Malformed response to request with offset larger than resultset in combination with Prefer:count=exact

[ChristianBisig1]

Environment

• PostgreSQL version: PostgreSQL 15.13
• PostgREST version: postgrest/postgrest:v13.0.8

Description of issue

An invalid response is returned to a request that has an offset query parameter specified that is larger than the number of entries in the underlying table. (only if the Prefer:count=exact header is set on the request)

curl -i "http://localhost:3000/items?offset=2000" -H "Prefer: count=exact"

Until postgrest version v12.2.12 the following response was returned
HTTP/1.1 416 Requested Range Not Satisfiable

Later postgrest versions return a malformed response that cannot be parsed

[laurenceisla]

Can confirm that it happens in the main branch, e. g. in our spec tests:

$ curl "http://localhost:3000/menagerie?offset=1" -H  "Prefer: count=exact" -i
HTTP/1.1 416 Requested Range Not Satisfiable
Date: Thu, 06 Nov 2025 22:26:38 GMT
Server: postgrest/15 (pre-release)
Content-Range: */0
Content-Location: /menagerie?offset=1
Content-Length: 2
Content-Type: application/json; charset=utf-8
Preference-Applied: count=exact

{" 

It returns a truncated JSON response. Which is strange, since our tests pass:

postgrest/test/spec/Feature/Query/RangeSpec.hs

Lines 279 to 292 in bccf7ac

	context "of invalid range" $ do
	it "refuses a range with nonzero start when there are no items" $
	request methodGet "/menagerie?offset=1"
	[("Prefer", "count=exact")] ""
	`shouldRespondWith`
	[json| {
	"message":"Requested range not satisfiable",
	"code":"PGRST103",
	"details":"An offset of 1 was requested, but there are only 0 rows.",
	"hint":null
	}|]
	{ matchStatus = 416
	, matchHeaders = ["Content-Range" <:> "*/0"]
	}

[taimoorzaeem]

This is certainly caused by the Content-Length header. Removing it from the response headers, and then doing the request again gives the correct response:

$ curl "http://localhost:3000/menagerie?offset=1" -H  "Prefer: count=exact" -i
HTTP/1.1 416 Requested Range Not Satisfiable
Transfer-Encoding: chunked
Date: Sat, 08 Nov 2025 10:15:53 GMT
Server: postgrest/15 (pre-release)
Content-Range: */0
Content-Location: /menagerie?offset=1
Content-Type: application/json; charset=utf-8
Preference-Applied: count=exact

{"code":"PGRST103","details":"An offset of 1 was requested, but there are only 0 rows.","hint":null,"message":"Requested range not satisfiable"}

I think this happens because wai only responds with the amount of bytes in Content-Length header.

In our spec tests, we use hspec-wai, which I guess does not respect Content-Length, which may be why the tests didn't fail. 🤔

[laurenceisla]

This is certainly caused by the Content-Length header. Removing it from the response headers, and then doing the request again gives the correct response:

Yup, can confirm. The regression was caused by 57ef998, the commit before didn't have this issue.