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Fix mod update checks failing on non-JSON responses (#931)

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The mod update and Find Mods feeds feed the raw HTTP body straight to
Json.decode. When the endpoint hands back something that is not JSON
(an HTML error page, a proxy/captive prompt, or a plain-text outage
message like "Exceeded secondary rate limit" -- usually still HTTP 200),
the decoder's "unexpected character 'E'" assert escaped through the
pcall and became the error message, blaming the parser instead of the
response.

Add Json.describeUnexpected() as a pre-decode content-type guard: it
returns nil for body shapes the endpoints actually publish (JSON object
or array) and otherwise a short message naming what the server sent
(HTML page / plain text / empty, with a preview). Wire it into
ModUpdate.parseReleases and ModIndex.parse, so both the sync and async
update-check paths surface the real answer instead of the parse error.
HTTP status was already checked upstream by HostShell.httpGet (non-2xx
becomes "HTTP <code> from <url> (...)"); this closes the remaining
"2xx but not JSON" gap everywhere, including bridge platforms that
expose no status or headers.

Add regression tests for plain-text, HTML, and empty bodies; strengthen
the ModIndex HTML soft-fail test.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
This commit is contained in:
Shane McGovern
2026-08-07 10:49:10 +01:00
parent 112120e8fe
commit d59a9522ee
5 changed files with 58 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/link/Json.lua
+22
View File
@@ -171,4 +171,26 @@ function Json.decode(s)
return nil, v return nil, v
end end
-- For an HTTP response that was meant to carry JSON but did not. Returns nil
-- when `s` starts like a JSON object or array (the only shapes the update and
-- index endpoints publish), otherwise a short message naming what the server
-- actually sent -- so callers surface "the response was an HTML page/plain
-- text, not JSON (it starts with ...)" instead of leaking the decoder's
-- low-level "unexpected character 'E'" assert at the first byte of an error
-- page or plain-text outage message.
function Json.describeUnexpected(s)
if type(s) ~= "string" then
return "the response had no body to decode"
end
local first = s:match("^%s*(.)")
if first == "{" or first == "[" then return nil end
local preview = s:gsub("%s+", " "):gsub("^%s+", ""):gsub("%s+$", "")
if preview == "" then
return "the response was empty, not JSON"
end
if #preview > 60 then preview = preview:sub(1, 57) .. "..." end
local kind = (first == "<") and "an HTML page" or "plain text"
return ("the response was %s, not JSON (it starts with %q)"):format(kind, preview)
end
return Json return Json
src/mods/ModIndex.lua
+3 -1
View File
@@ -191,8 +191,10 @@ end
-- Never throws: a truncated download, an HTML error page, or a feed from a -- Never throws: a truncated download, an HTML error page, or a feed from a
-- future schema all come back as a message the panel can print. -- future schema all come back as a message the panel can print.
function ModIndex.parse(jsonText, Json) function ModIndex.parse(jsonText, Json)
Json = Json or require("src.link.Json")
local notJson = Json.describeUnexpected(jsonText)
if notJson then return nil, notJson end
local ok, result, err = pcall(function() local ok, result, err = pcall(function()
Json = Json or require("src.link.Json")
local doc, decodeErr = Json.decode(jsonText) local doc, decodeErr = Json.decode(jsonText)
if type(doc) ~= "table" then if type(doc) ~= "table" then
return nil, decodeErr or "index.json is not an object" return nil, decodeErr or "index.json is not an object"
src/mods/ModUpdate.lua
+3 -1
View File
@@ -140,8 +140,10 @@ end
-- Decode a releases array (GET /repos/.../releases) into a sorted list -- Decode a releases array (GET /repos/.../releases) into a sorted list
-- (newest first). Releases without a .zip asset are dropped. Never throws. -- (newest first). Releases without a .zip asset are dropped. Never throws.
function ModUpdate.parseReleases(jsonText, modId, Json) function ModUpdate.parseReleases(jsonText, modId, Json)
Json = Json or require("src.link.Json")
local notJson = Json.describeUnexpected(jsonText)
if notJson then return nil, notJson end
local ok, result, err = pcall(function() local ok, result, err = pcall(function()
Json = Json or require("src.link.Json")
local doc, decodeErr = Json.decode(jsonText) local doc, decodeErr = Json.decode(jsonText)
if type(doc) ~= "table" then if type(doc) ~= "table" then
return nil, decodeErr or "releases json is not an array" return nil, decodeErr or "releases json is not an array"
tests/engine/mod_index_tests.lua
+5 -1
View File
@@ -142,7 +142,11 @@ do
index, err = ModIndex.parse(Json.encode({ mods = { NUZLOCKE } })) index, err = ModIndex.parse(Json.encode({ mods = { NUZLOCKE } }))
check(index == nil and err ~= nil, "a feed with no schema_version is refused") check(index == nil and err ~= nil, "a feed with no schema_version is refused")
index, err = ModIndex.parse("<!DOCTYPE html><html>404</html>") index, err = ModIndex.parse("<!DOCTYPE html><html>404</html>")
check(index == nil and err ~= nil, "an HTML error page soft-fails") check(index == nil and tostring(err):find("HTML", 1, true) ~= nil,
"an HTML error page is named, not blamed on the parser")
index, err = ModIndex.parse("Error: upstream unavailable")
check(index == nil and tostring(err):find("not JSON", 1, true) ~= nil,
"a plain-text error names the response")
index, err = ModIndex.parse('{"schema_version":1}') index, err = ModIndex.parse('{"schema_version":1}')
check(index == nil and err ~= nil, "a feed with no mods array soft-fails") check(index == nil and err ~= nil, "a feed with no mods array soft-fails")
end end
tests/engine/mod_update_tests.lua
+25
View File
@@ -76,6 +76,31 @@ do
check(path == nil and dlErr ~= nil, "empty url soft-fails") check(path == nil and dlErr ~= nil, "empty url soft-fails")
end end
-- the reported bug: a non-JSON answer (plain-text error, proxy/captive
-- prompt, outage message) used to leak the decoder's "unexpected character"
-- assert at the first byte of the body. The guard must name what the server
-- actually sent and never let that assert surface.
do
local list, err = ModUpdate.parseReleases("Error: API rate limit exceeded", "demo")
check(list == nil and err ~= nil, "plain-text error soft-fails")
check(tostring(err):find("not JSON", 1, true) ~= nil
and tostring(err):find("Error: API", 1, true) ~= nil,
"plain-text error names the response and previews what it said")
list, err = ModUpdate.parseReleases("<!DOCTYPE html><html>502 Bad Gateway</html>", "demo")
check(list == nil and tostring(err):find("HTML", 1, true) ~= nil,
"an HTML error page is named as such")
list, err = ModUpdate.parseReleases("", "demo")
check(list == nil and tostring(err):find("empty", 1, true) ~= nil,
"an empty response is named")
check(tostring(err):find("unexpected character", 1, true) == nil,
"the decoder's assert never leaks into the message")
list = ModUpdate.parseReleases(Json.encode({
{ tag_name = "v1.0.0", assets = {
{ name = "demo-1.0.0.zip", browser_download_url = "https://x/d.zip" } } },
}), "demo")
eq(#list, 1, "the guard lets real JSON through")
end
do do
local body = Json.encode({ local body = Json.encode({
tag_name = "v2.0.0", tag_name = "v2.0.0",