make precompile files writable (#41614)

(cherry picked from commit 6d58068)

Author: KristofferC

base/loading.jl

@@ -1354,8 +1354,8 @@ function compilecache(pkg::PkgId, path::String, internal_stderr::IO = stderr, in
             open(tmppath, "a+") do f
                 write(f, _crc32c(seekstart(f)))
             end
-            # inherit permission from the source file
-            chmod(tmppath, filemode(path) & 0o777)
+            # inherit permission from the source file (and make them writable)
+            chmod(tmppath, filemode(path) & 0o777 | 0o200)
 
             # Read preferences hash back from .ji file (we can't precompute because
             # we don't actually know what the list of compile-time preferences are without compiling)

test/precompile.jl

@@ -826,6 +826,10 @@ precompile_test_harness("Issue #25971") do load_path
     chmod(sourcefile, 0o600)
     cachefile = Base.compilecache(Base.PkgId("Foo25971"))
     @test filemode(sourcefile) == filemode(cachefile)
+    chmod(sourcefile, 0o444)
+    cachefile = Base.compilecache(Base.PkgId("Foo25971"))
+    # Check writable
+    @test touch(cachefile) == cachefile
 end
 
 precompile_test_harness("Issue #38312") do load_path