Limit of length of row for json parsing (fixes #22636 for most cases) (#22769)

Limit the length of the row for JSON parsing. This was the simplest way to make another attack at #22636.

src/metabase/driver/postgres.clj

@@ -190,21 +190,14 @@
   (binding [*enum-types* (enum-types driver database)]
     (sql-jdbc.sync/describe-table driver database table)))
 
-(def ^:const max-nested-field-columns
-  "Maximum number of nested field columns."
-  100)
-
 ;; Describe the nested fields present in a table (currently and maybe forever just JSON),
 ;; including if they have proper keyword and type stability.
 ;; Not to be confused with existing nested field functionality for mongo,
 ;; since this one only applies to JSON fields, whereas mongo only has BSON (JSON basically) fields.
 (defmethod sql-jdbc.sync/describe-nested-field-columns :postgres
   [driver database table]
-  (let [spec   (sql-jdbc.conn/db->pooled-connection-spec database)
-        fields (sql-jdbc.describe-table/describe-nested-field-columns driver spec table)]
-    (if (> (count fields) max-nested-field-columns)
-      #{}
-      fields)))
+  (let [spec   (sql-jdbc.conn/db->pooled-connection-spec database)]
+    (sql-jdbc.describe-table/describe-nested-field-columns driver spec table)))
 
 ;;; +----------------------------------------------------------------------------------------------------------------+
 ;;; |                                           metabase.driver.sql impls                                            |

src/metabase/driver/sql_jdbc/sync/describe_table.clj

@@ -208,6 +208,11 @@
   "Number of rows to sample for describe-nested-field-columns"
   500)
 
+(def ^:dynamic *nested-field-column-max-row-length*
+  "Max string length for a row for nested field column before we just give up on parsing it.
+  Marked as mutable because we mutate it for tests."
+  50000)
+
 (defn- flattened-row [field-name row]
   (letfn [(flatten-row [row path]
             (lazy-seq
@@ -237,10 +242,16 @@
                (into {} (map (fn [[k v]] [k (type-by-parsing-string v)]) flat-row))))))
 
 (defn- describe-json-xform [member]
-  ((comp (map #(for [[k v] %] [k (json/parse-string v)]))
+  ((comp (map #(for [[k v] %
+                     :when (< (count v) *nested-field-column-max-row-length*)]
+                 [k (json/parse-string v)]))
          (map #(into {} %))
          (map row->types)) member))
 
+(def ^:const max-nested-field-columns
+  "Maximum number of nested field columns."
+  100)
+
 (defn- describe-json-rf
   "Reducing function that takes a bunch of maps from row->types,
   and gets them to conform to the type hierarchy,
@@ -323,7 +334,6 @@
         field-hash   (apply hash-set (filter some? valid-fields))]
     field-hash))
 
-
 ;; The name's nested field columns but what the people wanted (issue #708)
 ;; was JSON so what they're getting is JSON.
 (defn describe-nested-field-columns
@@ -344,4 +354,6 @@
               query            (jdbc/reducible-query spec sql-args)
               field-types      (transduce describe-json-xform describe-json-rf query)
               fields           (field-types->fields field-types)]
-          fields)))))
+          (if (> (count fields) max-nested-field-columns)
+            #{}
+            fields))))))

test/metabase/driver/sql_jdbc/sync/describe_table_test.clj

@@ -82,6 +82,23 @@
       (is (= {} (#'sql-jdbc.describe-table/row->types arr-row)))
       (is (= {[:zlob "blob"] java.lang.Long} (#'sql-jdbc.describe-table/row->types obj-row))))))
 
+(deftest dont-parse-long-json-xform-test
+  (testing "obnoxiously long json should not even get parsed (#22636)"
+    ;; Generating an actually obnoxiously long json took too long,
+    ;; and actually copy-pasting an obnoxiously long string in there looks absolutely terrible,
+    ;; so this rebinding is what you get
+    (let [obnoxiously-long-json "{\"bob\": \"dobbs\"}"
+          json-map              {:somekey obnoxiously-long-json}]
+      (with-redefs [sql-jdbc.describe-table/*nested-field-column-max-row-length* 3]
+        (is (= {}
+               (transduce
+                 #'sql-jdbc.describe-table/describe-json-xform
+                 #'sql-jdbc.describe-table/describe-json-rf [json-map]))))
+      (is (= {[:somekey "bob"] java.lang.String}
+             (transduce
+               #'sql-jdbc.describe-table/describe-json-xform
+               #'sql-jdbc.describe-table/describe-json-rf [json-map]))))))
+
 (deftest describe-nested-field-columns-test
   (testing "flattened-row"
     (let [row       {:bob {:dobbs 123 :cobbs "boop"}}