[PATCH v4 08/11] tools/ocaml/libs/mmap: mark mmap/munmap as blocking and raise Unix_error on failure

[Edwin Török <edvin.torok@xxxxxxxxxx>]

These functions can potentially take some time,
so allow other OCaml code to proceed meanwhile (if any).

Also raise a Unix_error based on `errno` if `mmap` fails, instead of just
calling failwith (which would lose the error reason).

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---

Changes since v2:
* repost of lost patch from 2020 (posted to ML on 2021
  
https://patchwork.kernel.org/project/xen-devel/patch/294a60be29027d33b0a1d154b7d576237c7dd420.1620755942.git.edvin.torok@xxxxxxxxxx/)
---
 tools/ocaml/libs/mmap/xenmmap_stubs.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/tools/ocaml/libs/mmap/xenmmap_stubs.c 
b/tools/ocaml/libs/mmap/xenmmap_stubs.c
index bf864a7c32..5b8c240ef9 100644
--- a/tools/ocaml/libs/mmap/xenmmap_stubs.c
+++ b/tools/ocaml/libs/mmap/xenmmap_stubs.c
@@ -27,6 +27,8 @@
 #include <caml/custom.h>
 #include <caml/fail.h>
 #include <caml/callback.h>
+#include <caml/unixsupport.h>
+#include <caml/signals.h>
 
 #define Intf_val(a) ((struct mmap_interface *) Data_abstract_val(a))
 
@@ -35,7 +37,9 @@ static int mmap_interface_init(struct mmap_interface *intf,
                                int len, int offset)
 {
     intf->len = len;
+    caml_enter_blocking_section();
     intf->addr = mmap(NULL, len, pflag, mflag, fd, offset);
+    caml_leave_blocking_section();
     return (intf->addr == MAP_FAILED) ? errno : 0;
 }
 
@@ -64,17 +68,22 @@ CAMLprim value stub_mmap_init(value fd, value pflag, value 
mflag,
     if (mmap_interface_init(Intf_val(result), Int_val(fd),
                             c_pflag, c_mflag,
                             Int_val(len), Int_val(offset)))
-        caml_failwith("mmap");
+        uerror("mmap", Nothing);
     CAMLreturn(result);
 }
 
 CAMLprim value stub_mmap_final(value intf)
 {
     CAMLparam1(intf);
+    struct mmap_interface interface = *Intf_val(intf);
 
-    if (Intf_val(intf)->addr != MAP_FAILED)
-        munmap(Intf_val(intf)->addr, Intf_val(intf)->len);
+    /* mark it as freed, in case munmap below fails, so we don't retry it */
     Intf_val(intf)->addr = MAP_FAILED;
+    if (interface.addr != MAP_FAILED) {
+        caml_enter_blocking_section();
+        munmap(interface.addr, interface.len);
+        caml_leave_blocking_section();
+    }
 
     CAMLreturn(Val_unit);
 }
-- 
2.34.1