStatement countQuery =
@@ -178,3 +174,6 @@ createExplainStatement countQuery =
 
 unicodeStatement :: Text -> HE.Params a -> HD.Result b -> Bool -> H.Statement a b
 unicodeStatement = H.Statement . encodeUtf8
+
+decodeGucHeaders :: HD.Value (Either Text [GucHeader])
+decodeGucHeaders = first toS . JSON.eitherDecode . toS <$> HD.bytea

src/PostgREST/Types.hs

@@ -408,8 +408,8 @@ instance JSON.FromJSON GucHeader where
     _ -> mzero
   parseJSON _          = mzero
 
-toHeaders :: [GucHeader] -> [Header]
+gucHToHeader :: GucHeader -> Header
-toHeaders = map $ \(GucHeader (k, v)) -> (CI.mk $ toS k, toS v)
+gucHToHeader (GucHeader (k, v)) = (CI.mk $ toS k, toS v)
 
 {-|
   This type will hold information about which particular 'Relation' between two tables to choose when there are multiple ones.

test/Feature/PgVersion96Spec.hs

@@ -1,5 +1,6 @@
 module Feature.PgVersion96Spec where
 
+import Network.HTTP.Types
 import Network.Wai        (Application)
 
 import Test.Hspec
@@ -12,7 +13,7 @@ import SpecHelper
 spec :: SpecWith Application
 spec =
   describe "features supported on PostgreSQL 9.6" $ do
-    context "GUC headers" $ do
+    context "GUC headers on function calls" $ do
       it "succeeds setting the headers" $ do
         get "/rpc/get_projects_and_guc_headers?id=eq.2&select=id"
           `shouldRespondWith` [json|[{"id": 2}]|]
@@ -67,6 +68,50 @@ spec =
               "Set-Cookie" <:> "sessionid=38afes7a8; HttpOnly; Path=/",
               "Set-Cookie" <:> "id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT; Secure; HttpOnly"]}
 
+    context "GUC headers on all other methods via pre-request" $ do
+      it "succeeds setting the headers on GET and HEAD" $ do
+        request methodGet "/items?id=eq.1" [("User-Agent", "MSIE 6.0")] mempty
+          `shouldRespondWith` [json|[{"id": 1}]|]
+          {matchHeaders = [
+              matchContentTypeJson,
+              "Cache-Control" <:> "no-cache, no-store, must-revalidate"]}
+
+        request methodHead "/items?id=eq.1" [("User-Agent", "MSIE 7.0")] mempty
+          `shouldRespondWith` ""
+          {matchHeaders = ["Cache-Control" <:> "no-cache, no-store, must-revalidate"]}
+
+        request methodHead "/projects" [("Accept", "text/csv")] mempty
+          `shouldRespondWith` ""
+          {matchHeaders = ["Content-Disposition" <:> "attachment; filename=projects.csv"]}
+
+      it "succeeds setting the headers on POST" $
+        request methodPost "/items" [] [json|[{"id": 11111}]|]
+          `shouldRespondWith` ""
+          { matchStatus = 201
+          , matchHeaders = ["X-Custom-Header" <:> "mykey=myval"]
+          }
+
+      it "succeeds setting the headers on PATCH" $
+        request methodPatch "/items?id=eq.11111" [] [json|[{"id": 11111}]|]
+          `shouldRespondWith` ""
+          { matchStatus = 204
+          , matchHeaders = ["X-Custom-Header" <:> "mykey=myval"]
+          }
+
+      it "succeeds setting the headers on PUT" $
+        request methodPut "/items?id=eq.11111" [] [json|[{"id": 11111}]|]
+          `shouldRespondWith` ""
+          { matchStatus = 204
+          , matchHeaders = ["X-Custom-Header" <:> "mykey=myval"]
+          }
+
+      it "succeeds setting the headers on DELETE" $
+        request methodDelete "/items?id=eq.11111" [] mempty
+          `shouldRespondWith` ""
+          { matchStatus = 204
+          , matchHeaders = ["X-Custom-Header" <:> "mykey=myval"]
+          }
+
     context "Use of the phraseto_tsquery function" $ do
       it "finds matches" $
         get "/tsearch?text_search_vector=phfts.The%20Fat%20Cats" `shouldRespondWith`

