Use the date inlcuded in the query to calculate offset in MySQL [ci drivers]

Previously current date was used determine the offset for a given
timezone. This causes issues when the date being queried has a
different offset than today's date. Daylight savings time is an
example of that as depending on when the date is, the offset can be
different.

Fixes #7106

src/metabase/driver/mysql.clj

 (ns metabase.driver.mysql
   "MySQL driver. Builds off of the Generic SQL driver."
   (:require [clj-time
+             [coerce :as tcoerce]
              [core :as t]
              [format :as time]]
             [clojure
@@ -108,24 +109,30 @@
   (time/formatter "ZZ"))
 
 (defn- timezone-id->offset-str
-  "Get an appropriate timezone offset string for a timezone with `timezone-id`. MySQL only accepts these offsets as
-  strings like `-8:00`.
+  "Get an appropriate timezone offset string for a timezone with `timezone-id` and `date-time`. MySQL only accepts
+  these offsets as strings like `-8:00`.
 
-      (timezone-id->offset-str \"US/Pacific\") ; -> \"-08:00\"
+      (timezone-id->offset-str \"US/Pacific\", date-time) ; -> \"-08:00\"
 
-  Returns `nil` if `timezone-id` is itself `nil`."
-  [^String timezone-id]
+  Returns `nil` if `timezone-id` is itself `nil`. The `date-time` must be included as some timezones vary their
+  offsets at different times of the year (i.e. daylight savings time)."
+  [^String timezone-id date-time]
   (when timezone-id
-    (time/unparse (.withZone timezone-offset-formatter (t/time-zone-for-id timezone-id)) (t/now))))
+    (time/unparse (.withZone timezone-offset-formatter (t/time-zone-for-id timezone-id)) date-time)))
 
-(def ^:private ^String system-timezone-offset-str
-  (timezone-id->offset-str (.getID (TimeZone/getDefault))))
+(defn- ^String system-timezone->offset-str
+  "Get the system/JVM timezone offset specified at `date-time`. The time is needed here as offsets can change for a
+  given timezone based on the time of year (i.e. daylight savings time)."
+  [date-time]
+  (timezone-id->offset-str (.getID (TimeZone/getDefault)) date-time))
 
 ;; MySQL doesn't seem to correctly want to handle timestamps no matter how nicely we ask. SAD! Thus we will just
 ;; convert them to appropriate timestamp literals and include functions to convert timezones as needed
 (defmethod sqlqp/->honeysql [MySQLDriver Date]
   [_ date]
-  (let [report-timezone-offset-str (timezone-id->offset-str (driver/report-timezone))]
+  (let [date-as-dt                 (tcoerce/from-date date)
+        report-timezone-offset-str (timezone-id->offset-str (driver/report-timezone) date-as-dt)
+        system-timezone-offset-str (system-timezone->offset-str date-as-dt)]
     (if (and report-timezone-offset-str
              (not= report-timezone-offset-str system-timezone-offset-str))
       ;; if we have a report timezone we want to generate SQL like convert_tz('2004-01-01T12:00:00','-8:00','-2:00')

test/metabase/driver/mysql_test.clj

 (ns metabase.driver.mysql-test
-  (:require [expectations :refer :all]
+  (:require [clj-time.core :as t]
+            [expectations :refer :all]
             [metabase
              [sync :as sync]
              [util :as u]]
@@ -97,9 +98,18 @@
   (with-redefs [metabase.driver/execute-query (constantly {:rows [["2018-01-08 23:00:00.008 CET"]]})]
     (tu/db-timezone-id)))
 
-(expect (#'mysql/timezone-id->offset-str "US/Pacific")          "-08:00")
-(expect (#'mysql/timezone-id->offset-str "UTC")                 "+00:00")
-(expect (#'mysql/timezone-id->offset-str "America/Los_Angeles") "-08:00")
+
+(def before-daylight-savings (u/str->date-time "2018-03-10 10:00:00"))
+(def after-daylight-savings (u/str->date-time "2018-03-12 10:00:00"))
+
+(expect (#'mysql/timezone-id->offset-str "US/Pacific" before-daylight-savings) "-08:00")
+(expect (#'mysql/timezone-id->offset-str "US/Pacific" after-daylight-savings)  "-07:00")
+
+(expect (#'mysql/timezone-id->offset-str "UTC" before-daylight-savings) "+00:00")
+(expect (#'mysql/timezone-id->offset-str "UTC" after-daylight-savings) "+00:00")
+
+(expect (#'mysql/timezone-id->offset-str "America/Los_Angeles" before-daylight-savings) "-08:00")
+(expect (#'mysql/timezone-id->offset-str "America/Los_Angeles" after-daylight-savings) "-07:00")
 
 ;; make sure DateTime types generate appropriate SQL...
 ;; ...with no report-timezone set