Only retrieve GET quieries from cache (#573)

Author: hadley

NEWS.md

@@ -2,6 +2,7 @@
 
 # httr2 1.0.5
 
+* `req_cache()` no longer retrieves anything but `GET` requests from the cache.
 * New `resp_stream_aws()` to retrieve AWS's special streaming format. With thanks to <https://github.com/lifion/lifion-aws-event-stream/> for a simple reference implementation.
 * New `req_auth_aws_v4()` signs request using AWS's special format (#562, #566).
 * `req_perform_parallel()` and `req_perform_promise()` now correctly set up the method and body (#549).

R/req-cache.R

@@ -186,6 +186,12 @@ cache_pre_fetch <- function(req, path = NULL) {
     return(req)
   }
 
+  # Only GET requests should be retrieved from cache. It's not sufficient to
+  # only save GET requests, because the method is not part of the cache key
+  if (req_method_get(req) != "GET") {
+    return(req)
+  }
+
   debug <- cache_debug(req)
   cache_prune_if_needed(req, debug = debug)
 

tests/testthat/test-req-cache.R

@@ -6,6 +6,21 @@ test_that("nothing happens if cache not enabled", {
   expect_equal(cache_post_fetch(req, resp), resp)
 })
 
+test_that("never retrieves POST request from cache", {
+  req <- request("http://example.com") %>%
+    req_method("POST") %>%
+    req_cache(tempfile())
+
+  # Fake an equivalent GET request in the cache
+  resp <- response(200,
+    headers = "Expires: Wed, 01 Jan 3000 00:00:00 GMT",
+    body = charToRaw("abc")
+  )
+  cache_set(req, resp)
+
+  expect_equal(cache_pre_fetch(req), req)
+})
+
 test_that("immutable objects retrieved directly from cache", {
   req <- request("http://example.com") %>% req_cache(tempfile())
   resp <- response(200,