fix(manifest): actionable errors when USCOGDATA_URL is unset or returns non-JSON

`cog_gov_search()` (and every other verb) used to fail with a cryptic
`jsonlite` lexical error when the package's placeholder default URL was
hit and the server returned an HTML welcome page that got cached as
`manifest.json`. Three guards added:

1. `.check_url_configured()` aborts with class `uscogdata_url_not_configured`
   when the resolved URL is empty or still contains the
   `REPLACE_WITH_SHARE_TOKEN` sentinel. Message names both
   `Sys.setenv(USCOGDATA_URL = ...)` and `options(uscogdata.url = ...)`
   remediations and points at the bundled fixture.
2. `.fetch_or_cache_manifest()` parses the response body before persisting
   it. Non-JSON payloads raise class `uscogdata_invalid_manifest` (URL,
   Content-Type, parse error) and never touch the on-disk cache.
3. Cache writes are atomic via a sibling tempfile + `file.rename`, and
   existing caches with non-JSON content are silently refetched instead
   of returning a parse error to the caller.

Local-path manifests that aren't valid JSON now surface the same
`uscogdata_invalid_manifest` class with file context.

R/manifest.R

@@ -1,5 +1,66 @@
 # R/manifest.R
 
+# Sentinel substring baked into the placeholder default URL. If we see this
+# in the resolved URL, the user hasn't configured USCOGDATA_URL yet.
+.PLACEHOLDER_TOKEN <- "REPLACE_WITH_SHARE_TOKEN"
+
+#' Abort with actionable guidance when the resolved corpus URL is still the
+#' placeholder shipped with the package (or any URL containing the sentinel).
+#' Called from `cog_open()` before any I/O so users see a clear message
+#' instead of a downstream JSON parse error.
+#' @noRd
+.check_url_configured <- function(url) {
+  if (!is.character(url) || length(url) != 1L || !nzchar(url)) {
+    cli::cli_abort(c(
+      "USCOGDATA_URL is not configured.",
+      i = "Set the corpus location via one of:",
+      "*" = "{.code Sys.setenv(USCOGDATA_URL = \"<url-or-local-path>/\")}",
+      "*" = "{.code options(uscogdata.url = \"<url-or-local-path>/\")}",
+      i = "For an offline smoke test, use the bundled fixture: {.code system.file(\"extdata/fixture_corpus\", package = \"uscogdata\")}."
+    ), class = "uscogdata_url_not_configured")
+  }
+  if (grepl(.PLACEHOLDER_TOKEN, url, fixed = TRUE)) {
+    sentinel <- .PLACEHOLDER_TOKEN
+    cli::cli_abort(c(
+      "USCOGDATA_URL is not configured (placeholder URL detected).",
+      x = "Current value contains the sentinel {.val {sentinel}}: {.url {url}}",
+      i = "Set the corpus location via one of:",
+      "*" = "{.code Sys.setenv(USCOGDATA_URL = \"<url-or-local-path>/\")}",
+      "*" = "{.code options(uscogdata.url = \"<url-or-local-path>/\")}",
+      i = "For an offline smoke test, use the bundled fixture: {.code system.file(\"extdata/fixture_corpus\", package = \"uscogdata\")}.",
+      i = "For the live Civilytics corpus, request the Nextcloud share URL from the package maintainer."
+    ), class = "uscogdata_url_not_configured")
+  }
+  invisible(url)
+}
+
+#' Try to parse a JSON file. Returns parsed object on success, NULL on
+#' any parse failure (so callers can decide whether to refetch).
+#' @noRd
+.try_parse_manifest_file <- function(path) {
+  tryCatch(
+    jsonlite::fromJSON(path, simplifyVector = FALSE),
+    error = function(e) NULL
+  )
+}
+
+#' Abort with a clear, classified error when a manifest payload (string or
+#' file) cannot be parsed as JSON. Surfaces the URL, content-type if known,
+#' and the underlying parse error.
+#' @noRd
+.abort_invalid_manifest <- function(source, content_type = NA_character_, parse_error = NULL) {
+  ct <- if (is.na(content_type) || !nzchar(content_type)) "<unknown>" else content_type
+  pmsg <- if (is.null(parse_error)) "" else conditionMessage(parse_error)
+  cli::cli_abort(c(
+    "Corpus manifest is not valid JSON.",
+    x = "Source: {source}",
+    i = "Content-Type: {ct}",
+    i = "Likely causes: USCOGDATA_URL points at a login page, a 404 HTML page, or the wrong share; or the corpus has not been published yet.",
+    i = "Set USCOGDATA_URL to a directory (local path or HTTPS) that serves manifest.json directly.",
+    if (nzchar(pmsg)) c(">" = "Parse error: {pmsg}") else NULL
+  ), class = "uscogdata_invalid_manifest")
+}
+
 #' Fetch manifest.json from URL (or read from a local fixture path),
 #' cache locally, validate TTL.
 #' @noRd
@@ -11,23 +72,54 @@
     if (!file.exists(local_manifest)) {
       cli::cli_abort("Local fixture has no manifest.json at {local_manifest}")
     }
-    return(jsonlite::fromJSON(local_manifest, simplifyVector = FALSE))
+    return(tryCatch(
+      jsonlite::fromJSON(local_manifest, simplifyVector = FALSE),
+      error = function(e) .abort_invalid_manifest(source = local_manifest, parse_error = e)
+    ))
   }
 