test/Main.hs

@@ -78,6 +78,7 @@ main = do
       extraSearchPathApp   = return $ postgrest (testCfgExtraSearchPath testDbConn)   refDbStructure pool getTime $ pure ()
       rootSpecApp          = return $ postgrest (testCfgRootSpec testDbConn)          refDbStructure pool getTime $ pure ()
       htmlRawOutputApp     = return $ postgrest (testCfgHtmlRawOutput testDbConn)     refDbStructure pool getTime $ pure ()
+      responseHeadersApp   = return $ postgrest (testCfgResponseHeaders testDbConn)   refDbStructure pool getTime $ pure ()
 
   let reset, analyze :: IO ()
       reset = resetDb testDbConn
@@ -88,8 +89,7 @@ main = do
       actualPgVersion = pgVersion dbStructure
       extraSpecs =
         [("Feature.UpsertSpec", Feature.UpsertSpec.spec) | actualPgVersion >= pgVersion95] ++
-        [("Feature.PgVersion95Spec", Feature.PgVersion95Spec.spec) | actualPgVersion >= pgVersion95] ++
+        [("Feature.PgVersion95Spec", Feature.PgVersion95Spec.spec) | actualPgVersion >= pgVersion95]
-        [("Feature.PgVersion96Spec", Feature.PgVersion96Spec.spec) | actualPgVersion >= pgVersion96]
 
       specs = uncurry describe <$> [
           ("Feature.AuthSpec"                , Feature.AuthSpec.spec actualPgVersion)
@@ -165,6 +165,8 @@ main = do
       describe "Feature.ExtraSearchPathSpec" Feature.ExtraSearchPathSpec.spec
 
     -- this test runs with a root spec function override
-    when (actualPgVersion >= pgVersion96) $
+    when (actualPgVersion >= pgVersion96) $ do
       before rootSpecApp $
         describe "Feature.RootSpec" Feature.RootSpec.spec
+      before responseHeadersApp $
+        describe "Feature.PgVersion96Spec" Feature.PgVersion96Spec.spec

test/SpecHelper.hs

@@ -138,6 +138,9 @@ testCfgRootSpec testDbConn = (testCfg testDbConn) { configRootSpec = Just "root"
 testCfgHtmlRawOutput :: Text -> AppConfig
 testCfgHtmlRawOutput testDbConn = (testCfg testDbConn) { configRawMediaTypes = ["text/html"] }
 
+testCfgResponseHeaders :: Text -> AppConfig
+testCfgResponseHeaders testDbConn = (testCfg testDbConn) { configReqCheck = Just "custom_headers" }
+
 setupDb :: Text -> IO ()
 setupDb dbConn = do
   loadFixture dbConn "database"

test/fixtures/schema.sql

@@ -1642,3 +1642,22 @@ add constraint fst_shift foreign key           (fst_shift_activity_id, fst_shift
                          references activities (id, schedule_id),
 add constraint snd_shift foreign key           (snd_shift_activity_id, snd_shift_schedule_id)
                          references activities (id, schedule_id);
+
+-- for a pre-request function
+create or replace function custom_headers() returns void as $$
+declare
+  user_agent text := current_setting('request.header.user-agent', true);
+  req_path   text := current_setting('request.path', true);
+  req_accept text := current_setting('request.header.accept', true);
+begin
+  if user_agent similar to 'MSIE (6.0|7.0)' then
+    perform set_config('response.headers',
+      '[{"Cache-Control": "no-cache, no-store, must-revalidate"}]', false);
+  elsif req_path similar to '/(items|projects)' and req_accept = 'text/csv' then
+    perform set_config('response.headers',
+      format('[{"Content-Disposition": "attachment; filename=%s.csv"}]', trim('/' from req_path)), false);
+  else
+    perform set_config('response.headers',
+      '[{"X-Custom-Header": "mykey=myval"}]', false);
+  end if;
+end; $$ language plpgsql;