fix(mysql-connector-cpp): a CN mirror, and a success path that says so

TWO SMALL THINGS, both found while this package kept turning up in the
middle of something else.

CN MIRROR

The url was a plain upstream string, which docs/cn-mirror.md describes as
the fallback for when nobody has `mcpp-res` write access -- CN users reach
GitHub directly and wait. This package sits on the path of anything that
touches the graphics or database members, and a 4.8 MB tarball with no
mirror was a recurring tax on exactly the runs where something else was
being diagnosed.

`mcpp-res/mysql-connector-cpp@26.7.0` now carries the same bytes:

    CN http=200  size=4792083
    GLOBAL sha  b2299862eefc33fd71c0aac68328305671805fc955e6bd2578ef205c10f98550
    CN     sha  b2299862eefc33fd71c0aac68328305671805fc955e6bd2578ef205c10f98550

Same sha as the descriptor already declared, so no version's hash moves.

`return true` ON THE SUCCESS PATH

`install()` wraps its work in a pcall and returns whatever that yields.
Every failure path says `return false`; the success path ended on an
`io.writefile` and yielded `nil`. So a WORKING install has been logging

    install() result=nil

all along. In a hook log that is otherwise a list of failures, that reads
like one — and it did: during the huxerui work it was taken for the cause
of a failure whose real origin was an mcpp scanner regression three layers
away. The value was never wrong; it just never said it was right.

Author: Sunrisepeak

pkgs/c/compat.mysql-connector-cpp.lua

@@ -17,7 +17,10 @@ package = {
                 "xim:make@latest",
             },
             ["26.7.0"] = {
-                url = "https://github.com/mysql/mysql-connector-cpp/archive/refs/tags/26.7.0.tar.gz",
+                url    = {
+                    GLOBAL = "https://github.com/mysql/mysql-connector-cpp/archive/refs/tags/26.7.0.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/mysql-connector-cpp/releases/download/26.7.0/mysql-connector-cpp-26.7.0.tar.gz",
+                },
                 sha256 = "b2299862eefc33fd71c0aac68328305671805fc955e6bd2578ef205c10f98550",
             },
         },
@@ -28,7 +31,10 @@ package = {
                 "xim:cmake@latest",
             },
             ["26.7.0"] = {
-                url = "https://github.com/mysql/mysql-connector-cpp/archive/refs/tags/26.7.0.tar.gz",
+                url    = {
+                    GLOBAL = "https://github.com/mysql/mysql-connector-cpp/archive/refs/tags/26.7.0.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/mysql-connector-cpp/releases/download/26.7.0/mysql-connector-cpp-26.7.0.tar.gz",
+                },
                 sha256 = "b2299862eefc33fd71c0aac68328305671805fc955e6bd2578ef205c10f98550",
             },
         },
@@ -318,6 +324,14 @@ function install()
 
     io.writefile(path.join(prefix, "mcpp_mysql_connector_cpp_anchor.c"),
                  "int mcpp_compat_mysql_connector_cpp_anchor(void) { return 0; }\n")
+
+    -- EXPLICIT, because the last statement above is not a return and the
+    -- pcall would otherwise hand `nil` back as the success value. Every
+    -- failure path here says `return false`; success said nothing, so
+    -- `install() result=nil` is what a WORKING install has been logging all
+    -- along. In a log that is mostly failures that reads like one, and it
+    -- cost a wrong diagnosis once already.
+    return true
         end)
     if not ok then
         hook_log("UNCAUGHT Lua error: " .. tostring(result))