Precompile the test environment inside the serialized window

testprocess/TestItemServer/src/TestItemServer.jl

@@ -590,7 +590,7 @@
         setup_details = state.test_setups[(params.packageUri, Symbol(i))]
 
         if setup_details.kind==:module && !setup_details.evaled
             mod = Core.eval(Main.Testsetups, :(module $(Symbol(i)) end))
 
             code = string('\n'^(setup_details.line-1), ' '^(setup_details.column-1), setup_details.code)
 
@@ -1089,6 +1089,34 @@
             TestEnv.activate(params.packageName)
         end
 
+        # The controller serializes this request: one test process is nominated to activate
+        # while every other one waits, and they are released when it reports back. The point
+        # of that is to build the test environment's precompile caches exactly once, because
+        # Julia has no cache file locking before 1.10 — two processes precompiling the same
+        # package race, and on Windows the loser cannot replace a `.ji` the winner holds
+        # open, so it dies with "Cannot write cache file".
+        #
+        # From Julia 1.9 on, `TestEnv.activate` finishes with `Pkg._auto_precompile` on the
+        # sandbox environment, so the caches really are built inside that window. The older
+        # variants stop at `Pkg.activate`, and `Pkg.instantiate` did not precompile before
+        # Julia 1.6 either — so nothing was compiled until the first `using` inside a test
+        # item, which happens in every test process at once, after the gate has let them all
+        # go. Precompiling here puts that work back inside the serialized window.
+        @static if VERSION < v"1.9"
+            try
+                # `Pkg.precompile` does not exist before Julia 1.4 — it only became a
+                # forwarder to `Pkg.API.precompile` then, and before that the name resolved
+                # to `Base.precompile` through the implicit `using Base` and threw a
+                # MethodError. `Pkg.API.precompile()` has a zero-argument method in every
+                # version this branch runs on.
+                Pkg.API.precompile()
+            catch err
+                # Not worth failing activation over: whatever is wrong will come back with a
+                # better message when a test item loads the package.
+                @debug "Precompiling the test environment failed" exception = (err, catch_backtrace())
+            end
+        end
+
         return TestItemServerProtocol.ActivateEnvResult(
             status = "success",
             error = missing