make sure that the read task is done when precompiling (#29024)

Makefile

@@ -206,7 +206,7 @@ define sysimg_builder
 $$(build_private_libdir)/sys$1-o.a $$(build_private_libdir)/sys$1-bc.a : $$(build_private_libdir)/sys$1-%.a : $$(build_private_libdir)/sys.ji
 	@$$(call PRINT_JULIA, cd $$(JULIAHOME)/base && \
 	if ! $$(call spawn,$3) $2 -C "$$(JULIA_CPU_TARGET)" --output-$$* $$(call cygpath_w,$$@).tmp $$(JULIA_SYSIMG_BUILD_FLAGS) \
-		--startup-file=no --warn-overwrite=yes --sysimage $$(call cygpath_w,$$<) $$(call cygpath_w,$$(JULIAHOME)/contrib/generate_precompile.jl) $$(call cygpath_w,$$<); then \
+		--startup-file=no --warn-overwrite=yes --sysimage $$(call cygpath_w,$$<) $$(call cygpath_w,$$(JULIAHOME)/contrib/generate_precompile.jl); then \
 		echo '*** This error is usually fixed by running `make clean`. If the error persists$$(COMMA) try `make cleanall`. ***'; \
 		false; \
 	fi )

contrib/generate_precompile.jl

@@ -69,8 +69,7 @@ function generate_precompile_statements()
     end
 
     print("Generating precompile statements...")
-    sysimg = isempty(ARGS) ? joinpath(dirname(Sys.BINDIR), "lib", "julia", "sys." * Libdl.dlext) : ARGS[1]
-
+    sysimg = Base.unsafe_string(Base.JLOptions().image_file)
     mktemp() do precompile_file, _
         # Run a repl process and replay our script
         repl_output_buffer = IOBuffer()
@@ -87,31 +86,46 @@ function generate_precompile_statements()
         done = false
         withenv("JULIA_HISTORY" => tempname(), "JULIA_PROJECT" => nothing,
                 "TERM" => "") do
-            p = run(`$(julia_cmd()) -O0 --trace-compile=$precompile_file --sysimage $sysimg
-                    --compile=all --startup-file=no --color=yes
-                    -e 'import REPL; REPL.Terminals.is_precompiling[] = true'
-                    -i`,
-                    slave, slave, slave; wait=false)
-            readuntil(master, "julia>", keep=true)
-            @async begin
-                while true
-                    done && break
-                    write(repl_output_buffer, readavailable(master))
-                end
-            end
             if have_repl
-                for l in split(precompile_script, '\n'; keepempty=false)
-                    write(master, l, '\n')
+                p = run(`$(julia_cmd()) -O0 --trace-compile=$precompile_file --sysimage $sysimg
+                        --compile=all --startup-file=no --color=yes
+                        -e 'import REPL; REPL.Terminals.is_precompiling[] = true'
+                        -i`,
+                        slave, slave, slave; wait=false)
+                readuntil(master, "julia>", keep=true)
+                t = @async begin
+                    while true
+                        sleep(0.5)
+                        s = String(readavailable(master))
+                        write(repl_output_buffer, s)
+                        if occursin("__PRECOMPILE_END__", s)
+                            break
+                        end
+                    end
                 end
-            end
-            # TODO Figure out why exit() on Windows doesn't exit the process
-            if Sys.iswindows()
-                print(master, "ccall(:_exit, Cvoid, (Cint,), 0)\n")
+                if have_repl
+                    for l in split(precompile_script, '\n'; keepempty=false)
+                        write(master, l, '\n')
+                    end
+                end
+                write(master, "print(\"__PRECOMPILE\", \"_END__\")", '\n')
+                wait(t)
+
+                # TODO Figure out why exit() on Windows doesn't exit the process
+                if Sys.iswindows()
+                    print(master, "ccall(:_exit, Cvoid, (Cint,), 0)\n")
+                else
+                    write(master, "exit()\n")
+                    readuntil(master, "exit()\r\e[13C\r\n")
+                    @assert bytesavailable(master) == 0
+                end
+                wait(p)
             else
-                write(master, "exit()\n")
+                # Is this even needed or is this already recorded just from starting this process?
+                p = run(`$(julia_cmd()) -O0 --trace-compile=$precompile_file --sysimage $sysimg
+                        --compile=all --startup-file=no
+                        -e0`)
             end
-            wait(p)
-            done = true
         end
         close(master)