make tests ignore root more robustly (#42533)

(cherry picked from commit 536d35e)

base/libc.jl

@@ -402,6 +402,9 @@ Interface to the C `srand(seed)` function.
 """
 srand(seed=floor(Int, time()) % Cuint) = ccall(:srand, Cvoid, (Cuint,), seed)
 
+getuid() = ccall(:jl_getuid, Culong, ())
+geteuid() = ccall(:jl_geteuid, Culong, ())
+
 # Include dlopen()/dlpath() code
 include("libdl.jl")
 using .Libdl

src/sys.c

@@ -230,6 +230,24 @@ JL_DLLEXPORT double jl_stat_ctime(char *statbuf)
     return (double)s->st_ctim.tv_sec + (double)s->st_ctim.tv_nsec * 1e-9;
 }
 
+JL_DLLEXPORT unsigned long jl_getuid(void)
+{
+#ifdef _OS_WINDOWS_
+    return -1;
+#else
+    return getuid();
+#endif
+}
+
+JL_DLLEXPORT unsigned long jl_geteuid(void)
+{
+#ifdef _OS_WINDOWS_
+    return -1;
+#else
+    return geteuid();
+#endif
+}
+
 // --- buffer manipulation ---
 
 JL_DLLEXPORT jl_array_t *jl_take_buffer(ios_t *s)

test/file.jl

@@ -516,7 +516,10 @@ end
 
 if !Sys.iswindows()
     # chown will give an error if the user does not have permissions to change files
-    if get(ENV, "USER", "") == "root" || get(ENV, "HOME", "") == "/root"
+    uid = Libc.geteuid()
+    @test stat(file).uid == uid
+    @test uid == Libc.getuid()
+    if uid == 0 # root user
         chown(file, -2, -1)  # Change the file owner to nobody
         @test stat(file).uid != 0
         chown(file, 0, -2)  # Change the file group to nogroup (and owner back to root)

test/read.jl

@@ -465,7 +465,7 @@ rm(f)
 io = Base.Filesystem.open(f, Base.Filesystem.JL_O_WRONLY | Base.Filesystem.JL_O_CREAT | Base.Filesystem.JL_O_EXCL, 0o000)
 @test write(io, "abc") == 3
 close(io)
-if !Sys.iswindows() && get(ENV, "USER", "") != "root" && get(ENV, "HOME", "") != "/root"
+if !Sys.iswindows() && Libc.geteuid() != 0 # root user
     # msvcrt _wchmod documentation states that all files are readable,
     # so we don't test that it correctly set the umask on windows
     @test_throws SystemError open(f)
@@ -515,7 +515,7 @@ close(f1)
 close(f2)
 @test eof(f1)
 @test_throws Base.IOError eof(f2)
-if get(ENV, "USER", "") != "root" && get(ENV, "HOME", "") != "/root"
+if Libc.geteuid() != 0 # root user
     @test_throws SystemError open(f, "r+")
     @test_throws Base.IOError Base.Filesystem.open(f, Base.Filesystem.JL_O_RDWR)
 else