   cache_path <- file.path(cache_dir, "manifest.json")
   ttl <- as.integer(.cfg("manifest_ttl_secs"))
 
-  needs_fetch <- !file.exists(cache_path) ||
-                 difftime(Sys.time(), file.info(cache_path)$mtime, units = "secs") > ttl
+  cache_fresh <- file.exists(cache_path) &&
+    difftime(Sys.time(), file.info(cache_path)$mtime, units = "secs") <= ttl
 
-  if (needs_fetch) {
-    resp <- httr2::request(paste0(url, "manifest.json")) |>
-      httr2::req_error(is_error = function(r) httr2::resp_status(r) >= 400) |>
-      httr2::req_perform()
-    writeLines(httr2::resp_body_string(resp), cache_path)
+  # Honor a fresh cache only if its contents still parse as JSON. A previous
+  # version of this package could write HTML directly into the cache; treat
+  # such poisoned caches as if they were missing so the next call recovers.
+  if (cache_fresh) {
+    parsed <- .try_parse_manifest_file(cache_path)
+    if (!is.null(parsed)) return(parsed)
   }
 
-  jsonlite::fromJSON(cache_path, simplifyVector = FALSE)
+  resp <- httr2::request(paste0(url, "manifest.json")) |>
+    httr2::req_error(is_error = function(r) httr2::resp_status(r) >= 400) |>
+    httr2::req_perform()
+  body <- httr2::resp_body_string(resp)
+
+  # Parse BEFORE persisting. If the server returned HTML / a login page /
+  # any non-JSON body with a 2xx status, we must not write it to the cache.
+  parsed <- tryCatch(
+    jsonlite::fromJSON(body, simplifyVector = FALSE),
+    error = function(e) {
+      ct <- tryCatch(httr2::resp_content_type(resp), error = function(e2) NA_character_)
+      .abort_invalid_manifest(
+        source = paste0(url, "manifest.json"),
+        content_type = ct,
+        parse_error = e
+      )
+    }
+  )
+
+  # Atomic write: tmp file alongside cache_path (same filesystem -> no EXDEV)
+  # then rename. Ensures a partial write or interrupted process never
+  # replaces a previously-good cache.
+  if (!dir.exists(cache_dir)) dir.create(cache_dir, recursive = TRUE)
+  tmp <- paste0(cache_path, ".tmp.", Sys.getpid())
+  on.exit(if (file.exists(tmp)) unlink(tmp), add = TRUE)
+  writeLines(body, tmp)
+  file.rename(tmp, cache_path)
+  parsed
 }
 
 #' @noRd